(PCCP) Division WORKSHOP TAPASYA SHEET CHEMISTRY COURSE : OLYMPIAD CLASS-VII

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1 Resonance Pre-foundation Career Care (PCCP) Division Programmes WORKSHOP TAPASYA SHEET CHEMISTRY COURSE : OLYMPIAD CLASS-VII

2 SUBJECT : CHEMISTRY CLASS-VII(OLYMPIAD) S. No. Topics Page No. 1. Acids, Bases and Salts General Chemistry 5-10 Copyright reserved. All right reserved. Any photocopying, publishing or reproduction of full or any part of this study material is strictly prohibited. This material belongs to only the enrolled student of RESONANCE. Any sale/resale of this material is punishable under law. Subject to Kota Jurisdiction only. 13RPCCP

3 ACIDS AND BASES INTRODUCTION Curd, vinegar, tamarind and lemon juice etc. all have sour taste. Green mangoes, tomatoes, karvanda (corinda) and unripened grapes also taste sour. Many fruits (specially citrus fruits) and some vegetables are sour tasting because of the presence of substances known as acids, which have formed naturally in them. On the other hand, there are some substances, such as, washing soda, lime (chuna) and materials like soap whose solutions have a soapy touch and a bitter taste. These materials are either bases or alkalis. An acid and a base react together forming water and a new compound, known as salt. ACIDS The term acid is derived from the Latin word acidus meaning sour. Lemons, oranges and grapes taste sour because they contain citric acid. Tamarind and vinegar contain tartaric acid and acetic acid respectively. Definition : An acid is a compound which contains one or more hydrogen atoms replaceable partially or completely by a metal or a positive radical to produce a salt. e.g. Zn H 2 (dil.) Zn H 2 Zinc Sulphuric Zinc Sulphate Hydrogen acid Hydrochloric acid (HCl), nitric acid (HNO 3 ), acetic acid (CH 3 COOH), formic acid (HCOOH) etc. have one and sulphuric acid (H 2 ) has two replaceable hydrogen atom, thus they are acids. Vitamin C which is very important for our body is also an organic acid known as ascorbic acid. Classification of Acids: (a) On the basis of occurrence: (i) Mineral acids : Acids which are obtained from the minerals present in earth s crust are called mineral acids. e.g. HCl, H 2, HNO 3 etc. (ii) Organic acids : Acids that are found in animals and plants are known as organic acids. e.g. Lactic acid, citric acid, tartaric acid, acetic acid and formic acid. The sharp pain caused by the sting of ants and bees is due to formic acid, which they push into the body or spray on the skin. Acids like conc. H 2 and conc. HNO 3 are corrosive in nature. They destroy organic matter like clothes, paper, wood and cause burn to human skin. (b) On the basis of strength: (i) Strong Acids : Acids, which almost completely ionise (break up into ions) in water, are called strong acids. e.g. Hydrochloric acid (HCl), sulphuric acid (H 2 ), nitric acid (HNO 3 ) etc. HCl Water H 2 Water (ii) Weak Acids: H (aq) Cl (aq) 2H 2 (aq) (aq) Acids, which partially ionise in water, are called weak acids. e.g. Carbonic acid (H 2 CO 3 ), phosphoric acid (H 3 PO 4 ), formic acid (HCOOH), acetic acid (CH 3 COOH). CH 3 COOH Water CH 3 COO (aq) H (aq) (Partially ionised) In general mineral acids are strong while organic acids are weak. (c) On the basis of concentration : (i) Concentrated acid : The acid containing very less amount of water is called concentrated acid. HCl is prepared by dissolving HCl gas in water. The solution of this acid is called conc. HCl. (ii) Dilute acid : The acid containing excess amount of water is called dilute acid. Strength can be decreased by dissolving the acid in more water. In a laboratory, we generally use either concentrated acid or it s solution diluted to a definite strength. Dilution of acids : It is always desirable to add acid to water, keeping the solution continuously stirred, while preparing dilute solutions of acids, specially mineral acids. We should always slowly add acid to water; otherwise, so much heat is produced during the dilution process that the container, specially that of glass, may break. The hot contents may also cause an explosion and spill on our clothes and body. This may result into serious acid burns. PAGE # 1

