CHEMISTRY Standard X PART-1

Transcription

1 PDF Compressor Free Version CHEMISTRY Standard X PART-1 Government of Kerala Department of Education State Council of Educational Research and Training (SCERT) Kerala 2016

2 PDF Compressor Free Version THE NATIONAL ANTHEM Jana-gana-mana adhinayaka jaya he Bharatha-bhagya-vidhata, Punjab-Sindh-Gujarat-Maratha Dravida-Utkala-Banga Vindhya-Himachala-Yamuna-Ganga Uchchala-Jaladhi-taranga Tava subha name jage, Tava subha asisa mage, Gahe tava jaya gatha. Jana-gana-mangala-dayaka jaya he Bharatha-bhagya-vidhata, Jaya he, jaya he, jaya he, Jaya jaya jaya jaya he! PLEDGE India is my country. All Indians are my brothers and sisters. I love my country, and I am proud of its rich and varied heritage. I shall always strive to be worthy of it. I shall give my parents, teachers and all elders respect, and treat everyone with courtesy. To my country and my people, I pledge my devotion. In their well-being and prosperity alone lies my happiness. State Council of Educational Research and Training (SCERT) Poojappura, Thiruvananthapuram , Kerala Website : scertkerala@gmail.com Phone : , Fax : Typesetting and Layout : SCERT Printed at : KBPS, Kakkanad, Kochi-30 Department of Education, Government of Kerala

3 PDF Compressor Free Version Dear children, As students of Class X, the highest class in a high school you are at the portals of the higher secondary education. This 'Chemistry' text has accordingly, been designed to cater to the demands of the next academic level. The text provides opportunities for student's active participation in the classrooms. We have tried to organise these activities giving due emphasis to the level of your competence through investigative learning. Scientific activity, while enabling social progress, must also uphold eco-friendly values. This should happen at deeper levels of any science enquiry and activity. The text has tried to incorporate such ideologies to the possible extent and to discuss emerging areas like Green Chemistry. The initial units focus on explaining peculiarities of elements related to their electronic configuration, identifying the relationship between mass of substances and their number of molecules and also identifying the significance of mole concept in chemistry. Following this, discussions on the rate of reactions and equilibrium and chemical reactivity of metals and their stages of production are made. Exposing the students to the basic concepts of Organic Chemistry, the book has introduced naturally occuring and man made materials including medicines, polymers etc. which are inevitable to human progress. It is the duty of each one of you to assimilate ideas in this text, carry out the activities effectively and attain your goal. Hope that your journey be fruitful in this through active interactions and appropriate activities. Wishing you the best... Dr P.A. Fathima Director, SCERT

4 PDF Compressor Free Version Textbook Development Team Members Anil M.R. HSST (HG), GGHSS, Karamana, Thiruvananthapuram Anilkumar P.K. HSA, CHMHSS, Kavumpady, Thillenkery, Kannur Babu Payyath BPO, BRC, Meladi, Kozhikkode Premachandran K.V. HSA, GHSS, Maniyoor, Kozhikkode Dr Subhash Babu Deputy Director (Rtd.), Collegiate Education, Thiruvananthapuram Sebastian Lukose T.J. Selection Grade Lecturer of Chemistry (Rtd.), University College, Thiruvananthapuram Abhilash Thiruvoth GVHSS, Payyoli,Kozhikkode Experts Pushpa N. HSA, GGHSS, Attingal Thiruvananthapuram Santhoshkumar V.G. HSA, BYKV HSS, Valavannur, Malappuram Aloysius E. HSA, St. Joseph HSS, Thiruvananthapuram Annie Varghese HSA, GHSS, Kudamalloor, Kottayam Dr Alaudeen M. Principal (Rtd.), Govt. College, Elarithattu, Kasaragode Bimalkumar S. GBHSS, Thevalli, Kollam English Version Artists Academic Coordinator Dr Sobha Jacob Research Officer, SCERT, Kerala Dr Vishnu V.S. Assistant Professor, Department of Chemistry, Govt. Arts College, Thiruvananthapuram Dr Abraham George Head, Dept. of Chemistry (Rtd.), Mar Ivanious College, Thiruvananthapuram Dr Sushil Kumar R. Associate Professor of English, DB College, Sasthamcotta, Kollam Rathy S Nair HSST English, Big Bazar HSS, Palakkad Raakhy Radhakrishnan HSA English, RRVBHSS, Kilimanoor, Thiruvananthapuram Sonia S Ravi HSA English, Concordia HSS, Peroorkada, Thiruvananthapuram Mooza Mustajib E.C. MMETHSS, Melmuri, Malappuram Lohithakshan K. Assisi HSS for deaf, Malaparambu, Malappuram

5 PDF Compressor Free Version 1 PERIODIC TABLE AND ELECTRONIC CONFIGURATION MOLE CONCEPT RATE OF CHEMICAL REACTIONS AND CHEMICAL EQUILIBRIUM REACTIVITY SERIES AND ELECTROCHEMISTRY... 71

6 PDF Compressor Free Version THE SYMBOLS USED IN THE TEXTBOOK Additional Information (Need not be assessed) ICT Possibilities for Concept Clarity Significant Learning Outcomes Let Us Assess Extended Activities

7 PDF Compressor Free Version s p d What is s, p, d, f? Hints Gases Liquid Synthetic element f Don't you also have the same doubt? Let us try to find more details relating to the periodic table. You have learned in the lower class about the modern periodic table in which the elements are comprehensively classified. What is the importance of the periodic table? What is the basis of the classification of elements in this table? Don't you think that elements are classified in such a way that their properties can be precisely and clearly analysed and predicted? Is there any relation between atomic structure and the classification of elements in the periodic table? Let us examine.

8 You have already learned about different atom PDF Compressor Free Version models. You also know that according to the Bohr model of the atom, electrons are arranged in Subshells different shells or energy levels around the The names s, p, d, f are given to nucleus in the increasing order of energy. subshells based on words indicating As the distance from the nucleus increases, certain properties related to the the energy of the electrons in the shells atomic structure of elements. increases and the attractive force between the s sharp, p principal, nucleus and the electrons decreases. d diffuse, f fundamental. According to modern theories of atomic structure, electrons revolve in a three dimensional region around the nucleus. Each main energy level has sub levels or sub shells. In these subshells, there are regions where there is a high probability of finding electrons. These are known as orbitals. The maximum number of electrons that can be accommodated in an orbital is 2. The s subshell has only one orbital with a spherical shape. The p subshell has 3 orbitals which are dumb-bell shaped; the d subshell has 5 orbitals and the f subshell has 7 orbitals. The shapes of these orbitals are more complex. 8 s subshell p subshell The electronic configuration of lithium ( 3 Li) is 2, 1. Like this, write the electronic configuration of sodium and argon and complete Table 1.1. Element Shells K L M Na Ar Table 1.1 $ How many electrons are present in the M shell of argon which is its outermost shell? $ How many more electrons can be accommodated in the M shell? The element after argon, potassium ( 19 K) has one electron more than that of argon. The electronic configuration of potassium is 2, 8, 8, 1. Why does the last electron of potassium go to the 4 th shell instead of occupying the third shell, even though the third shell has the capacity to accommodate ten more electrons? Based on further studies related to atomic structure, scientists have formulated certain assumptions and laws to explain this. Accordingly, shells such as K, L, M, N are considered principal energy levels and each principal energy level contains sub energy levels

9 or subshells. All main energy levels or shells except K have more than one PDF sub Compressor shell. K shell Free has Version only one subshell. The subshells are named s, p, d and f. Each of the main energy level contains the same number of subshells as its serial number. Thus the K shell, which is the first shell, has 1 subshell. L shell has 2 sub shells, and so on. $ What will be the number of subshells in the shells M and N? M =..., N =... See the subshells present in each shell shown in Table 1.2. Shell number Using the KALZIUM software of school Edubuntu, check the accuracy of the entries in Table 1.1. Subshells s s, p s, p, d s, p, d, f Table 1.2 Which subshell is common to all shells? How can you identify the shell to which each subshell belongs? Let's attach the serial number of the shell to the symbol of the subshell. For example, '1s' to indicate the s subshell of the first shell and '2s' for the s subshell of second shell and so on. Fill in Table 1.3. Shell number Subshell s s p s p d s p d f Representation of the subshell 1s p d - Table 1.3 Filling of Electrons in the Subshell Have you found out the subshells present in each shell from the table? You also know the maximum number of electrons that can be accommodated in a shell. What will be the number of electrons that can be accommodated in each subshell? 9

10 In an atom, the first shell contains s subshell, the second shell PDF Compressor has Free s and Version p subshells, the third shell has s, p and d subshells, and the fourth shell has s, p, d and f subshells. The maximum number of electrons that can be accommodated in each sub shell is 2 in s, 6 in p, 10 in d and 14 in f. Shell number The maximum number of electrons in each of the first 4 shells is given. Find the number of electrons in the subshells of each and complete Table 1.4 given below. Subshell 1s 2s 2p 3s 3p 3d 4s 4p 4d 4f The maximum number of electrons in each sub shell The maximum number of electrons in each shell Figure Table 1.4 When electrons are added to the subshells of the atoms, they are filled in the order from subshells of lower energy to those of higher energy. This is called the subshell electronic configuration of atoms. Figure 1.1 will help you in finding the order in which the energy of various subshells increases. Note the direction of the arrow head indicating this order. Write the subshells in the increasing order of their energy. 1s < 2s < 2p < 3s <... <... <... <... Now let us learn to write the subshell wise electronic configuration. Let us see how the electronic configuration of lithium ( 3 Li) is written. $ What is the atomic number of lithium? 10

11 The atomic number of an element will be the same as the number of electrons PDF in Compressor one atom of Free the element. Version Electrons are filled in 1s followed by 2s in the increasing order of energy. This can be represented as 1s 2 2s 1. This is the subshell electronic configuration of lithium. While writing the subshell electronic configuration, the number written on the left side of the subshell shows the shell number and the number shown as super script on the right shows the number of electrons. The electronic configuration of lithium is to be read as one-s-two, two-s-one (1s 2 2s 1 ). Complete Table 1.5, writing the subshell electronic configuration of the given elements. Element Number of Subshell electronic configuration electrons 7 N 7 1s2 2s 2 2p 3 9 F 9 1s... 2s... 2p Na Cl Ar - - Table 1.5 $ Write the subshell electronic configuration of potassium ( 19 K). Chemistry $ Standard - X Using the KALZIUM software of School Edubuntu check the accuracy of entries in Table 1.5. $ What about its shell wise electronic configuration? See the graph (Figure 1.2) showing the relationship of the subshells with their energies. $ Compare the energies of 1s and 2s subshells. Which has lower energy? $ Of the 3s and 3p subshells, which has higher energy? What about 3d and 4s? Thus, when examined on the basis of the energies of subshells, it can be understood why, after 8 electrons get filled up in the M shell of potassium, the remaining one electron goes to the N shell? This must be due to the lower energy of 4s than that of 3d. Energy 1s 2p 2s 3d 3p 3s 4p 4s K L M N Figure

12 $ The electronic configuration of scandium ( 21 Sc) is 2, 8, 9, 2. How PDF Compressor can we Free write Version its subshell electronic configuration? Electrons are added to the subshells of scandium in the order 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1. But this is to be written as 1s 2 2s 2 2p 6 3s 2 3p 6 3d 1 4s 2, i.e. in the order of the shells. Based on the increasing order of energy, once the 4s is filled, the next electron should go to 3d. This is how Sc gets the electronic configuration 2, 8, 9, 2. $ Now write the electronic configuration of 22 Ti and 23 V, the two elements after Sc. Let us examine another method of representing subshell electronic configuration. While writing the subshell electronic configuration of elements with higher atomic numbers, the symbol of the noble gas preceding that element may be shown within square brackets followed by the electronic configuration of the remaining subshells. For example potassium ( 19 K) has the electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1. The preceding noble gas argon has the electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 6. Hence we may write the configuration of potassium using the symbol of argon as [Ar] 4s 1. Likewise, the configuration of sodium may be written as [Ne] 3s 1. Write the subshell electronic configuration of the elements in this way and complete Table 1.6. Using the KALZIUM software of School Edubuntu check the accuracy of the entries. Element Subshell electronic configuration 21 Sc [Ar] 3d1 4s 2 Ca Mg Co Table 1.6 Peculiarity of the Electronic Configurations of Chromium (Cr) and Copper (Cu) $ Write the subshell electronic configuration of 24 Cr. 12