4 (d) On the basis of basicity : (i) Monobasic Acids : When one molecule of an acid on complete ionisation produces one hydronium ion (H 3 O ) in aqueous solution, the acid is said to be a monobasic acid. Examples of Monobasic Acids. Some examples of monobasic acids are : (i) Hydrochloric acid (HCl) (ii) Hydrobromic acid (HBr) (iii) Nitric acid (HNO 3 ) (iv) Acetic acid (CH 3 COOH) (v) Formic acid (HCOOH) (ii) Dibasic Acids : When one molecule of an acid on complete ionisation produces two hydronium ions (H 3 O ) in aqueous solution, the acid is said to be a dibasic acid. Examples of Dibasic Acids : Some examples of dibasic acids are : (i) Sulphuric acid (H 2 ) (ii) Sulphurous acid (H 2 SO 3 ) (iii) Carbonic acid (H 2 CO 3 ) (iv) Oxalic acid [(COOH) 2 ] (v) Hydrofluoric acid (HF) (iii) Tribasic Acids : When one molecule of an acid on complete ionisation produces three hydronium ions (H 3 O ) in aqueous solution, the acid is said to be a tribasic acid. An example of tribasic acids is Phosphoric acid (H 3 PO 4 ). (iv) Tetrabasic Acids : When one molecule of an acid on complete ionisation produces four hydronium ions (H 3 O ) in aqueous solution, the acid is said to be a tetrabasic acid. An example of tetrabasic acids is silicic acid (H 4 SiO 4 ) Examples of some important acids and their uses: Name Type Where found or used Carbonic acid Mineral acid Present in soft drinks and in stomach as gastric juice, used in tanning industry. Nitric acid Hydrochloric acid Sulphuric acid Mineral acid Mineral acid Mineral acid Used in the manufacture of explosives (TNT, Nitroglycerine), fertilizers (Ammonium nitrate, calcium nitrate) and in purification of Au, Ag. Used in purification of common salt, in textile industry as bleaching agent, in cleaning kitchen sink and sanitary wares and to make aqua regia. Commonly used in car batteries, in the manufacture of fertilizers (Ammonium phosphate, Super phosphate) detergents etc., in paints, plastics, drugs, in manufacture of artificial silk and in petroleum refining. Phosphoric acid Mineral acid Used in antirust paints and in fertilizers. Formic acid Organic acid Found in the stings of ants and bees, used in tanning leather, in medicines for treating gout. Acetic acid Organic acid Found in vinegar, used as solvent in the manufacture of dyes and perfumes. Lactic acid Organic acid Responsible for conversion of milk in curd. Benzoic acid Organic acid Used as a food preservative. Citric acid Organic acid Present in lemons, oranges and citrus fruits. Tartaric acid Organic acid Present in tamarind. The atmosphere of Venus is made up of thick white and yellow clouds of Oil of Vitriol (H 2 ). BASES Bases are the substances which have bitter taste and are soapy in touch. Definition : A base is a compound which contains one or more replaceable hydroxyl group(oh ). It produces salt on reaction with an acid e.g. (a) Alkalis : Bases which dissolves in water are called alkalis. e.g. KOH, NaOH. All alkalis are bases but all bases are not alkalis. e.g. Al(OH) 3 is a base, but not an alkali Bases (e.g, Al(OH) 3, Zn(OH) 2 all neutralise acids Alkalis are water soluble bases (e.g. NaOH, KOH) Sodium hydroxide NaOH Calcium hydroxide Ca(OH) 2 Aluminium hydroxide Al(OH) 3 PAGE # 2

5 (b) Classification of Bases : (i) On the basis of strength : (1) Strong Bases : Bases which are almost completely dissociated in water, are known as strong bases. e.g. Sodium hydroxide (NaOH), potassium hydroxide (KOH), barium hydroxide Ba(OH) 2 etc. NaOH(s) Water KOH(s) Water Na (aq) OH (aq) K (aq) OH (aq) (2) Weak Bases : Bases which dissolve in water only slightly and produce a low concentration of hydroxide ions are called weak bases. e.g. Ammonium hydroxide (NH 4 OH), silver hydroxide (AgOH) etc. (ii) On the Basis of their Concentration : By the term concentration, we mean the amount of water present in the given sample of alkali solution in water. On the basis of concentration, the alkalis can be classified as under : (1) Concentrated alkali : A solution of alkali having a relatively high percentage of alkali in its aqueous solution is known as concentrated alkali. (2) Dilute alkali : A solution of alkali having a relatively low percentage of alkali in its aqueous solution is known as a dilute alkali. If the concentration of alkali in the solution is less than 1 mole per litre, then it is considered to be a dilute alkali. (iii) On the Basis of their Acidity : Before we discuss this classification, let us understand the meaning of the word acidity of an alkali. The number of hydroxyl (OH ) ions produced by one molecule of an alkali on complete dissociation in water or the number of hydrogen ions (of an acid) with which a molecule of that alkali reacts to produce salt and water only is known as acidity of an alkali. For water insoluble hydroxides, acidity of the base is equal to the number of OH ions present in one molecule of that base. On the basis of acidity, the bases can be classified as under : (1) Monoacidic Bases (or alkalis) : W hen one molecule of the base on complete ionisation produces one hydroxyl (OH ) ion in aqueous solution, the base or alkali is said to be monoacidic OR A monoacidic base (or alkali) may be defined as one whose one molecule reacts with one hydrogen (H ) ion completely to form salt and water as the only products. Examples of Monoacidic Bases (or alkalis) : Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Ammonium hydroxide (NH 4 OH). All these substances produce only one hydroxyl ion on complete ionisation in aqueous solution. NaOH(aq.) KOH(aq.) Na (aq) OH (aq) K (aq) OH (aq) The dissociation of monoacidic bases or alkalis takes place in a single step. (2) Diacidic Bases (or alkalis) : When one molecule of a base or alkali on complete ionisation produces two hydroxyl (OH ) ions in aqueous solution, the base or alkali is said to be diacidic. Examples of Diacidic Bases Calcium hydroxide [Ca (OH) 2 ] and magnesium hydroxide [Mg (OH) 2 ] Ca(OH) 2(aq) Mg(OH) 2(aq) Ca 2 (aq) 2OH (aq) Mg 2 (aq) 2OH (aq) One molecule of both the bases are producing 2OH ions in aqueous solution, therefore, these are termed as diacidic bases. (3) Triacidic Bases : When one molecule of a base or alkali on complete ionisation produces three hydroxyl (OH ) ions in aqueous solution, the base or alkali is said to be triacidic base. Examples of Triacidic Bases : Aluminium hydroxide [Al(OH) 3 ], Ferric hydroxide [Fe (OH) 3 ] Al (OH) 3(aq) Al 3 (aq) 3OH (aq) Al (OH) 3 3HCl (aq) AlCl 3 3H 2 O In the above equations, one molecule of Al (OH) 3 is producing three OH ions and one molecule of Al (OH) 3 is reacting with three hydrogen (H ) ions to form salt and water only, therefore, it is termed as a triacidic base. PAGE # 3