13 Chemistry Standard - X The stable electronic configuration of Cr is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 PDF 4s 1 Compressor Free Version Find out the reason by analysing the information given below. The d subshell can accommodate a maximum of 10 electrons. The completely filled configuration (d 10 ) or the half filled configuration (d 5 ) of this subshell is more stable than the others. Based on this, atoms which are expected to have d 4 s 2 or d 9 s 2 configuration will have some changes in the filling of electrons. Likewise, for f subshell, f 7 and f 14 arrangements are more stable. In the same way, identify the correct electronic configuration of Cu from those given below. 29 1s 2 2s 2 2p 6 3s 2 3p 6 3d 9 4s 2 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 1 For the subshell electronic configurations of chromium and copper, the configurations with half filled d subshell or completely filled d subshell show greater stability. o If the subshell wise electronic configuration of an atom is 1s 2 2s 2 2p 6 3s 2, find answers to the following: How many shells are present in this atom? Which are the subshells of each shell? Which is the subshell to which the last electron was added? What is the total number of electrons in the atom? What is its atomic number? How can the subshell electronic configuration be written in a short form? Subshell Electronic Configuration and Blocks Based on the subshell electronic configuration, elements are classified into four blocks s, p, d and f in the modern periodic table. Use the KALZIUM software of School Edubuntu for further clarification. 13

14 The periodic table, with the blocks labelled, is given in Figure PDF Compressor 1.3. Analyse Free Version it and complete Table 1.7. s-block p-block d-block Lanthanoids Actinoids f-block Figure 1.3 Element Atomic Subshell The subshell to Block number electronic configuration which the last electron is added Li Mg N Sc Table 1.7 $ Which is the subshell of lithium to which the last electron was added? Using the KALZIUM software of School Edubuntu check the accuracy of the activity in Table 1.7. $ What about the subshell to which the last electron of nitrogen was added? $ What is the relation between the subshell to which the last electron was added and the block to which the element belongs? 14 $ Write the subshell electronic configuration of the following elements and find the blocks to which they belong. a. 4 Be b. 26 Fe c. 18 Ar

15 The block to which the element belongs will be the same as the PDF subshell Compressor to which Free the Version last electron is added. In the periodic table, elements in groups 1 and 2 belong to s block, those in groups 13 to 18 belong to the p block and those in groups 3 to 12 belong to the d block. The elements in the f block are placed at the bottom of the periodic table in two separate rows. The period and the group can be found out on the basis of subshell electronic configuration. You know how to find the period number of an element on the basis of its shell electronic configuration. Let us see how to find the period from subshell electronic configuration. Complete Table 1.8. Element Subshell The highest shell number Period electronic in the configuration number configuration 4 Be 1s2 2s C 1s2 2s 2 2p Na 1s2 2s 2 2p 6 3s K 1s2 2s 2 2p 6 3s 2 3p 6 4s Table 1.8 The period number is the same as the highest shell number in the subshell electronic configuration. The Group Number of s Block Elements The group number of elements can be found on the basis of subshell electronic configuration. Some elements are given in Table 1.9. With the help of the periodic table (Figure 1.4) complete Table 1.9. Element Subshell electronic Block Group configuration number Li 1s 2 2s Na 1s 2 2s 2 2p 6 3s 1 s 1 Mg 1s 2 2s 2 2p 6 3s 2-2 Ca 1s 2 2s 2 2p 6 3s 2 3p 6 4s Table

16 16 Hints Gases Liquids Synthetic Elements PDF Compressor Free Version Periodic Table Atomic number Symbol Name Name in English (Name in Latin/Greek) Chemistry $ Standard - X Figure 1.4

17 $ In Table 1.9, which is the subshell in which the last electron of all the PDF elements Compressor is filled? Free Version $ To which block do these elements belong? $ Do all these elements belong to the same group? $ How can you relate the number of electrons in the outermost sub shell of s block elements to their group number? Using the KALZIUM software of School Edubuntu check for more detials in the periodic table. For s block elements the number of electrons in the outermost s subshell will be the group number. It is now clear that the elements in groups 1 and 2 belong to s block. You have learned some properties of these elements. $ What is the common name of group 1 elements? $ What is the common name of group 2 elements? $ When s block elements react chemically, do they donate or receive electrons? $ Why do these elements show positive (+) oxidation state? When alkali metals undergo chemical reactions, they show +1 oxidation state, and alkaline earth metals show +2 oxidation state. These elements show definite oxidation states only. Are the oxides of these elements acidic or basic? Let us examine. $ Mg ribbon on burning gives MgO. Which are the oxides and hydroxides of s block elements available in your science lab? 17

18 $ Find the nature of NaOH and KOH by conducting litmus test PDF Compressor and Free record Version your conclusions. Take a note of certain characteristics of s block elements that are given below. Low ionization energy. Low electronegativity. Metallic nature. Lose electrons in chemical reactions. Compounds are mostly ionic. The 1 st group elements of s block have greater reactivity in their respective periods. Oxides and hydroxides are basic in nature. Highest atomic radius in the respective periods. Reactivity increases down the group. The Group Number of p Block Elements $ Which are the groups included in p block? Examine Table 1.10 showing some elements included in the p block. Element Subshell electronic Group configuration number 6 C 1s2 2s 2 2p Al 1s2 2s 2 2p 6 3s 2 3p Cl 1s2 2s 2 2p 6 3s 2 3p 5 17 Table 1.10 $ You can find out the number of electrons in the s and p sub shells of the outermost shell from the table. What is the relation between this number and group number? 18 The p block consists of elements of groups 13 to 18. The group number of p-block elements can be obtained by adding 10 to the total number of electrons in the s and p subshells of the outermost shell.

19 Characteristics of p Block Elements PDF Compressor Free Version The outermost p subshell of the p block elements contain 1 to 6 electrons. Elements in the solid, liquid and gaseous states at room temperature are present in p block. Using ICT, prepare a table of these elements on the basis of their physical state. Physical state Solid Liquid Elements Gas Table 1.11 Don't you find metals as well as nonmetals among them? $ Write the subshell electronic configurations of 10 Ne and 18 Ar. Chemistry $ Standard - X Analyse the electronic configuration and chemical reactivity of the 18 th group elements and on that basis find out how they differ from the other p block elements and prepare a note. $ Among the p block elements, the highest reactivity is shown by group 17 elements in each period. Find the reason for this on the basis of the size of the atom and electronic configuration, and prepare a note. Fluorine (F) is the most reactive element in group 17. Elements showing positive (+) oxidation state and negative (-) oxidation state are present among the p block elements. Al 3+, Sn 2+, Pb 2+, Cl, F, O 2 are examples. Prepare a note on how the properties such as ionization energy, electro negativity, metallic character, and oxidation state of the p block elements vary in each group and periods. $ The atomic number of an element is 16. Find the following. o Subshell configuration : o Group : o Period : o Block : 19

20 $ The outermost electronic configuration of an element is PDF Compressor 3sFree 2 3p 5. Version What details about the element are obtained from this? o complete subshell electronic configuration : o atomic number : o period number : o metal/nonmetal : o valency : The Group Number of d Block Elements $ Where are the d block elements placed in the periodic table? $ From which period does d block start? See Table 1.12 that shows the electronic configuration of some d block elements. Element Subshell electronic configuration Group number 21 Sc 1s2 2s 2 2p 6 3s 2 3p 6 3d 1 4s Fe 1s2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 8 Cr 24 1s2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 1 6 Table 1.12 Are you not able to find the group number of the d block elements on the basis of their electronic configuration? The group number of the d block elements will be the same as the sum obtained by adding the total number of electrons in the outermost s subshell and the number of electrons in the preceding inner d subshell. Characteristics of d Block Elements You know that the d block elements are those in which the last electron enters the d subshell during filling of electrons. These elements are also known as Transition Elements. 20

21 Put a tick mark "ü' against the statements below, which are applicable to d block PDF elements. Compressor Free Version These are metals. The last electron is filled in the shell preceding the outermost shell. In the case of these elements in the 4 th period, the last electron is added to 4s. These are found in groups 3 to 12 of the periodic table. You have already learned that representative elements show similarity in their respective groups. They belong to the s and p blocks, don't they? This is because the elements of the same group have the same number of electrons in the outermost shells. The number of electrons in the 3d and 4s subshells of the d block elements of the 4 th period are given in Table Analyse the table and examine whether the transition elements show similarity in a period. Table 1.13 Chemistry $ Standard - X Group number Electronic configuration 3d 1 4s 2 3d 2 4s 2 3d 3 4s 2 3d 5 4s 1 3d 5 4s 2 3d 6 4s 2 3d 7 4s 2 3d 8 4s 2 3d 10 4s 1 3d 10 4s 2 The outermost shell electronic configuration of transition elements is generally the same in a group and also along a period. Hence, they show similarity in properties not only in a group but also in a period. The Oxidation State of d Block Elements You know that valency is the number of electrons lost or gained or shared by atoms during chemical bond formation. The concept of oxidation state is also known to you. Look at the names and chemical formulae of two different chlorides of iron (Fe) given below. o Ferrous chloride þ FeCl 2 o Ferric chloride þ FeCl 3 The oxidation state of chlorine is ( 1). Find the oxidation state of Fe in these compounds and then complete Table

22 Compound Oxidation The subshell electronic PDF Compressor Free Version state of Fe configuration of the ions of Fe FeCl 2 FeCl 3 Table 1.14 When d block elements undergo chemical reactions along with the electrons in the outermost shell, the electrons in the penultimate d subshell also take part. In the d block elements, electrons are not lost in the same order as they are filled up i.e. electrons are first lost from the outermost s subshell. Look at the subshell electronic configuration of 26 Fe. $ How does Fe change to Fe 2+? 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2 $ Write down the subshell electronic configuration of Fe 2+. In FeCl 3, iron loses three electrons to form Fe 3+ ion. There is only a small difference of energy between the outermost s subshell and the penultimate d subshell of transition elements. $ If so, which will be the subshell from which iron loses the third electron? $ Write the electronic configuration of Fe 3+ on the basis of this. 22 Manganese (Mn) is the element with atomic number 25. MnCl 2, MnO 2, Mn 2 O 3 and Mn 2 O 7 are different compounds of manganese. Complete Table 1.15 by writing the oxidation state of manganese in each of these and also the subshell electronic configuration of each. Compound Oxidation Subshell wise state of Mn Table 1.15 electronic configuration MnCl 2-1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 MnO Mn 2 O Mn 2 O 7 - -

23 When PDF s and Compressor p block elements Free Version take part in chemical reactions, the electrons in the outermost shell are involved. But in the case of transition elements the difference in energy between the outer most s subshell and the penultimate d subshell is very small. Hence under suitable conditions the electrons in d subshell also take part in chemical reactions. This accounts for the variable oxidation states shown by the transition elements. Coloured Compounds See some compounds of the transition elements listed below. $ Copper sulphate $ Cobalt nitrate $ Potassium permanganate $ Ferrous sulphate $ Examine these compounds available in the science lab and find out their colours. Find more coloured compounds and extend the list. Most of the coloured salts are compounds of transition elements. The colour is due to the presence of transition element ions in these compounds. List out instances in which compounds of transition elements are used to impart colour. $ $ Characteristics of f Block Elements The f block elements are the elements placed in two rows at the bottom of the periodic table. The last electrons are filled up in these elements in the antepenultimate shell (i.e. the shell just inside the penultimate shell). The elements in the first row are called Lanthanoids and those in the second row are called Actinoids. These elements belong to the 6 th and 7 th periods respectively. The Catalytic Property of Transition Elements Catalysts are substances which influence the speed of a chemical reaction without themselves undergoing any chemical change. Generally transition elements and their compounds are widely used as catalysts. Vanadium pentoxide (V 2 O 5 ) in Contact Process, spongy iron in Haber Process, and nickel (Ni) in hydrogenation of vegetable oils to obtain vanaspathi are some examples. One reason for the catalytic activity of the d block elements is that they are capable of showing different oxidation states and hence can act as oxidising agents or reducing agents at the same time. 23

24 Examine the electronic configurations of one element from each PDF Compressor row given Free Version in Table Element Subshell Period The shell in The subshell in electronic number which the last which the last configuration electron enters electron enters 58 Ce [Xe]4f1 5d 1 6s f 90 Th [Rn]5f1 6d 1 7s f The Mineral Wealth of Kerala The distribution of minerals is not uniform throughout the earth. Our state, Kerala, is blessed with abundant deposits of certain minerals. The beach sands of Kerala are rich sources of minerals like monazite, ilmenite, zircon, rutile, silimanite etc. Ilmenite is the raw material for the production of titanium dioxide (TiO 2 ) which is widely used in daily life. The mineral monazite is the source of thorium (Th) which is considered to be useful in breeder reactors. Monazite is also the raw material for the production of the metal neodymium (Nd). Neodymium is widely used for the production of powerful and light weight magnets. Monazite is also the mineral of Cerium (Ce) metal used for making flint stones. We should judiciously utilise this valuable mineral wealth. Table 1.16 Which is the subshell in which the last electron enters? Lanthanoids :... subshell Actinoids :... subshell See that the filling of electrons takes place in the 4f subshell for the f block elements of the 6 th period and 5f subshell for the f block elements of the 7 th period. Take note of certain characteristics and uses of f block elements given below. Most of these show variable oxidation states like the d block elements. Most of the actinoids are radioactive and are artificial elements. Uranium (U), Thorium (Th), Plutonium (Pu) etc. are used as fuels in nuclear reactors. Many of them are used as catalysts in the petroleum industry. 24

25 Significant PDF Compressor Learning Free Version Outcomes The learner identifies that electrons in an atom are arranged in the various shells around the nucleus and that each shell has subshells. identifies that shells have subshells which differ in their energy levels and writes the subshell electronic configuration of the elements. identifies the group, period and block of elements based on filling of electrons in subshells and writes them down. identifies the characteristics of s, p, d and f block elements and lists them. Let Us Assess 1. Which are the subshells in which electrons are filled in the s block elements of the third period? Write the subshell electronic configuration of the last element of this period. 2. When the last electron of an atom was filled in the 3d subshell, the subshell electronic configuration was recorded as 3d 8. Answer the questions related to this atom. Complete subshell electronic configuration Atomic number Block Period number Group number 3. Pick out the wrong ones from the subshell electronic configurations given below. a) 1s 2 2s 2 2p 7 b) 1s 2 2s 2 2p 2 c) 1s 2 2s 2 2p 5 3s 1 d) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 2 4s 1 e) 1s 2 2s 2 2p 6 3s 2 3p 6 3d 2 4s 2 4. The element X in group 17 has 3 shells. If so, a) Write the subshell electronic configuration of the element. b) Write the period number c) What will be the chemical formula of the compound formed if the element X reacts with element Y of the third period which contains one electron in the p subshell? 25

26 5. The element Cu with atomic number 29 undergoes chemical PDF Compressor reaction Free to Version form an ion with oxidation number +2. a) Write down the subshell electronic configuration of this ion. b) Can this element show variable valency? Why? c) Write down the chemical formula of one compound formed when this element reacts with chlorine ( 17 Cl). 6. Certain subshells of an atom are given below. 2s, 2d, 3f, 3d, 5s, 3p a) Which are the subshells that are not possible? b) Give the reason. Extended Activities 1. A part of the periodic table is given below. The symbols of elements given in the columns are not real E G A B D C H 18 F a) Which are the elements having only one electron in the 4s subshell? b) Which is the element in s block with the smallest atomic radius? c) Which are the elements likely to form coloured compounds? d) Which is the metal with the highest reactivity? e) Which is the element with the least reactivity? f) Which is the element in which the last electron is filled in the 4p sub shell? Find its atomic number. g) Frame as many questions as possible to get each element in this table as the answer. 26

27 PDF Compressor Free Version There are molecules in all these bags. Count them if you have any doubt. My goodness! What a huge number! Can the molecules be accurately counted? Let us analyse some activities and try to find the answer. Take a long magnesium ribbon. Clean it well by scraping it with a knife. Cut the magnesium ribbon into small pieces which are less than half a centimetre long. Put one piece of this magnesium ribbon carefully into 5 ml dilute hydrochloric acid taken in a test tube. Observe the reaction. Observation : Didn't you see the hydrogen gas produced by the reaction between the metal and the acid bubbling out and then subsiding after some time? Is there any magnesium remaining in the test tube?

28 Test with blue litmus to see if any acid remains in the solution. PDF Compressor $ Free What Version change of colour occurred to the litmus? $ Why did the reaction stop even when the acid remained? Let us conduct the same experiment in another manner. Take just 1mL of acid in the test tube. Put the pieces of magnesium one by one into this. $ Didn't the reaction stop at a certain stage even when magnesium remained in the test tube? What may be the reason for this? Doesn't the reaction stop when any one of the reactants is completely used up during a chemical reaction? The unreacted portion of the other reactants will remain. If we take exact quantities of all reactants, can't we convert them all completely into products? When hydrogen reacts chemically with chlorine, it is hydrogen chloride that is always formed. Observe the balanced chemical equation of this reaction and the symbolic illustration. H 2 + Cl 2 2HCl Haven't you seen that two hydrogen chloride (HCl) molecules are formed from one molecule each of hydrogen (H 2 ) and chlorine (Cl 2 )? What will be the result if the number of hydrogen molecules is 2? 28 One hydrogen molecule remains unreacted after the reaction. Analyse some instances like this and try to complete Table 2.1.

29 Chemistry Standard - X Hydrogen (H 2 ) Chlorine (Cl 2 ) Number of HCl The reactant molecules molecules PDF Compressor molecules Free Version molecules formed remaining after the reaction H 2 2 Cl 2 2 HCl 1 Cl 2 2 H 2 2 Cl 2 4 HCl Nothing left 3 H 2 2 Cl H 2 8 Cl H 2 20 H HCl Nothing left Table 2.1 Thus, in the chemical reaction leading to the formation of HCl, if equal number of molecules of hydrogen and chlorine are taken, the reaction gets completed with none of the reactants remaining unreacted. That means for a complete chemical reaction to take place, hydrogen and chlorine molecules should be taken in the ratio 1:1. Will the ratio between the reactant molecules be the same in all chemical reactions? Examine the balanced chemical equation for the manufacture of ammonia. N 2 + 3H 2 2NH 3 What is the ratio between the number of hydrogen and nitrogen molecules in this case? What about the reaction between hydrogen and oxygen producing water? Balanced chemical equation Ratio between the molecules, hydrogen : oxygen Now try to complete Table 2.2. Balanced chemical equation The ratio between reactant molecules H 2 + Cl 2 2HCl... N 2 + 3H 2 2NH H 2 + O 2 2H 2 O... 2Mg + O 2 2MgO... Table 2.2 You must have realized that in any chemical reaction a definite number of reactant molecules react to give products and that the ratio between 29

30 the number of molecules will be different in different reactions. PDF Compressor Don't Free you Version think this is true not only for the simple experiments you have done in the school laboratory but also for major chemical industries manufacturing important products? In fact, there are crores of extremely minute particles present even in the very small quantities of reactants used in chemical reactions. Is it possible to count the exact number of such minute particles that consist of atoms and molecules? Why? Mass Instead of Number 5.50 kg Have you thought of the time taken in financial institutions like banks to count large number of coins of the same kind and find their exact number? For instance, suppose one rupee coins of the same type, worth one lakh rupees are to be counted. Think how much time is required for this. if the mass of a coin is known? 30 Figure 2.1 Size of the Atom One drop (0.05 ml) of water contain molecules, i.e molecules. You know that each molecule of water contains three atoms, of which two are hydrogen atoms and one is an oxygen atom. Therefore, the total number of atoms will be three times this number. The space occupied by this number of atoms is just as small as the size of a drop of water. Now just imagine how small the atoms are! Would this be possible if instead of coins of the same kind, the one rupee coins were of different kinds and mass? You have understood that in the case of objects that are identical, even if their number is quite large, that number can be ascertained from the mass. Finding the Number of Minute Particles The atoms of an element have the same mass. The molecules of a pure substance (element or compound) also have the same mass. Hence, the number of minute particles like atoms, molecules etc. can be found out using their mass. The exact mass of even minute particles has been determined using modern techniques. However, the relative mass method is used to express their mass. In this method, the mass of an atom is compared with the mass of another atom and expressed as a

31 number which shows how many times it is heavier than the PDF other Compressor atom. Free Version The atomic mass of elements is expressed by considering 1/12 the mass of an atom of carbon -12 isotope as one unit. This mass is termed unified mass or 'u'. 1 u = 1 mass of a carbon-12 atom 12 One u, when expressed in kilogram is kg. When average atomic masses of elements are calculated considering the abundance of isotopes of elements, the atomic masses of several elements may have fractional values. By analyzing the details in the boxes given below you can understand how this happens. However, for practical purposes and calculations, most of these values are taken as whole numbers. The atomic masses of certain elements are given. Element Elements Atomic mass (u) H 1 He 4 C 12 N 14 O 16 P 31 Cl 35.5 Average atomic mass (u) Atomic mass considered for practical purposes (u) Hydrogen Helium Carbon Nitrogen Oxygen Phosphorus Chlorine Isotopes Most of the elements contain atoms with different number of neutrons. Such atoms of the same element with different mass numbers are called isotopes. Naturally their atomic masses are different. While expressing the atomic masses of elements, the average atomic mass is calculated by considering the atomic masses of all the isotopes based on their relative abundance in nature. For example, three isotopes of Neon (Ne) are found in nature viz. 20 Ne, 21 Ne and 22 Ne. The relative abundance of these in nature is 90.48%, 0.27%, and 9.25% respectively. The average atomic mass = ( ) + ( ) + ( ) 3 = 20.18u Most of the elements have fractional atomic masses since the average atomic mass is calculated like this. Avogadro Number While expressing a group of certain objects, terms such as pair (2 numbers), dozen (12 numbers), gross (144 numbers) etc. are used. Similarly, in chemistry, Avogadro Number is used to express the number of minute particles. Its value is N A or N 0 is the symbol used to indicate this number. 31

32 Gram Atomic Mass and Avogadro Number PDF Compressor Free Version The mass of an element in grams numerically equal to the atomic mass of the element is one gram atomic mass (Gram Atomic Mass - GAM) of that element. One gram atomic mass of carbon is 12 g. One gram atomic mass of hydrogen is 1 g and one gram atomic mass of oxygen is 16 g. The number of atoms present in 12 g of carbon-12 is This is indicated as N A. One gram atomic mass of any element contains the same number of atoms as is present in one gram atomic mass of carbon-12 (GAM of C = 12 g). The mass of N A atoms = 1 GAM Gram atomic mass is also known as gram atom. GAM of hydrogen = 1 g. 1 g of hydrogen has 'N A ' number of atoms. Does 1 g of helium contain the same number of atoms? $ How many grams of helium is required to obtain 'N A ' number of the atoms? $ What is the mass corresponding to 'N A ' atoms of carbon? $ What will be the mass for 'N A ' chlorine atoms? 32 Your findings can be consolidated like this. 1 g of hydrogen, 4 g of helium, 12 g of carbon and 35.5 g of chlorine contain the same number of atoms. Here, haven't we taken each element in grams with numerical values equal to their respective atomic masses? When different elements are taken with mass in grams numerically the same as their atomic masses, the number of atoms will be equal. The amount of an element in grams numerically equal to its atomic mass is called gram atomic mass - GAM. One gram atomic mass of any element will contain Avogadro number of atoms.