6 (c) Examples of some important bases and their uses: Chemical Name Sodium hydroxide Potassium hydroxide Calcium hydroxide Magnesium hydroxide Aluminium hydroxide Ammonium hydroxide Commercial Name Caustic soda Caustic potash Slaked lime Milk of magnesia Chemical Formula NaOH KOH Ca(OH) 2 Mg(OH) 2, Other uses : CaO is used to neutralize acidic nature of soil. Ca(OH) 2 is used to prepare mortar, bleaching powder and to neutralize acid in water supplies. KOH (caustic potash) is used to conduct electricity between two electrodes. Both NaOH and KOH are deliquescent in nature which means that they absorb moisture from air. INDICATORS An indicator is a substance which indicates the nature of particular solution whether acidic, basic or neutral. Hence they indicate the change in nature of the solution from acidic to basic and vice versa. Indicators are basically coloured organic substances. (a) Different types of indicators : (i) Litmus : Litmus is a purple dye which is extracted from a plant lichen. It can also be applied on paper in the form of strips and is available as blue and red strips. A blue litmus strip, when dipped in an acid solution acquires red colour. Similarly a red strip when dipped in a base solution becomes blue. (ii) Phenolphthalein : It is also an organic dye. In neutral or acidic solution, it remains colourless while in the basic solution, the colour of indicator changes to pink. (iii) Methyl Orange : Methyl orange is an orange coloured dye and keeps this colour in the neutral or basic medium. In the acidic medium the colour of indicator becomes red. (iv) Red Cabbage Juice : It is purple in colour in neutral medium and turns red or pink in the acidic medium. In the basic or alkaline medium, its colour changes to green. (v) Turmeric juice : It is yellow in colour and remains as such in the neutral and acidic medium. In the basic medium its colour becomes reddish or deep brown. (vi) China Rose : Extract of china rose (Gudhal) petals is of pink colour. It will change into dark pink (magenta) in acidic solution and green in basic solution. Uses In manufacture of soap, paper pulp,rayon, refining of petroleum etc. In alkaline storage batteries, manufacture of soap, absorbing CO 2 gas etc. In manufacture of bleaching powder, softening of hard water etc. As an antacid to remove acidity from stomach. As foaming agent in fire Al(OH) 3 extinguishers. In removing grease stains NH 4 OH from clothes and in cleaning window panes. NEUTRALISATION The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat. Evolved heat is known as heat of neutralisation. Acid Base e.g. Salt Water HCl NaOH NaCl H 2 O 57.1 kj Hydrochloric Sodium Sodium water acid hydroxide chloride Where 57.1kJ energy is the heat of neutralisation for above reaction this value remains same if both acid and base are strong. If one out of these is weak then amount of energy released will be lesser than 57.1 kj (a) Neutralisation in everyday life: (i) Indigestion : People particularly of old age suffer from acidity problems in the stomach which is caused mainly due to release of excessive gastric juices containing HCl. The acidity is neutralised by antacid tablets which contain sodium hydrogen carbonate (baking soda), magnesium hydroxide etc. (ii) Ant bite : The stings of bees and ants contain formic acid. Its corrosive and poisonous effect can be neutralised by rubbing soap which contains NaOH (an alkali) or by rubbing baking soda (NaHCO 3 ) or by calamine solution (ZnCO 3 ). The stings of wasps contain an alkali and its poisonous effect can be neutralised by an acid like acetic acid (present in vinegar). (iii) Soil treatment : Farmers generally neutralize the effect of acidity in the soil caused by acid rain by adding slaked lime (Calcium hydroxide ) to the soil. (iv) Factory wastes : The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances. PAGE # 4

7 GENERAL CHEMISTRY INTRODUCTION Chemistry is defined as that branch of science which deals with the composition and properties of matter and the changes that matter undergoes by various interactions. Anything that occupies space, has mass and offers resistance is called matter. ATOM An atom is the smallest particle of an element that can take part in a chemical reaction. The size of an atom is indicated by its radius which is called "atomic radius" (radius of an atom). Atomic radius is measured in "nanometres"(nm). 1 metre = 10 9 nanometres or 1nm = 10-9 m. Hydrogen atom is the smallest atom of all having an atomic radius 0.037nm or 0.37 Å (1 Å = metre) Maharishi Kanad told that if we keep dividing matter (padartha) on and on, we will get the smallest particle called as parmanu (which was later termed as atom by John Dalton). SYMBOL "A symbol is a brief representation of the name of the element". Berzelius, a Swedish chemist, was the first to introduce the system of using letters as symbols for the elements. (a) Symbols may be derived from the first letter of the English name of the element. e.g. Hydrogen (H), Oxygen (O), Nitrogen (N) etc. (b) Symbols may be derived from the first letter and another significant letter of the name of element. e.g. Aluminium (Al), Bromine (Br), Calcium (Ca) etc. (c) Symbols may be derived from their Latin names of the elements. e.g. Copper (Cuprum-Cu), Iron (Ferrum-Fe), Silver (Argentum-Ag), Sodium (Natrium- Na), Potassium (Kalium -K),Gold ( Aurum -Au) etc. Dalton was the first to use symbols to represent elements. DALTON'S ATOMIC THEORY (i) Matter consists of small indivisible particles called atoms. (ii) All atoms of an element are identical. (iii) The atoms of an element are different from the atoms of any other element. (iv) A compound is formed by combination of atoms of two or more elements in simple ratio. e.g. Ratio between H and O in water is 2 : 1 by volume. (v) Atoms take part in chemical reactions. (vi) Atoms can neither be created nor be destroyed. MERITS (i) Dalton s theory explains the law of conservation of mass and law of constant proportion. (ii) Atoms of elements take part in chemical reaction this is true till today. DEMERITS (i) The atom is no longer supposed to be indivisible. (ii) He could not explain that why do atoms of same element combined with each other. (iii) Atoms of the same element may not necessarily be identical in all aspects. e.g. isotopes. (iv) Atoms of different elements may not necessarily be different in all aspects. e.g. isobars. MODERN ATOMIC THEORY (a) Structure of an atom : An atom consists of two parts - (i) Nucleus : Nucleus is situated in the centre of an atom. All the protons & neutrons are situated in the nucleus, therefore, the entire mass of an atom is almost concentrated in the nucleus. The overall charge of nucleus is positive due to the presence of positively charged protons. The protons & neutrons are collectively called nucleons. The radius of the nucleus of an atom is of the order of cm and its density is of the order of g/cm 3. (ii) Extra nuclear region : In extra nuclear part electrons are present which revolve around the nucleus in orbits of fixed energies. The maximum number of electrons that can be accommodated in a shell is given by the formula 2n 2.(n = number of shells i.e. 1,2, ) First energy level Second energy level Third energy level Fourth energy level Nucleus K L M N Electron shells Maximum number of electrons which can be accommodated in the various shells (b) Composition of an atom : It consists of three elementary particles electron, proton and neutron. These are known as sub-atomic fundamental particles. PAGE # 5

8 Property Electron Proton Neutron 1. Discovery J.J. Thomson E. Goldstein James Chadwick 2. Symbol e p n 3. Nature Negatively charged Positively charged Neutral 4. Relative charge Absolute charge C C Relative mass Absolute mass g g g ATOMIC NUMBER (Z) Z = no. of protons = no. of electrons (in electrically neutral atoms). e.g. 13 Al Z = 13 p = 13 e = 13 Each element has a unique atomic number. MASS NUMBER (A) e.g. A = no. of protons no. of neutrons (total no. of nucleons) A = p n A = Z n Al Z = 13 A = 27 p = 13 e = 13 n = A Z = = 14 (a) Atomic Mass of an Element : Actual masses of the atoms of the elements are very, very small. For