33 Thus, we can find the number of gram atomic mass corresponding to a certain mass of the element and the number of atoms present in it. Number of gram atomic masses (gram atoms) = 1 GAM = atoms = Mass in grams equal to atomic mass PDF Compressor Free Version 1 GAM Hydrogen = numbers of H atoms = 1 g 1 GAM Helium = numbers of He atoms = 4 g 1 GAM Oxygen = numbers of O atoms = 16 g 1 GAM Chlorine = numbers of Cl atoms = 35.5 g If 2 GAM is considered, won't the quantity be doubled? 2 GAM Hydrogen = 2 N A H atoms = 2 1 g = 2 g 2 GAM Chlorine = 2 N A Cl atoms = g = 71 g 3 GAM Chlorine = 3 N A Cl atoms = g = g Mass in grams GAM of the element Find the number of GAM and the number of atoms present in the samples of each element given below. (Atomic mass H = 1, He = 4, O = 16, C = 12) g hydrogen g helium g oxygen g carbon Gram Molecular Mass 1 GAM of hydrogen contains atoms. We know that hydrogen gas exists in nature as diatomic (H 2 ) molecules. What will be the number of molecules corresponding to hydrogen atoms present in 1 g of hydrogen? How many grams of hydrogen is required to get molecules of hydrogen? Molecular Mass The mass of one molecule of an element or a compound is termed its molecular mass. This is also expressed in terms of 'u'. The molecular mass of a substance can be found out by finding the sum of the atomic masses of the total number of different types of atoms present in one molecule of the substance. For example, (atomic mass H = 1 u, O = 16 u, S = 32 u and C = 12 u) Molecular mass of oxygen (O 2 ) = atomic mass of O 2 = 16 u 2 = 32 u Molecular mass of ethane (C 2 H 6 ) = (atomic mass of C 2) + (atomic mass of H 6) = (12 u 2) + (1 u 6) = 30 u Molecular mass of H 2 SO 4 = (1 u 2) + (32 u 1) +(16 u 4) = 98 u As hydrogen is diatomic, 2 g of hydrogen which is twice the GAM of hydrogen is required to give Avogadro number of molecules. 33

34 Similarly, oxygen, nitrogen, chlorine etc are also diatomic. Carbon, PDF Compressor helium, Free sodium Version etc. are considered as monoatomic. 16 g of oxygen contains Avogadro Number of atoms. But 32 g of oxygen is required to get Avogadro Number of molecules. The GAM of chlorine is 35.5 g. How many grams of chlorine is required to get chlorine molecules (Cl 2 ) equal to Avogadro Number? 34 You have understood that when each element is taken in grams equivalent to the numerical values of their molecular masses, Avogadro Number of molecules of each is obtained. The mass in grams equal to the molecular mass of an element or compound is called its gram molecular mass (GMM). 1 GMM substance is also called one gram molecule of the substance. One gram molecular mass of any substance contains Avogadro Number of molecules Hydrogen atoms (H) = 1 g Hydrogen molecules (H 2 ) = 2 g Oxygen atoms (O) = 16 g Oxygen molecules (O 2 ) = Carbon dioxide molecules (CO 2 ) = Water molecules (H 2 O) =... Is it possible to calculate the number of molecules present in a certain substance by finding the number of GMM present in it? Number of gram molecular masses (Gram molecules) = Mass in grams GMM of the substance The GMM of water is 18 g. What is the number of gram molecules present in 90 g of water? How many water molecules are present in it? o Number of gram molecules = = Mass in grams GMM 90 g 18 g = 5 o Number of molecules =

35 $ Samples of some compounds are given. PDF Compressor Free Version 85 g 90 g 88 g NH 3 P Q R S o Calculate the gram molecular mass of each substance. o How many GMM of the substance is present in each sample? o Arrange the samples in the increasing order of the number of molecules present in each. (Indicators: Gram atomic masses H = 1 g, C = 12 g, N = 14 g, O = 16 g) Mole Concept C 6 H 12 O 6 (Glucose) CO 2 One gram atomic mass (1 GAM) of any element contains atoms. One gram molecular mass (1GMM) of any element or compound contains molecules. The number (Avogadro Number) is very important in chemistry while expressing the number of minute particles. The amount of any substance containing particles is called one mole (1 mole or 1 mol). You know that while expressing the amount of Avogadro number of minute particles, it will be equal to 1 GAM in the case of atoms and 1 GMM in the case of molecules. Hence when we express the amount of a substance in moles, it is very important to specify the type of particle (i.e. atom or molecule). 1 mol hydrogen atoms (H) = 1 g = atoms 1 mol hydrogen molecules (H 2 ) = 2 g = molecules = atoms 1 mol oxygen atoms (O) = 16 g = atoms 1 mol oxygen molecules (O 2 ) = 32 g = molecules = atoms 1 mol ozone molecules (O 3 ) = 48 g = molecules = atoms 50 g H 2 Chemistry $ Standard - X Gram Formula Mass (GFM) In the case of ionic compounds, each positive ion is attracted by the surrounding negative ions, and each negative ion is attracted by the surrounding positive ions. Hence, usually they do not exist as discrete molecules. However on the basis of the ratio between the number of positive and negative ions which is related to the valencies of the ions, they are considered as formula units. For example, in sodium chloride the ratio between the positive and negative ions is 1:1. Therefore its formula is NaCl. In calcium chloride this ratio is 1:2 and its formula is CaCl 2. The relative mass of the formula unit is used to express their masses. The mass in grams numerically equal to the formula unit mass is called the gram formula mass. The number of formula units in one GFM will be equal to the Avogadro number ( ). 35

36 1 mol helium atoms (He) = 4 g = PDF Compressor 1 mol Free helium Version molecules (He) = 4 g = Can you guess why the mass of one mole of helium atoms is equal to one mole of its molecules? Suppose, we consider one mol each of different compounds. 1 mol H 2 O = 18 g = water molecules 1 mol CO 2 = 44 g = molecules 1 mol NH 3 =... =... 1 mol C 6 H 12 O 6 =... =... When 1 mole is expressed as a number it will be the same (Avogadro's Number) for atoms or molecules (or any other minute particle). But how do you express 1 mole as mass? 1 mol atom = 1 GAM 1 mol molecule = 1 GMM Look at what one mole means in terms of number and mass. International Mole Day Mole is an important unit in chemistry. Realizing the importance of 'mole', chemists celebrate 23 October as International Mole Day. The mole day extends from 6.02 am to 6.02 pm on 23 October every year. On this day, chemists pay respect to the 'mole', raising the flames of the Bunsen burner in the laboratory. In the value , 6.02 denotes the time, 10 denotes the month and 23 denotes the date mol Number Mass Atoms Molecules particles is 1 mol. How many moles is particles? Let us see how to find the number of moles if the number of particles is known. Number of moles = Number of particles Avogadro Number GAM 1 GMM See how the number of moles corresponding to 10,000 water molecules is calculated The number of moles of water molecules =

37 Find the number of moles in the following. a. PDF Compressor CO 2 molecules Free Version b H 2 molecules c glucose molecules Thus it is possible to find the number of mol atoms in a definite mass of an element. The number of moles of atoms in 16 g oxygen = 1 The number of moles of atoms in 32 g oxygen = 2 The number of moles of atoms in 160 g of oxygen =.. Number of moles of atoms = Mass in grams Gram atomic mass What would be the value if molecules of elements or compounds are considered instead of atoms? The number of moles of molecules = Mass in grams Gram molecular mass Notice how the mol number of the following is calculated. a. The number of moles of oxygen atoms in 160 g of oxygen, Number of moles of atoms = Mass in grams GAM 160 g = 16 g = 10 b. The number of moles of oxygen molecules in 160 g of oxygen. Number of moles of oxygen molecules = = 160 g 32 g = 5 c. The number of moles of 160 oxygen molecules Number of moles = Massin grams GMM Given number of molecules Avogadro Number = 23 (Remember that the kind of particle - atom, molecule etc. is not relevant if its total number is given). Find the answers to the questions given below. a. Number of moles of CO 2 molecules in 220 g of CO 2. b. Number of moles of nitrogen atoms and number of moles of nitrogen molecules in 700 g of nitrogen (N 2 ). 37

38 38 c. Number of moles of sugar molecules in 1 kg sugar PDF Compressor Free (CVersion 12 H 22 O 11 ) and the number of moles of carbon atoms present in it. d. Number of moles of carbon atoms. You have seen how to convert the number of particles or the mass of a substance into number of moles. Similarly, we can convert the amount in moles to number of particles or mass. Look at the formulae used in such cases. Number of particles = number of moles N A Mass of substance = number of moles GAM (for atoms) Mass of substance Work out the following. = number of moles GMM (for molecules) o No. of particles in 4 mol of water Number of particles = Number of moles... = =... o The mass of 20 mol oxygen atoms Mass =... GAM = =... o The mass of 20 mol oxygen molecules Mass = Number of moles... = =... You have now understood how to convert the mass of a substance or the number of particles into number of moles and vice versa. Now, if a definite mass of a substance is given, you can find the number of atoms or molecules present in it. Likewise, if the number of particles is known, the mass corresponding to the number of particles can also be calculated. $ The gram molecular mass of water (H 2 O) is 18 g. See the method of calculating the number of molecules and the total number of atoms in 180 g of water. The number of moles in 180 g of water = 180 g 18 g = 10 The number of molecules in 10 mol water =

39 One molecule of water has 2 hydrogen atoms and one oxygen PDF atom Compressor which add Free up Version to a total of 3 atoms. Therefore, Total number of atoms = number of molecules 3 = $ How can we find the masses of atoms and molecules in grams? Mass of N A atoms = GAM Mass of one atom = Mass of N A molecules Mass of one molecule = GAM N A = GMM GMM $ Find answers to the questions given below. 1. What will be the mass in grams of molecules of water? 2. How many molecules are present in 710 g of chlorine gas (Cl 2 )? What will be the total number of atoms in it? 3. Calculate the number of molecules and the total number of atoms in 90 g of glucose. 4. Calculate the mass of each of the following in grams: one nitrogen atom, one nitrogen molecule and one sulphuric acid molecule (Given : gram atomic masses: H = 1 g, C=12 g, S = 32 g, O = 16 g, Cl = 35.5 g, N = 14 g) The Volume of Gases and Number of Moles We are aware of the distinctive properties of gases that make them different from solids and liquids. In gases, molecules are at a larger distance from one another. Compared to the size of the molecules the distance between them is several times greater. We know that the volume of solids and liquids is the space actually occupied by their molecules. But in the case of gases, at a given temperature and pressure, the volume of the gas is the space used by its molecules for their constant motion. Generally as the entire space inside a vessel is thus taken by the molecules, the volume of the vessel itself is considered as the volume of the gas. N A Chemistry $ Standard - X 39

40 40 As the distance between the molecules is very large, PDF Compressor Free Version the volume of the gas does not depend on the size of its molecules. On the other hand, the volume depends on the number of molecules and their motion. If pressure and temperature are constant, the molecules with smaller mass move faster and the molecules with larger mass move at a slower speed. Hence the space required for motion will be the same for each molecule. Volume of Gases and Gas Laws If a gas is taken in a vessel which can expand or contract on its own, (e.g. a cylinder with a piston which can move freely, rubber balloon ) the pressure and temperature outside the vessel will influence the volume of the gas (e.g. the balloon expands on heating; on applying pressure the balloon contracts) Certain Gas laws relating to the volume, pressure, temperature and number of molecules of gases are given below. Boyle's Law Boyle's law states that for a fixed mass of gas, at constant temperature, the volume is inversely proportional to its pressure. Charles' Law Charles' law deals with the volume of gases at constant pressure. The law states that for a fixed mass of gas, at constant pressure, the volume is directly proportional to the temperature in Kelvin scale. As the volume of a gas changes with pressure and temperature, while expressing the volume of a gas, the pressure and temperature also should be specified to make it meaningful. Avogadro's Law At constant pressure and temperature, the volume of a gas is directly proportional to the number of molecules. That is, the volume of a gas depends on the number of molecules and not on the type or size of the molecules. Hence, whatever be the gas taken at constant pressure and temperature, if the number of molecules are equal, the volume also will be equal. From the details given in the box, it is clear how the volume changes with a change in pressure and temperature. In the case of gases, if the pressure and temperature remain constant, equal number of molecules will have equal volumes. As the number of molecules in one mol of any gas is equal, their volumes also will be equal. This is called molar volume. But will this be true if temperature and pressure are changed? By analyzing the information given in the box, it is clear to you that the volume of a gas will change if temperature or pressure is changed. At constant pressure and temperature equal volumes of gases will contain equal number of molecules. Scientists have proved by experiments that if temperature is fixed as 273 K and pressure as 1 atmosphere (1 atm), molecules (1 mol molecules) of any gas will have a volume of 22.4 L. 273 K temperature and 1 atm pressure are known as standard temperature and pressure or STP. That is, at STP one mol of any gas will have a volume of 22.4 L. This is called molar volume at STP.