example, one atom of hydrogen (H) has a mass of x gram. To avoid the inconvenience in using such small and complicated figures in our calculations, it was necessary to define atomic mass in such a way that we get simple figures for them. Carbon-12 atom is that atom of carbon which has 6 protons and 6 neutrons in its nucleus, so that its mass number is 12. Carbon-12 atom has been assigned an atomic mass of exactly 12 atomic mass units, written as 12 u. Definition of atomic mass : Atomic mass express as to how many time mass of an atom of an element is 1 heavier than the mass of carbon -12 atom. 12 Atomic Mass Unit (u) = One-twelfth the mass of a Carbon-12 atom. 1u = g. One atomic mass unit (1u) is defined as exactly onetwelfth the mass of an atom of Carbon-12.The atomic mass of an element is the relative mass of its atom as compared with the mass of a Carbon-12 atom taken as 12 units. The arrangement of electrons in different shells is known as the electronic configuration of the element. If the outermost shell has its full quota of 8 electrons it is said to be an octet. If the first shell has its full quota of 2 electrons, it is said to be duplet. (a) Bohr - Bury scheme for distribution of electrons in various shells : Name of shells K L M N Number of shells Maximum Number of electrons (2n 2 ) PAGE # 6

9 (b Electronic Configuration of some Elements - Atomic number Symbols of the element Name of the element 1 H Hydrogen 1 2 He Helium 2 3 Li Lithium 2,1 4 Be Beryllium 2,2 5 B Boron 2,3 6 C Carbon 2,4 7 N Nitrogen 2,5 8 O Oxygen 2,6 9 F Fluorine 2,7 10 Ne Neon 2,8 11 Na Sodium 2,8,1 12 Mg Magnesium 2,8,2 13 Al Aluminium 2,8,3 14 Si Silicon 2,8,4 15 P Phosphorus 2,8,5 16 S Sulphur 2,8,6 17 Cl Chlorine 2,8,7 18 Ar Argon 2,8,8 19 K Potassium 2,8,8,1 20 Ca Calcium 2,8,8,2 Electronic configuration (c) Significance of Electronic Configuration : (i) Electronic configuration of an atom helps us to understand the chemical reactivity of the element. (ii) When the outermost shell of an atom is completely filled as per Bohr-Bury scheme then the element is unreactive. (iii) When the outermost shell of an atom is not completely filled according to Bohr-Bury rule, the element is reactive. An atom can get the noble gas electronic configuration in three ways - By losing one or more electrons. By gaining one or more electrons. By sharing one or more electrons with other atom or atoms. The outermost shell of an atom is known as the valence shell. The electrons present in the valence shell of an atom are known as valence electrons. The remainder of the atom i.e. the nucleus and other electrons is called the core of the atom. Electrons present in the core of an atom are known as core electrons. e.g. The electronic configuration of the sodium (Na) atom is :- Na (11) K L M Thus, valence electrons in Na atom = 1 and core electrons in Na atom = 2 8 = 10 IONS (a) Valency : Valency of an element is the combining capacity of the atoms of the element with atoms of the same or different elements. The valency of an element = number of valence electrons (when number of valence electrons are from 1 to 4) The valency of an element = 8 number of valence electrons. (when number of valence electrons are more than 4) (b) Variable Valency : Certain elements (metals and non - metals ) exhibit more than one valency. (i) Among the metals iron, copper, silver etc. show variable valency. For lower valency a suffix -ous and for higher valency a suffix -ic is attached at the end of the name of the metal. e.g. Ferrous = Fe 2 Ferric = Fe 3 (ii) Among the non - metals nitrogen, phosphorus, sulphur etc. show variable valency. (c) How Do Atoms Exist? The atoms of only a few elements called noble gases (such as helium, neon, argon and krypton etc.) are chemically unreactive and exist in the free state (as single atoms). Atoms of most of the elements are chemically very reactive and do not exist in the free state (as single atoms). Atoms usually exist in two ways: (i) in the form of ions and (ii) in the form of molecules When atoms form molecules or ions, they become stable (because in doing so they acquire the stable electron arrangement of noble gases). The charged particles formed by an atom on the gain or loss of one or more electron(s) are called ions. Cation Ion Anion (a) Cation The loss of an electron by an atom leads to the formation of a cation. e.g. Na e Na 11e 10e (b) Anion : The gain of an electron by an atom leads to formation of an anion. e.g. Cl e Cl 17e 18e PAGE # 7

10 RADICALS A molecule of an inorganic compound is made up of two electrically charged species which are known as radicals. Types of radicals : (a) On basis of charge : (i) Basic radicals : The positively charged radical is known as the basic radical E.g.. K, Ca 2, Fe 3, etc. (ii) Acidic radicals : The negatively charged radical is called acidic radical. E.g. Cl, 2, Br etc. (b) Number of element : (i) Simple radicals : When a radical consists of only one element, it is called simple radical. e.g. Ag, Ba 2, Cl, Br, S 2 etc. (ii) Compound radicals: When a radical consists of more than one type of elements, it is called a compound radical. e.g. NO 3, 2, NH 4, BO 3 3, PO 4 3 etc. LIST OF COMMON ELECTROVALENT POSITIVE RADICALS Monovalent Electropositive Bivalent Electropositive Trivalent Electropositive Tetravalent Electropositive 1. Hydrogen H 1. Magnesium Mg 2 1. Aluminium Al 3 1. Stannic [Tin (IV)] Sn 4 2. Ammonium NH 4 2. Calcium Ca 2 2. Ferric [Iron (III)] Fe 3 2. Plumbic [Lead (IV)] Pb 4 3. Sodium Na 3. Zinc Zn 2 3. Chromium Cr 3 4. Potassium K 4. Plumbous [Lead (II)] Pb 2 5. Cuprous [(Copper (I)] Cu 5. Cupric [(Copper) (II)] Cu 2 6. Argentous [Silver (I)] Ag 6. Argentic [Silver(II)] Ag 2 7. Mercurous [Mercury(I)] Hg Stannous [Tin (II)] Sn 2 8. Ferrous [Iron (II)] Fe 2 9. Mercuric [Mercury (II)] Hg Barium Ba 2 LIST OF COMMON ELECTROVALENT NEGATIVE RADICALS Monovalent Electronegative Bivalent Electronegative Trivalent Electronegative Tetravalent Electronegative 1. Fluoride F 1. Sulphate 2. Chloride Cl 2. Sulphite SO 3 1. Nitride N 3-1. Carbide C 4-2. Phosphide P 3-3. Bromide Br 3. Sulphide S 3. Phosphite PO Iodide I 4. Thiosulphate S 2 O 3 5. Hydride H 5. Zincate ZnO 2 6. Hydroxide OH 6. Oxide O 7. Nitrite NO 2 8.Nitrate NO 3 9. Bicarbonate or Hydrogen carbonate HCO Bisulphite or Hydrogen sulphite HSO Bisulphide or Hydrogen sulphide HS 12. Bisulphate or Hydrogen sulphate H 13. Acetate CH3COO 7. Peroxide O 2 8. Dichromate Cr 2 O 7 9. Carbonate CO Silicate SiO 3 4. Phosphate PO 4 3- PAGE # 8

11 MOLECULES A molecule is an electrically neutral group of two (or more) atoms chemically bonded together by means of attractive forces. Or A molecule is the smallest particle of a substance (element or compound) which has the properties of that substance and can exist in the free state. Molecules can be formed either by the combination of atoms of the "same element" or of "different elements". There are two types of molecules: molecules of elements and molecules of compounds. (a) Molecules of Elements : The molecule of an element contains two (or more) similar atoms chemically bonded together. For example, a molecule of hydrogen contains 2 hydrogen atoms combined together and it is written as H 2 representing Hydrogen gas. Similarly, Ozone gas has 3 oxygen atoms combined together, so ozone exists in the form of O 3. The noble gases like helium, neon, argon and krypton etc., exist as single atoms He, Ne, Ar and Kr respectively. So, their atoms and molecules are just the same. (b) Molecules of Compounds : The molecule of a compound contains two (or more) different types of atoms chemically bonded together.for example, molecule