41 Now in the case of gases at STP, can't we give an additional PDF meaning Compressor to one Free mol? Version At STP one mol gas = 22.4 L Shall we write down the meanings of the mole that we have understood so far? 22.4 L of gas at STP = 1 mol 1 mol Mass Number Atoms Molecules Volume of gas at STP Units of Pressure, Temperature and Volume Pressure SI Unit - Pascal Other units commonly used: Atmosphere (atm), bar, 1 atm = Pa 1 bar = 10 5 Pa 1 atm = bar Temperature SI Unit - Kelvin Other units commonly used: Degree Celsius ( C), Degree Gas 1 mol hydrogen (H 2 ) at STP 1 mol nitrogen (N 2 ) at STP 1 mol CO 2 at STP 44.8 L of gas at STP = 2 mol 224 L of gas at STP =... In the case of gases at STP, Volume 22.4 L 22.4 L 22.4 L Farenheit ( F) C temperature = Kelvin temperature þ 273 F temperature = ( x C temperature) 5 Volume SI Unit - Cubic meter (m 3 ) Other units commonly used: Litre (L), Millilitre (ml), Cubic centimetre (cm 3 or cc), 1 1 L = 1000 m3 1 1 ml = 1 cm 3 = 1000 L 1 L = 1000 cm 3 = 1000 ml Number of moles = Volume at STP (Litres) Molar Volume at STP (Litres) Volume at STP (Litres) = 22.4 L Let us consolidate the methods used for finding the number of moles. 41

42 If the number of particles is given, PDF Compressor Free Version $ Number of moles = If the mass is given, $ Number of moles of atoms = $ Number of moles of molecules = If the volume of gas at STP is given, $ Number of moles = Mole Concept - In Balanced Chemical Equations Balanced chemical equations are those equations in which the number of each type of atoms is made equal on the reactant side and the product side. Each chemical reaction can be represented by a balanced chemical equation. Examine the balanced chemical equation for the manufacture of ammonia. N 2 + 3H 2 2NH 3 One nitrogen molecule reacts with three hydrogen molecules to produce two molecules of ammonia. Suppose there are nitrogen molecules instead of one molecule then, N H NH 3 1 mol N mol H 2 2 mol NH 3 The quantity in mol can also be expressed as mass or volume. N 2 + 3H 2 2NH 3 28 g 3 2 g 2 17 g (Mass) At STP 22.4 L L L (Volume) That is, if 28 g of nitrogen is allowed to react with sufficient quantity of hydrogen then 34 g of ammonia will be obtained. If so, can you predict the number of grams of ammonia produced when 140 g of nitrogen is allowed to react? o NH 3 obtained from 28 g N 2 = 34 g o NH 3 obtained from 1 g N 2 = o NH 3 obtained from 140 g N 2 = 34 g g 28 42

43 By applying the mole concept to balanced chemical equations, can you predict quantities PDF Compressor like the Free amount Version of hydrogen required to react with a definite amount of nitrogen or the amount of reactants required to obtain a definite mass or a definite volume of ammonia? Calculate the amount of hydrogen required to react with 500 g of nitrogen during the manufacture of ammonia. o The amount of H 2 required to react with 28 g of N 2 o The amount of H 2 required to react =6 g with 1 g of N 2 = 6 g 28 o The amount of H 2 required to react with 500 g of N 2 = g 28 $ How many grams of N 2 is required to obtain 1000 L of ammonia at STP? o The mass of N 2 required to obtain 44.8 L of NH 3 = 28 g o The mass of N 2 required to obtain 1 L of NH 3 at STP = o The mass N 2 required to get 1000 L of NH 3 at STP = Chemistry $ Standard - X 28 g g 44 8 Think about the great advantages of making such calculations in advance in industries. The Number of Moles in Solutions Several chemical substances including acids and alkalies are used in the form of solutions in laboratories. In such situations, the concentration of solutions is usually expressed in terms of the number of moles of the solute dissolved in a definite volume of the solution. This method of expressing the concentration of solutions is termed molarity. Molarity is used to indicate the number of moles of solute in one litre of the solution. If one litre of the solution contains 1 mol of solute, it is called 1 molar (1M) solution. If one litre of the solution contains 2 mols of solute then it is a 2 molar (2 M) solution. If the amount of solute in one litre of solution is 0.5 mol (or 1 mol in 2 L solution) then it is 0.5 M solution. 43

44 PDF Compressor Molarity Free of Version solution = Do more activities using the molarity app in PhEt of School Edubuntu. Number of moles of solute Volume of solution (litres) or M = n V Procedure of Preparing 1 M Solution Suppose a 1 M solution of NaCl is to be prepared. You know that 1 mol NaCl has a mass of 58.5 g. Two students were given 58.5 g (1 mol) each of NaCl. Analyse the figure showing the procedure they adopted for preparing 1 M solution L 1 L g of NaCl is dissolved in one litre of water 1 L 1 L 58.5 g of NaCl is dissolved in water and the volume is made up to 1 L. Which of these is certain to have a total volume of 1000 ml (1 L) of the solution and 1 mol of the solute at the end of the process? Which is the correct method to prepare 1 M solution of NaCl? Instead of 1 mol of solute in one litre of solution, a 2 mol solute in two litres of solution will also give a 1 M solution. Analyse the following table. Number of moles of solute (n) Total volume of the solution (V) Molar concentration n V 44 1 mol 1 L 1 M 2 mol 2 L 1 M 0.5 mol 0.5 L 1 M 0.5 mol 1 L 0.5 M 0.5 mol 2 L 0.25 M 2 mol 10 L 1 5 M From this you might have understood how different volumes of solutions with definite molarity are prepared.

45 Significant Learning Outcomes PDF Compressor Free Version The learner proves by experiment that in reactions involving more than one reactant the reaction stops when one of the reactants is completely consumed; and attains the ability to explain this on the basis of balanced chemical equations. explains that there exists a definite ratio between the number of reactant atoms in any chemical reaction. defines relative mass, atomic mass and molecular mass and attains the ability to analyse examples. defines gram atomic mass and gram molecular mass and attains the ability to solve simple problems pertaining to them. realises that the number of particles of each type is equal in one GAM and GMM respectively; and explains Avogadro Number. defines 'mole' and realises the importance of the unit of 'mole' in chemistry and attains the ability to explain mole in terms of number and mass. solves simple problems for finding the number of moles. realises that the volume of a gas is the space in which gas molecules are distributed and establishes through examples that equal number of molecules of different gases have the same volume at the same pressure and temperature. explains the need to mention pressure and temperature while expressing the volumes of gases. explains STP and molar volume at STP and solves simple problems. applies mole concept in balanced chemical equations, to relate this to mass and volume and to solve simple problems. prepares 1 M solution by understanding the method of expressing the quantity of chemicals in their solutions. solves simple problems relating to preparation of solutions of different molarities. Chemistry $ Standard - X 45

46 Let Us Assess PDF Compressor Free Version 1. Suppose 20 molecules of hydrogen are allowed to react with 20 molecules of oxygen to produce water. a. Which of the reactant molecules gets consumed first? b. The molecules of which reactant will remain unreacted? How many? 2. Which, among the following, is used as the basis for expressing atomic mass? (Hydrogen, Carbon - 12, Carbon - 14, Oxygen - 16) 3. The atomic mass of helium is 4 and that of oxygen is 16. How many grams of helium is required to get as many number of atoms as are present in 40 g of oxygen. (40 g, 160 g, 10 g, 4 g) 4. Find the number of gram atomic masses in each of the following. a. 100 g of helium b. 200 g of oxygen c. 70 g of nitrogen d. 1 g of calcium (Atomic mass He = 4, O = 16, N = 14, Ca = 40) 5. Calculate the gram molecular mass/gram formula mass of the following. a. HNO 3 b. CaCl 2 c. Na 2 SO 4 d. NH 4 NO 3 (Gram atomic masses H = 1 g, N = 14 g, O = 16 g, Na = 23 g, S = 32 g, Cl = 35.5 g, Ca = 40 g) 6. Given below are a few samples. a. 400 g of water (H 2 O) b. 400 g of carbon (C) c. 400 g of helium (He) d. 400 g of hydrogen (H 2 ) e. 400 g of glucose (C 6 H 12 O 6 ) (i) Find the number of moles in each. (ii) Arrange the samples in the increasing order of their number of moles. (Gram molecular masses He = 4 g, C = 12 g, H 2 = 2 g, H 2 O = 18 g, C 6 H 12 O 6 = 180 g) 7. Calculate the following. p. Number of moles in 1 kg of water. 46

47 q. Number of moles in 500 g of CaCO 3. r. PDF Number Compressor of molecules Free Version and total number of atoms in 88 g of CO 2. s. Volume of 170 g ammonia at STP. t. The mass of 112 L CO 2 gas at STP and the number of molecules. (GMM/GFM: H 2 O - 18 g, CaCO g, CO 2-44 g, NH 3-17 g) Extended Activities How many grams of carbon and oxygen are required to get the same number of atoms as is present in one gram of helium? Examine the samples given: a. 20 g of He b L of NH 3 at STP c L of N 2 at STP d. 1 mol of H 2 SO 4 e. 180 g of water (i) Arrange the samples in increasing order of the number of molecules in each. (ii) What will be the ascending order of the total number of atoms in each sample? (iii) What will be the masses of samples b, c and d? In 90 gram of water a. How many molecules are present? b. What will be the total number of atoms? c. What will be the total number of electrons in this total number of particles? You are given 100 g of NaOH, 200 ml water, beakers and weighing balance. How will you prepare a 1 molar (1 M) solution of NaOH taking sufficient quantities from these? 500 ml of a 1M solution of common salt is taken. a. How many grams of common salt are dissolved in this? b. If the solution is diluted with water to a volume of 2 L, what will be the molarity? Chemistry $ Standard - X 47

48 PDF Compressor Free Version You might have observed a variety of chemical reactions in your daily life. What are the situations shown in the picture? Now, expand the list given below identifying more examples known to you. Burning of firewood Rusting of iron The burning of firewood and rusting of iron are two different chemical reactions. Do they occur at the same rate? 48 There are situations when the speed of chemical reactions is to be increased or decreased.

49 Have you ever thought of reducing the rate of the rusting of iron considerably? PDF Compressor Similarly, don't Free we Version wish the firewood to burn faster? What are the methods adopted to make firewood burn faster? Provide more air Doesn't this indicate that some factors influence the rate of chemical reactions? Let us study the rate of chemical reactions and the important factors influencing it. Concentration and Speed of Chemical Reactions Let us do a simple experiment to find the rate of chemical reactions. Examine how magnesium reacts with hydrochloric acid (HCl) of different concentrations. What are the materials required? Chemistry $ Standard - X Isn't it necessary to have magnesium ribbons of equal mass? What about the volume of HCl? Now, we will do the experiment. Observe Figure 3.1. Take magnesium ribbons of equal mass in both the test tubes. Add concentrated HCl to one test tube and dilute HCl to the other in equal volumes. Concentrated HCl H 2 Dilute HCl Mg ribbon Mg ribbon 1 2 Figure

50 Record your observation. PDF Compressor Free Version Test tube 1 : Collision Theory According to this theory reactant molecules have to collide against each other for chemical reaction to occur. All collisions between reactant molecules need not result in chemical reaction. Only when the reactant molecules have energy greater than a minimum, they undergo effective collisions which result in product formation. An increase in the number of molecules and an increase in their energy will result in an increase in the number of effective collisions within a specified time. Test tube 2 : Write the balanced chemical equation for the reaction. In which test tube is the rate of reaction greater? Which test tube contains more HCl molecules per unit volume? Does the increase in the number of molecules lead to an increase in the speed of reaction? Explain this on the basis of the details of collision theory given in the box. 50 As the concentration of reactants increases, the number of molecules per unit volume and the number of effective collisions increase. Consequently, the speed of reaction increases. In the earlier experiment the amounts of magnesium were equal, while the concentrations of HCl were different. In which of the test tubes was magnesium consumed first? (Put a 'ü' for the correct one) Test tube with concentrated HCl Test tube with dilute HCl Isn't the faster consumption of magnesium an indication of increased reaction speed? If so, can't we relate the speed of the reaction to the time in which the reactant gets consumed? The rate of reaction can be expressed in terms of the amount of any one of the reactants being consumed in unit time.