of hydrogen chloride (HCl) contains one atom of hydrogen (H) chemically bonded with one atom of chlorine (Cl). Some more examples of the molecules of compounds are : sulphur dioxide (SO 2 ), methane (CH 4 ) and ammonia (NH 3 ). Every compound is a molecule, but every molecule is not a compound. (c) Atomicity: The number of atoms present in one molecule of an element or compound is called its atomicity. For example, the atomicity of noble gases is 1, that of hydrogen, nitrogen, oxygen etc. is 2 each and of ozone is 3. Thus, noble gases, hydrogen and ozone are respectively monoatomic, diatomic, and triatomic molecules. Atomicity of phosphorus is 4 and sulphur is 8. A compound which consists of molecules and not ions, is called a molecular compound. CHEMICAL FORMULA Molecule of an element or a compound may be represented by symbols of the elements present in one molecule of the compound. It is known as a chemical formula. E.g. HCl is the formula of hydrogen chloride and NaCl is that of sodium chloride. (a) Significance of a Chemical Formula : (i) Names of the elements present in the compound. (ii) Number of atoms of each element. (iii) Molecular weight of the compound. (iv) The relative proportion of weights of the elements. (b) Writing a Chemical Formula : Step-I Write the symbol of the positive ion or the radical to the left and that of the negative ion or radical to the right. Step-II Put the valence number of each radical or the ion on its top right. Divide the valence number by the highest common factor, if any, to get a simple ratio. Now ignore the () and ( ) signs. Interchange the valence numbers of radicals or ions. Step-III Shift the valence numbers to the lower right side of the radical or ion. If the compound radical receives number more than 1, enclose it within brackets. Do not enclose simple radicals within brackets. Names of compounds PAGE # 9

12 S.No. Compounds Common Names Chemical Names 1 CaO Lime Calcium oxide 2 NaHCO 3 Baking soda Sodium hydrogen carbonate 3 Na 2 CO 3.10H 2 O Washing soda Sodium carbonate decahydrate 4 CaCO 3 Limestone Calcium carbonate 5 Ca(OH) 2 Slaked lime Calcium hydroxide 6 Cu. 5H 2 O Blue vitriol Copper sulphate pentahydrate 7 NaCl Common salt Sodium chloride 8 Na 2 CO 3 Soda ash Sodium carbonate 9 NaOH Caustic soda Sodium hydroxide 10 KOH Caustic potash Potassium hydroxide 11 CaOCl 2 Bleaching powder Calcium oxychloride 12 Ca.1/2 H 2 O Plaster of paris Calcium sulphate hemihydrate 13 Ca. 2H 2 O Gypsum Calcium sulphate dihydrate 14 Fe. 7H 2 O Green vitriol Ferrous sulphate heptahydrate 15 H 2 Oil of vitriol Sulphuric acid MOLECULAR MASS The molecular mass of a substance may be defined as the relative mass of its molecule as compared to the mass of an atom of carbon (carbon-12) taken as 12 units. OR Molecular mass expresses as to how many times a molecule of a substance is heavier than 1/12 th of the mass of an atom of carbon (carbon-12). Eg. A molecule of water is 18 times heavier than 1/12 th of the mass of carbon atom. Therefore, the molecular mass of water is 18u. Similarly, the molecular mass of CO 2 is 44u. Calculation : The molecular mass is equal to sum of the atomic masses of all the atoms present in one molecule of the substance. Eg. The molecular mass of Sulphuric Acid (H 2 ) can be calculated as follows: Molecular mass of H 2 = Mass of 2 H atoms Mass of 1 S atom Mass of 4 O atoms = (2 x 1) (1 x 32) (4 x 16) = = 98u. Thus, the molecular mass of Sulphuric acid is 98u. List of atomic masses of some common elements : Atomic Number Element Symbol Atomic mass 1 Hydrogen H 1 2 Helium He 4 3 Lithium Li 7 4 Beryllium Be 9 5 Boron B 11 6 Carbon C 12 7 Nitrogen N 14 8 Oxygen O 16 9 Fluorine F Neon Ne Sodium Na Magnesium Mg Aluminium Al Silicon Si Phosphorus P Sulphur S Chlorine Cl Argon Ar Potassium K Calcium Ca 40 PAGE # 10