51 Then, Rate of reaction PDF Compressor = Free Version Amount of reactant used Time taken for consumption of the reactant Examine the chemical equation of this experiment. Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) What are the products of the reaction? Can we collect the H 2 formed in the reaction? Can't the rate of the reaction be determined from the amount of this hydrogen? The rate of a reaction can also be expressed in terms of the amount of any one of the products formed within a specific period of time. Then, Rate of reaction = Amount of product formed Time taken for the formation of the product Nature of the Reactants and the Rate of Chemical Reaction Are the rates of reaction of zinc (Zn) and magnesium (Mg) with dilute HCl the same? Let us do an experiment. What are the materials required for the experiment? Chemistry $ Standard - X Physical State of Substances in Chemical Equations While writing chemical equations, letters are used as subscripts along with chemical formulae such as 's' for solid, 'l' for liquid' 'g' for gas and 'aq' for aqueous solution and they specify their physical states. Isn't it necessary to take Zn and Mg in equal amounts and in almost the same shapes? Note down the procedure of this experiment. Which gas is formed during the reaction? Try to write the chemical equations. Experiment 1 (When Zn was added): Experiment 2 (When Mg was added): Which reaction was faster? Is there any difference in the concentration of the acid used in these experiments? 51

52 What is the factor that influenced the rate of reaction here? PDF Compressor Free Version The nature of reactants is one of the factors which influences the rate of chemical reaction. Pressure and Rate of Reaction Some examples of reactions in which gaseous reactants react together to give gaseous products are given below. Nitrogen and hydrogen combine to form ammonia. Hydrogen and chlorine combine to form hydrogen chloride. Nitrogen and oxygen combine to form nitric oxide. Figure 3.2 shows nitrogen and hydrogen taken in a cylinder. The pressure can be varied using the piston. 1 atm N 2 2 atm H 2 H2 N 2 Figure 3.2 What happens to the volume when pressure is increased to 2 atm? Is there any change in the number of molecules? When the pressure in the chemical reaction involving gases increases, the molecules come closer. Did any change occur in the number of reactant molecules per unit volume due to the change in pressure? How is the rate of collision of molecules affected on increasing the pressure? 52

53 Why do the rate of gaseous reactions increase on an increase of pressure? PDF Prepare Compressor a short Free note Version the basis of the pictorial representation given below. Use Gas properties App for more clarity in School Edubuntu, PhET. In reactions involving gases On increasing pressure o Molecules come closer. o Number of molecules per unit volume increases. o Rate of collision increases Rate of reaction increases Surface Area of Solids and Rate of Reaction Let us see how dilute HCl react with a marble piece and marble powder of equal mass. Analyse Figure 3.3 and write the materials required and the procedure for the experiment. Dilute HCl Marble piece Dilute HCl Marble powder Let us write the chemical equation for this reaction. CaCO 3 (s) + 2HCl (aq) CaCl 2 (aq) + H 2 O (l) + CO 2 (g) What is the observation? Figure 3.3 Is there any difference in the rate of reaction in the two beakers? Is there any difference in the concentration of acid? Is there any difference in the mass of marble? 53

54 What about the surface area of marble? PDF Compressor Free Version In which case is there a chance for greater number of acid molecules to get in contact with marble, at a given time? What is the change in the rate of collision when surface area increases? What will happen to the rate of this reaction if marble is further split into small grains or powdered? Threshold Energy To take part in a chemical reaction, molecules should attain a certain minimum kinetic energy. This energy is called threshold energy. In the case of reactions involving solids, the surface area is a factor which influences the rate of reaction. When solids are made into small pieces or powder, their surface area increases. As a result the number of molecules undergoing effective collisions also increases. Hence the rate of reaction increases. Is it clear now, why firewood burns faster when cut into small pieces? Identify more examples to illustrate the increase in rate of reaction with the increase in surface area. Temperature and Rate of Reaction Let us see the role of temperature in the reaction between sodium thiosulphate and hydrochloric acid. Materials required: Sodium thiosulphate, hydrochloric acid, water, boiling tube, spirit lamp. Procedure : Prepare the dilute solution of sodium thiosulphate in a beaker. Take equal volumes of this solution in two boiling tubes. Heat one boiling tube for some time. Add dilute hydrochloric acid in equal amounts in both the boiling tubes. Record the observation. 54

55 In which of the boiling tubes is the precipitate formed faster? PDF Compressor Free Version What is the colour of the precipitate formed? The colour change is due to the precipitation of sulphur in both the boiling tubes. See the chemical equation. Na 2 S 2 O 3 (aq) + 2HCl (aq) 2NaCl (aq) + H 2 O (l) + SO 2 (g) + S (s) From this experiment, you can understand which factor has influenced the rate of reaction. Energy as well as speed of molecules increases when reactants are heated. That is, as temperature increases the number of molecules with threshold energy increases. As a result, the number of effective collisions increases and thus rate of reaction also increases. Temperature is an important factor which influences the rate of reaction. As temperature increases the rate of reaction also increases. Catalyst and Rate of Reaction Hydrogen peroxide (H 2 O 2 ) is a compound which undergoes decomposition. Usually its aqueous solution is used in chemical reactions. See the given chemical equation of its decomposition. 2H 2 O 2 (aq) 2H 2 O (l) + O 2 (g) Take some hydrogen peroxide solution in a test tube. Show a glowing incense stick into the test tube. What is the observation? Is there any difference occuring in the way in which the incense stick burns? Now add some manganese dioxide (MnO 2 ) into the test tube. Again show the glowing incense stick. Record the observation. Doesn't it indicate that when manganese dioxide is added the rate of reaction increases and oxygen is formed faster? Filter the solution using a filter paper when the reaction is complete. The substance remaining in the filter paper is manganese dioxide itself. When examined carefully it becomes clear that there is no change in its amount or property. 55

56 The presence of manganese dioxide has increased the rate of the PDF Compressor reaction. Free Manganese Version dioxide acts as a catalyst in this reaction. Catalysts are substances which alter the rates of chemical reactions without themselves undergoing any chemical change. Enzymes - the Biocatalysts Biological processes occuring in living cells are chemical reactions. These chemical reactions sustain life. These reactions are catalysed by complex protein molecules known as enzymes. The enzyme amylase, present in saliva, converts starch into maltose. amylase Starch + water maltose In this reaction, manganese dioxide acted as a catalyst by increasing the rate of the reaction. Such catalysts are called positive catalysts. You have understood that hydrogen peroxide decomposes to form water and oxygen. Then, isn't it necessary to reduce its rate of decomposition during its storage? For this purpose some phosphoric acid (H 3 PO 4 ) is added to hydrogen peroxide. Here phosphoric acid acts as a negative catalyst as it reduces the decomposition rate of hydrogen peroxide. In the manufacture of sulphuric acid vanadium pentoxide and in the manufacture of ammonia iron are used as positive catalysts. Light and the Rate of Chemical Reaction There are many reactions in which light energy has an influence. List down those photochemical reactions you are familiar with. Formation of HCl by the combination of H 2 and Cl 2. The decomposition of silver bromide is a photochemical reaction. The formation of sulphuryl chloride used for the manufacture of insecticides occurs readily in the presence of light. Sunlight SO 2 (g) + Cl 2 (g) SO 2 Cl 2 (g) (Sulphuryl chloride) 56

57 Light energy is a factor which influences the rate of reaction. You are PDF now Compressor familiar with Free the Version various factors influencing the rate of reaction. Enlist them. Prepare a small write up on the influence of these factors in the daily life and present it in a seminar. Chemical Equilibrium Consider the following chemical equation. What are the reactants? NaOH (aq) + HCl (aq) NaCl (aq) + H 2 O (l) What are the products? Will you get NaOH and HCl when the products sodium chloride and water react together? Let us do an experiment. Dissolve some sodium chloride in water and test it with a litmus paper. The presence of neither acid nor alkali can be observed in the solution. It indicates that NaCl and H 2 O do not react to form back the reactants. Consider some more reactions which we have discussed earlier. Zinc reacts with dilute HCl. Magnesium burns in air. Marble (CaCO 3 ) reacts with dilute HCl. Write the products formed in each reaction. $ Zn (s) + HCl (aq) $ $ In these reactions, reactants change to products, but the products cannot be converted back to the reactants. 57

58 PDF Compressor That is, in all Free these Version reactions, the reaction occurs only in one direction. Chemical reactions in which reactants give products but the products do not give back the reactants under the same conditions are called irreversible reactions. See more examples given for irreversible reactions. NaCl (aq) + AgNO 3 (aq) NaNO 3 (aq) + AgCl (s) C (s) + O 2 (g) CO 2 (g) Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) Are all the reactions irreversible? Let us do an experiment. Heat a little ammonium chloride (NH 4 Cl) taken in a boiling tube. Don't you experience a characteristic odour? What may be the gas formed? Show a moist red litmus paper at the mouth of the boiling tube. What change do you observe? Doesn't it indicate the basic nature of the gas? It is clear that the gas with a pungent smell and basic in nature is ammonia (NH 3 ). Keep the litmus at the mouth of the boiling tube for some more time and observe the change. What do you observe? It is the presence of hydrogen chloride (HCl) gas which resulted in the moist litmus turning red again. When ammonium chloride (NH 4 Cl) is heated, lighter NH 3 gas comes out first. The denser HCl gas comes out later. Write the chemical equation for this reaction. 58

59 Did you observe a white powder sticking on the sides of the test tube? This is PDF ammonium Compressor chloride. Free It Version is formed by the combination of NH 3 and HCl gas coming out. Let us do another experiment to confirm this. See Figure 3.4 Take a glass tube. Place a piece of cotton dipped in HCl at one end of the glass tube and another piece dipped in ammonia solution at the other end of the glass tube, such that these are well inside the glass tube. Close both ends of the glass tube tightly using a cork. Observe the changes inside the glass tube. Chemistry $ Standard - X Cotton dipped in HCl Cotton dipped in ammonia solution Thick white fumes of NH 4 Cl Figure 3.4 Do you observe the thick white fumes? It is due to the combination of HCl vapour and NH 3 gas. Heat the region of the glass tube where the white powder of ammonium chloride has been stuck. What happens to the white powder on heating? See the chemical equations for the decomposition of ammonium chloride while heating and the recombination of products formed. NH 4 Cl (s) NH 3 (g) + HCl (g) N H 3 (g) + HCl (g) NH 4 Cl (s) We can combine these equations into NH 4 Cl (s) U NH 3 (g) + HCl (g) The sign " U " indicates that the reaction takes place in both directions. Reactions taking place in both directions are called reversible reactions. In a reversible reaction the reaction in which the reactants change to the products is called the forward reaction and that in which the products change back to the reactants is called the backward reaction. Watch the video of the reaction between ammonia and hydrogen chloride from the page 'Some Non-metallic Compounds', opening 'Chemistry for Class X' in 'School Resources' of School Edubundu. 59

60 Examine the chemical equations given below and write the forward PDF Compressor and backward Free Version reactions in each. $ N 2 (g) + 3H 2 (g) U 2NH 3 (g) $ 2SO 2 (g) + O 2 (g) U 2SO 3 (g) $ H 2 (g) + I (g) U 2HI (g) 2 Equilibrium in Reversible Reactions Let us do an experiment. Take the solutions of potassium nitrate (KNO 3 ), potassium thiocyanate (KCNS) and ferric nitrate (Fe(NO 3 ) 3 ). Observe the colour of the solutions taken and complete Table 3.1. Solution Take a little of dilute ferric nitrate solution in a test tube and add a few drops of potassium thiocyanate solution to it. What is the observation? Colour KCNS... Fe(NO 3 ) 3... KNO 3... Table See the chemical equation for the reaction. Fe(NO 3 ) 3 (aq) Ferric nitrate Red Colour of Ferric Thiocyanate Ferric thiocyanate is formed in three stages. Fe 3+ + CNS U Fe(CNS) 2+ Fe(CNS) 2+ + CNS U Fe(CNS) + 2 Fe(CNS) CNS U Fe (CNS) 3 Among these Fe(CNS) 2+ causes the deep red colour. + 3KCNS (aq) Fe(CNS) 3 (aq) + 3KNO 3 (aq) Potassium Ferric Potassium thiocyanate thiocyanate nitrate The formation of Fe(CNS) 3 by the combination of Fe(NO 3 ) 3 and KCNS is the cause of red colouration. Keep the solution without disturbing for some time. Does the red colour intensify? Observe the colour again after some time. There is no colour change, isn't it? Dilute the solution and observe it after some time. Do you see any change? Transfer the dilute solution to three test tubes in equal amounts.

61 Test tube 1 (a) Add PDF Fe(NO Compressor 3 ) 3 to test Free tube Version 1. What is the observation? Which substance is responsible for this colour change? Test tube 2 (b) Add some KCNS to the second test tube. What is the observation? What is the cause of this colour change? Test tube 3 (c) Add a drop of concentrated KNO 3 to the third test tube. Doesn't the intensity of colour of the solution decrease significantly? Activity performed Observation Reason Added Fe(NO 3 ) 3 Added KCNS The red colour of the solution intensifies. The red colour of the solution intensifies. Fe(NO 3 ) 3 reacted with the remaining KCNS to form the product. KCNS reacted with the Fe(NO 3 ) 3 remaining in the solution to form more product. A drop of the concentrated solution of KNO 3 is added Intensity of the red colour of the solution is reduced significantly. The product Fe(CNS) 3 reacted with KNO 3 to form reactants. Table 3.2 What can be understood from these experiments? Both the products and the reactants are present in the solution. This is a reversible process. Already there were reactants along with the products in the system. Even then colour didn't change. What can be the reason? Is it due to the fact that even though products are formed, they are converted back to reactants? There was no colour change because the rate at which the reactants change to the products is equal to the rate at which the products change to the reactants. 61

62 That is, there was no colour change because the rate of forward PDF Compressor reaction Free is equal Version to the rate of backward reaction. Closed System Closed system is one in which nothing new is added to a system and nothing is removed from the system. Equilibrium is possible only in a closed system. Chemical equilibrium is the stage at which the rate of the forward reaction becomes equal to the rate of the backward reaction in a chemical reaction. The characteristics of equilibrium identified through the experimental observations conducted so far are given below: At equilibrium both the reactants and the products coexist. The rates of forward and backward reactions become equal at equilibrium. Chemical equilibrium is dynamic at the molecular level. The system can attain equilibrium even when the concentration of the reactants and products are not equal. Once equilibrium has been attained, there will be no change in the concentration of the reactants as well as that of the products. Chemical equilibrium is attained in closed systems. Try to find answers to the given questions by analysing the graph of the reversible process (Figure 3.5). Rate of Chemical Reaction Forward Reaction B C Backward reaction A Equilibrium state Time Figure 3.5 What happens to the rates of forward and backward reactions as time progresses? 62

63 Identify the point at which the rates of both forward and backward PDF Compressor reactions become Free Version equal? We have seen that in all systems in equilibrium, both reactants and products coexist. At equilibrium the forward and backward reactions will not stop. This is why chemical equilibrium is said to be dynamic at the molecular level. List the conditions in which there is a change in the equilibrium state in the experiment conducted using Fe(NO 3 ) 3 and KCNS. Are there some other factors which influence equilibrium? The scientist Le Chatelier proposed an important principle related to chemical equilibrium. Chemical Equilibrium is Dynamic Equilibrium A system reaches equilibrium not because the reaction stops but due to the rates of forward and backward reactions becoming equal. Even at equilibrium, the reactant molecules react to form product molecules and product molecules react to form reactant molecules. Hence chemical equilibrium is said to be dynamic at the molecular level. Le Chatelier's Principle When the concentration, pressure or temperature of a system at equilibrium is changed, the system will readjust itself so as to nullify the effect of that change and attain a new state of equilibrium. Influence of Concentration in Equilibrium System The chemical equation for the manufacture of ammonia is given below. N 2 (g) + 3H 2 (g) U 2NH 3 (g) In this, The rate of which reaction increases when the concentration of nitrogen is increased? Forward reaction/backward reaction ('ü' which is correct) What happens if the concentration of ammonia is increased? 63

64 What will be the effect of removing ammonia continuously from PDF Compressor the Free system? Version Complete Table 3.3 writing the effect of change in concentration in the system at equilibrium. Action Change of concentration Change in rate ÿmore hydrogen is added More ammonia is added Ammonia is removed More nitrogen is added Increases the concentration of reactant Increases the concentration of the product Decreases the concentration of the product Increases the concentration of the reactant Table 3.3 Pressure and Chemical Equilibrium Rate of forward reaction increases You know that pressure has a significant influence in the case of gases. Let us examine the influence of pressure in the manufacture of ammonia. N 2 (g) + 3H 2 (g) U 2NH 3 (g) In this equation what is the total number of moles of the reactant molecules? What about the products? 64 Here, both reactants and products are gases. Forward reaction : 4 mole reactant molecules 2 mole product molecules (Volume decreases) Backward reaction :. mole product molecules.. mole reactant molecules (Volume ) In a gaseous system, decrease in the number of molecules helps to decrease pressure. According to Le Chatelier, when pressure of a system at equilibrium is increased the system will try to attain equilibrium by reducing pressure.

65 In the manufacture of ammonia, the reaction in which direction results PDF in Compressor the decrease Free in the Version number of molecules? What happens when the pressure of the system is increased? What if the pressure of the system is decreased? In the manufacture of ammonia, why is a pressure of atm used? Analyse the chemical equation for the gaseous reaction given below. H 2 (g) + I 2 (g) U 2HI (g) What is the total number of moles of the reactants? What about the products? There is no change in the number of moles of the reactants and the products here as a result of forward and backward reactions. If there is no change in the number of reactant or product molecules as a result of forward and backward reactions, in gaseous reactions, pressure will not have any effect on the equilibrium state. Temperature and Equilibrium We have seen that there is energy change during chemical reactions. You know that endothermic reactions are those in which heat is absorbed and exothermic reactions are those in which heat is liberated. Take a little concentrated HNO 3 in a test tube and add some copper filings/newspaper balls. Doesn't a gas come out? What is its colour? Collect the gas formed in another test tube. Close this test tube with a cork (Figure 3.6). Ice N 2 O 4 Warm water NO 2 NO2 At room temperature Figure 3.6 Heat 65

66 66 When the test tube is placed in a vessel containing ice the brown colour PDF Compressor fades. The Free brown Version colour intensifies when it is placed in a vessel filled with hot water. Endothermic N 2 O 4 (g) 2NO 2 (g) Exothermic Let us analyse this reaction using Le Chatelier's principle. Here the brown coloured gas is NO 2. When the temperature is decreased by placing the test tube in the vessel containing ice, the exothermic reaction occurs and NO 2 gas changes to the reactant dinitrogen tetroxide (N 2 O 4 ). On heating, the rate of endothermic reaction increases to form more NO 2 gas and thus the brown colour increases. Let us examine the role of temperature in the manufacture of ammonia gas. You know that NH 3 is industrially manufactured by Haber process. Here, the forward reaction is exothermic in nature. N 2 (g) + 3H 2 (g) U 2 NH 3 (g) + heat On increasing the temperature, the system tries to reduce it and as a result, the rate of endothermic reaction increases. As a result ammonia decomposes to form N 2 and H 2. Hence, according to Le Chatelier principle, for the formation of a larger amount of NH 3, the temperature has to be reduced. But at low temperature, since the rates of forward and backward reactions get very much reduced, the system will take more time to reach equilibrium. Hence in the manufacture of ammonia, 450 o C is taken as the optimum temperature. Catalyst and Chemical Equilibrium You know that positive catalysts are those substances which increase the rate of reactions. In reversible reactions there is a forward as well as a backward reaction. A catalyst cannot increase the rate of anyone of these reactions alone. What then is the role of a catalyst in reversible reactions? In reversible reactions the catalyst increases the rates of both the forward and the backward reactions to the same extent. As a result the system reaches equilibrium at a faster rate. Once equilibrium has been reached, the rate of chemical reaction can be controlled by altering other factors. Is it beneficial to add a catalyst in a system which has already attained equilibrium?

67 We have learnt the influence of different factors on equilibrium state. Analyse PDF the given Compressor chemical Free equation Version for one of the different stages of the manufacture of sulphuric acid by contact process. Identify how the given factors influence this reaction. 2SO 2 (g) + O 2 (g) U 2SO 3 (g) + heat Increase in the amount of oxygen. Increase in pressure. Maintains the temperature at the optimum level. Catalyst (V 2 O 5 ) is added. SO 3 is removed. Chemistry $ Standard - X Significant Learning Outcomes The learner conducts various chemical reactions, compares the rates of chemical reactions and formulates conclusions. formulates and writes the practical definition for the rate of chemical reaction. identifies nature of reactants and the concentration as the two factors which influence the rate of reaction and explains their effect. recognises through experiments that pressure, temperature and the presence of catalyst are factors influencing the rate of chemical reaction and explains their effect. identifies irreversible and reversible reactions and explains using examples. experiments and identifies that chemical equilibrium is dynamic in nature. identifies and explains methods for high yield of products in reactions which attain equilibrium, by applying Le Chatelier principle. explains the effect of catalysts in reversible reactions. 67

68 Let Us Assess PDF Compressor Free Version 1. a) In which of the following reversible reactions does the change in pressure not influence equilibrium? What is the reason? i) H 2 (g) + I 2 (g) U 2HI (g) ii) N 2 (g) + 3H 2 (g) U 2NH 3 (g) b) What is the use of applying high pressure during formation of ammonia from nitrogen and hydrogen? heat 2. C (s) + H O (g) 2 U CO (g) + H (g) 2 a) Identify the reactants and products. b) Products are frequently removed from the system. Explain the reason. Endothermic 3. N 2 O 4 (g) Exothermic 2NO 2 (g) a) Which gas is brown in colour? b) How does an increase in temperature influence the state of equilibrium? c) How can the yield of the product NO 2 be increased? 4. The chemical equation for the reaction between marble and dilute HCl is given. CaCO 3 (s) + 2HCl (aq) CaCl 2 (aq) + CO 2 (g) + H 2 O (l) a) Which is the gas formed here? b) Suggest two methods for increasing the rate of reaction. Explain the reason for your suggestions. 5. 2NO (g) + O 2 (g) U 2NO 2 (g) + heat In this reaction how do the following changes influence the amount of the product? a) Decrease in temperature b) Increase in pressure c) Increase in the concentration of oxygen 68

69 6. N 2 (g) + 3H 2 (g) U 2NH 3 (g) + heat a) PDF In this Compressor reaction Free what Version happens to the volume when the reactants change to the products? b) What change in pressure is required for the maximum yield of the product? c) What is the change in concentration required for increasing the rate of the forward reaction? d) What is the catalyst used in this reaction? What is its effect? Chemistry $ Standard - X Extended Activities 1. Some apparatus and chemicals are given. Zn, Mg, dilute HCl, CaCO 3, test tube, water a) Design an experiment to prove that the nature of reactants can influence the rate of reaction. b) Write the equations for the chemical reactions. c) Write the expression for the rate of the reaction. 2. The graph for the reaction N 2 (g) + 3H 2 (g) U 2NH 3 (g) + heat is given below. Rate of Chemical Reaction Reaction C Reaction D A Equilibrium Time a. Identify and write the reactions C and D. b. What happens to the position of point A in the graph when a catalyst is used? Redraw the graph accordingly. 69

70 3. The experiments conducted by two students are given below. PDF Compressor Experiment Free Version ml of sodium thiosulphate solution is taken in a test tube, heated and to it 2 ml of HCl solution is added. Experiment ml of sodium thiosulphate solution is taken in a test tube and to it 2 ml of HCl solution is added. a) In which experiment is the precipitate formed first? Justify your answer. b) Write the balanced equation for the reaction. 4. Some materials available in the laboratory are given below. Magnesium ribbon, marble powder, marble pieces, dilue HCl, concentrated HCl. a) Which are the materials required for the preparation of more CO 2 in less time? b) Write the balanced chemical equation for the reaction. 70

71 PDF Compressor Free Version Didn't you notice the picture? What do you see in it? A few instances of the use of electricity and a few sources of electricity are shown in the picture. Generation of electricity from the chemical reactions of certain metals and separation of metals as well as non metals from solutions by passage of electric current are involved in these processes. The reason for using different metals in the same cell is the difference in their chemical reactivity. Metals are utilised in various chemical reactions. While some metals engage in chemical reactions vigorously, others react sluggishly in the same reactions. By comparing this nature of their reactivity, metals of greater reactivity and lesser reactivity can be found out. Let us see how we can utilise the difference in chemical reactivity of different metals once we identify them.

72 Chemical Reactivity of Metals PDF Compressor Each metal Free is Version different in its capacity to react. Figure 4.1 Kerosene Sodium Gas bubbles Water Let us carry out the reaction of certain metals with water. Take a little water in a test tube and add two drops of phenolphthalein to it. Pour equal volume of kerosene oil to this mixture. Put a small piece of sodium metal into it and note down the observation (Figure 4.1). $ Haven't you seen gas bubbles sticking on the surface of sodium? Which is the gas formed? See the video clip based on 'Sodium Muthalaya Lohangaljalavumayulla rasapravarthanam' given in the page 'Lohangal' of 'Chemistry for Class X' in the 'School Resources' of Edubuntu of School $ Did the colour of the water change? Find the reason by examining the chemical equation. 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2 (g) Isn't the reason for the formation of the pink colour in water clear now? Take a little cold water in one test tube and a little hot water in another test tube. After adding one drop of phenolphthalein in each test tube put some magnesium in both of them and note down the observation. $ From which one does hydrogen gas come out fast? Look at the chemical equation. Mg (s) + 2H 2 O (l) Mg(OH) 2 (aq) + H 2 (g) $ Why pink colour is formed quickly in hot water? Do iron and copper react with cold water and produce hydrogen? Think on the basis of day-to-day experiences. Iron enters into a reaction with steam heated to high temperature (super heated steam). Complete the equation. Fe (s) + H 2 O (g) Fe O (s) Copper does not react even with steam. Arrange the metals Cu, Mg, Fe and Na in decreasing order of their reactivity with water. One of the characteristics of metals is their metallic lustre. Cut a piece of sodium using a knife. Observe the freshly cut portion. Don't you see that its shining lusture fades after sometime? 72

73 Can you find the reason for this by examining the equations given below? PDF Compressor Free Version 4Na (s) + O 2 (g) 2Na 2 O (s) 2Na (s) + 2H 2 O (1) 2NaOH (s) + H 2 (g) 2NaOH (s) + CO 2 (g) Na 2 CO 3 (s) + H 2 O (1) This is due to the conversion of sodium into its compounds by reacting with oxygen, water and carbon dioxide in the atmosphere. Haven't you noticed a fresh magnesium ribbon losing its lustre when kept open in the air for some days? This is also due to its reaction with atmospheric air. 2Mg (s) + O 2 (g) It can be seen that the lustre of aluminum vessels diminishes as time passes by. In the case of copper vessels, it takes months for the loss of its lusture by the formation of verdigris. Does the shining of gold fade even after a long time? Doesn't this indicate that metals react with air at different rates? $ Which metal among magnesium, copper, gold, sodium and aluminum, lose its lustre at a faster rate? Chemistry $ Standard - X $ List the above metals in the decreasing order of losing lustre by reacting with air. Generally metals react with dilute HCl and produce hydrogen. Let us plan an experiment to compare the rates of reaction of the metals Mg, Pb, Zn, Fe and Cu with dilute HCl (Figure 4.2). Arrange the test tubes in the decreasing order of their reaction rates Figure

74 It is understood from these experiments that metals differ in PDF Compressor their Free reactivity. Version The series obtained by arranging some of the metals in the decreasing order of their reactivity is given in Table 4.1. This is known as the reactivity series. Note that hydrogen is also included in this series for comparison of chemical reactivity. Potassium Sodium Calcium Magnesium Aluminum Zinc Iron Nickel Tin Lead Hydrogen Copper Silver Gold Table 4.1 K Na Ca Mg Al Zn Fe Ni Sn Pb H Cu Ag Au Displaces Hydrogen by reacting with dilute acids. Does not displace Hydrogen by reacting with dilute acids. Reactivity Series and Displacement Reactions Prepare some CuSO 4 solution in a beaker. Dip a Zn rod in it. (Figure 4.3). Observe the changes and complete the observation note by filling Table 4.2. To be observed Before the After the Experiment Experiment Zn rod Zn 2+ Colour of Zn rod Zn Colour of CuSO solution CuSO 4 solution Cu Cu Table 4.2 Figure 4.3

75 Let us examine the reason for the change that has occured to the zinc rod. PDF Compressor Free Version The blue colour of CuSO 4 solution is due to the presence of Cu 2+ ions. But the colour of CuSO 4 solution faded on keeping the Zn rod immersed in it. What is the reason for this? See the chemical reaction taking place here. Zn (s) + CuSO 4 (aq) ZnSO 4 (aq) + Cu (s) $ Which metal is displaced here? Chemistry $ Standard - X $ Which among Zn and Cu is more reactive? $ On the basis of the position of Zn and Cu in the reactivity series can you explain why Cu had been displaced? Isn't it because zinc (Zn) has a higher reactivity than copper (Cu)? Let us rewrite the above equation showing the nature of the ions, as below. Zn 0 + Cu 2+ SO 4 2 Zn 2+ SO Cu 0 The change that occurred in Zn : Zn 0 Zn e $ This process is known as oxidation. What may be the reason for this? The change that occurred for Cu 2+ : + $ What is this process known as? Why? Zn is oxidized and Cu 2+ is reduced, i.e. oxidation and reduction take place simultaneously. This is a redox process. $ Observe what happens when a copper wire is kept immersed in silver nitrate solution (Figure 4.4 ). Copper $ What happened to the copper wire? Find the reason. $ What is the change that occured to the colour of the silver nitrate solution? Why? Ag NO 3 solution Cu Cu 2+ Ag Ag + Figure

76 $ Complete the chemical equation. PDF Compressor 2AgNO Free 3 (aq) Version + Cu (s) Cu(NO 3 ) 2 (aq) + $ Write the equation showing the ionic nature. $ Find the metal which was oxidized and the metal ion which was reduced. $ Write the equation for oxidation and the equation for reduction. Oxidation : Reduction : The metals Mg, Cu, Zn, Fe and Ag and their salt solutions are given in Table 4.3. With the help of a spotting tile, dip the metals in the salt solutions and observe. Complete the table by giving 'ü' to those in which metals are displaced and 'x' mark to the others. For more experience, open the interactive animation in the 'Adesa rasa pravarthanangal' in the page 'Lohangal' in 'Chemistry for class X' of 'School Resources' of School Edubuntu. Metal Solution Magnesium sulphate Copper sulphate Zinc sulphate Ferrous sulphate Silver nitrate Mg Cu Zn Fe Ag Table 4.3 Metals with low reactivity are displaced from their salt solutions by metals of high reactivity. $ Arrange the metals given above in the decreasing order of their ability to react by losing electrons. Compare the experiments conducted earlier with the reactivity series and examine whether the results are in agreement with the given sequence. Metals, to Produce Electricity too-galvanic Cell Generally metals with high reactivity show a tendency to form positive ions by losing electrons in aqueous solution. We have already seen that the ability to lose electrons is different for different metals. Let us do an experiment. See Figure

77 ZnSO 4 Solution Zn Rod PDF Compressor Free Version Voltmeter Salt bridge Figure 4.5 Cu Rod CuSO 4 Solution Take two beakers and add 100 ml of 1 M ZnSO 4 solution into one beaker and 100 ml of 1 M of CuSO 4 solution into the other. Immerse a Zn rod in ZnSO 4 solution and a Cu rod in CuSO 4 solution. Connect the negative terminal of a voltmeter to the Zn rod and the positive terminal to the Cu rod. Connect the solutions in two beakers by a salt bridge (A long strip of filter paper soaked in KCl solution can be used instead of salt bridge). Now, observe the change in the voltmeter reading. Here electricity is produced by a chemical reaction. The device used for the conversion of chemical energy to electrical energy through redox reaction is called Galvanic Cell or Voltaic Cell. From previous experiments, haven't you understood that Zn has more reactivity than Cu. $ In the cell made by these metals, which electrode can lose electrons? $ Which one can gain electrons? Which of the reactions given below takes place at the Zn electrode? Find the correct one and put "ü' against it. Zn (s) Zn2+ (aq) + 2e Zn 2+ (aq) + 2e Zn (s) The process taking place here is an oxidation process. Zn loses two electrons to form Zn 2+. Such an electrode at which oxidation occurs is called anode. For better clarity, open the video clip 'Interactive animation on Galvanic Cell' in the page 'Metal' of 'Chemistry for Class X' in the 'School Resources' of School Edubuntu. Salt Bridge Salt bridge is a U-tube filled with a paste made by mixing gelatin or agar agar gel and a salt like KCl, KNO 3 or NH 4 Cl. This completes the circuit by transfer of ions and maintains the electrical neutrality of the cell. 77

78 Zn rod turn negatively charged. PDF Compressor Free Version The electrons liberated from Zn rod reach the copper electrode through the external circuit and these electrons are received by copper ions in the solution changing it into copper. Direction of Electron Flow and the Direction of Flow of Electricity In a Galvanic cell, electrons flow from the negative electrode (anode) to the positive electrode (cathode). But the flow of electric current is considered to be always from positive to negative. In earlier times, it was assumed that electricity moved from positive to negative and on the basis of this assumption many rules and equations were formulated. Owing to the difficulty in correcting these, it has been considered as 'conventional current' and the direction of the movement of electrons as electron current. Try to write the chemical equation for the reaction at Cu electrode. This is a reduction reaction. The electrode at which reduction occurs is the cathode. Copper rod turn positively charged. The electrode at which oxidation occurs is the anode and that at which reduction occurs is the cathode. Shall we write the combined equation for the reactions at Zn and Cu electrodes? In Zn electrode: Zn (s) Zn 2+ (aq) + 2e In Cu electrode: 2+ - Cu (aq)+2e Cu (s) Zn (s)+cu (aq) Zn (aq)+cu (s) This is a redox reaction. The transfer of electrons produced by this redox reaction causes the flow of electric current in the cell. You have noticed that the direction of electron flow is from anode to cathode. Shall we construct a cell using silver and copper electrodes? Materials required: Silver wire, copper rod, two beakers, copper sulphate, silver nitrate, salt bridge, voltmeter, copper wire, water etc. $ Sketch the cell constructed. 78

79 $ Note down the reaction of the Galvanic cell. PDF Compressor Free Version $ Mark the direction of electron flow in the cell illustrated. Write the reactions taking place at cathode and anode. At cathode : At anode : Didn't copper act as the cathode in the cell constructed using zinc and copper? But what happened when silver and copper were used? See the redox reaction taking place here. Cu (s) + 2Ag + (aq) Cu 2+ (aq) + 2Ag (s) Corrosion of Metals The process of conversion of a metal into its compounds by continuous interaction with atmospheric air and water vapour is termed corrosion of metals. This is an electrochemical reaction in which metals lose electrons and oxygen accepts them. Metals such as iron, zinc, copper, etc., undergo corrosion in this way. Metals with high reactivity like potassium and sodium undergo corrosion by the direct reaction. You have used three metals Zn, Cu and Ag. How many cells can be produced using these? Complete Table 4.4 by writing anode and cathode in each. Zn - Cu Cell Anode Cathode Table 4.4 We are now familiar with the chemical reactions in which chemical energy is converted into electrical energy. If so, chemical reactions can also be carried out using electric energy. Different types of Chemical Cells Chemical cells produce electrical energy by making use of the basic principle of the conversion of chemical energy into electrical energy. Dry cell and Mercury cell are cells which cannot be used again by recharging. They belong to the category of Primary Cells. But Secondary Cells such as Lead storage cell and Nickel - Cadmium cell (Ni- Cd cell) when discharged can be used again by recharging. Lithium - ion cell used in mobile phones is also of this type. Hydrogen - Oxygen fuel cell is a modern concept of Galvanic cells. 79

80 Graphite Rod You have done the process of separation of hydrogen and oxygen PDF Compressor by Free electrolyzing Version water. 4.5 V + - V Graphite Rod Let us see similar set up. Electrolytic Cells See the experiment given in Fig Take a little cupric chloride (CuCl 2 ) in a beaker. Using two graphite rods, connect a 4.5 V battery as shown in the figure. $ Does CuCl 2 conduct electricity? Figure 4.6 CuCl 2 Solution Add some water to dissolve CuCl 2. $ Now, does the solution conduct electricity? Electrolytes are substances which conduct electricity in molten states or in aqueous solutions and undergo a chemical change. Acids, alkalis and salts are electrolytes in their molten state or in aqueous solution. In molten state or in aqueous solution, ions of the electrolytes can move freely. These ions are responsible for the conduction of electricity by the electrolytes. Isn't the electrical conduction of aqueous CuCl 2 due to the formation of ions? See the equation for its ionisation. CuCl 2 Cu Cl $ What is the change occurring at each electrode when electricity is passed for some time? Note down the observation. From the smell of the gas liberated at the electrode connected to the positive pole of the battery (positive electrode), you can identify it as chlorine. Take out the electrode attached to the negative pole of the battery (negative electrode) and observe its change. $ Which is the metal that is deposited on the graphite rod? 80