Lecture outline: Section 3

Transcription

1 Lecture outline: Section 3 Chemical reactions: chemical changes that occur when substances react to form new substances 1. Chemical equations 2. Atomic and molecular mass 3. Chemical calculations 1

2 Law of conservation of mass: atoms are not created or destroyed during chemical reactions. They simply rearrange. Mass before = mass after 2

3 Stoichiometry: the quantitative nature of chemical formulas and reactions - The study of quantities of materials consumed and produced in chemical reactions. 3

4 Chemical Equations + reacts with to produce + Left of arrow: starting substances, reactants right of arrow: substances produced, products Conservation of mass: atoms, left side = atoms, right side 4

5 Chemical Equations + + 2H 2 + O 2 2H 2 O Stoichiometric coefficients: the number of compounds involved in the reaction Subscript after an element in a compound: number of atoms of that element in the compound 5

6 Note the difference in meaning for a coefficient i in front of a formula vs. a subscript to a formula 6

7 Balancing a chemical Equation Identify the reactants and products Write the unbalanced chemical equation Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation Balance first the element(s) that t occur in only one substance on each side of the equation 7

8 Sodium reacts with chlorine gas (Cl 2 ) to produce sodium chloride Potassium reacts with water to produce potassium hydroxide and hydrogen gas (H 2 ) Identify the reactants and products Write the unbalanced chemical equation Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation 8

9 Write a balanced chemical equation for the reaction of methane gas (CH 4 4) with molecular oxygen (O 2 ) to produce water and carbon dioxide Identify the reactants and products Write the unbalanced chemical equation Balance equation by addition of proper coefficients in front of reactants and products so that the atoms balance on both sides of equation Balance first the element(s) that occur in only one substance on each side of the equation 9

10 Additional information from balanced chemical equations Physical states of reactants and products Reactants or products that are dissolved in water 10

11 Common types of chemical reaction Combustion Combination decomposition 11

12 Combustion: A chemical reaction where something burns in oxygen, producing heat Combustion of metals Combustion of diatomic molecules Combustion of nonmetal compounds Combustion of hydrocarbons Combustion of more complex organic molecules 12

13 Hydrocarbon combustion Reactants are a hydrocarbon (C x H y ) and O 2, products are CO 2 and H 2 O 13

14 Write the balanced chemical equation for the combustion of butane (tetracarbon decahydride) Identify the reactants and products Write the unbalanced chemical equation Start by balancing an element that only occurs in one compound on each side of the equation 14

15 Common types of chemical reaction Combustion Combination decomposition 0resources/media/CONTAC%7E1.MOV 15

16 Patterns of chemical reactivity Alkali metal + water Alkali metal + halogen 16

17 Concept check Chemical formulas subscripts Chemical equations coefficients Balancing equations Common reactions Combustion Combination Decomposition 17

18 Atomic mass Formula mass Chemical calculations The mole (mol) Empirical and molecular formulas Chemical equations and balancing 18

19 Atomic Mass: The mass of an atom is proportional to the number of p +, n, and e - the atom has subatomic mass in grams mass in amu particle proton x g amu neutron x g amu electron 9.11 x g 5.49 x amu = x g 19

20 1 amu = x g 1 amu 1/12 the mass of 12 C the mass of 1 atom of 12 C 12 amu 20

21 By definition, the mass of 1 atom of 12 C 12 amu Sum the masses of the subatomic particles of 12 C and what mass do you get? n p+ n p+ n p+ n p+ n p+ n p+ subatomic particle mass in grams mass in amu proton x g amu neutron x g amu electron 9.11 x g 5.49 x

22 The masses of the elements are determined experimentally. They cannot be accurately predicted by the masses of the constituent parts 22

23 The greater the mass defect, the more stable an isotope of an element is 23

24 Average atomic masses Most elements in nature are mixtures of more than 1 isotope Average atomic mass incorporates the relative abundance of each isotope in nature n p+ n n p+ n n p+ n p+ n p+ p+ n p+ n p+ n p+ n p+ n p+ n p+ n p+ n n p+ p+ n p+ n n n p+ n p+ 6 protons 6 neutrons 6 protons 7 neutrons 6 protons 8 neutrons C C C % % Trace amount (<0.001%) (stable) (stable) (radioactive) 24

25 Average atomic mass of C found in nature n p+ n n p+ n n p+ n p+ n p+ p+ n p+ n p+ n p+ n p+ n p+ p+ p+ n p+ n n n p+ n p+ n n n p+ n p+ 6 protons 6 protons 6 protons 6 neutrons 7 neutrons 8 neutrons 12 C % 12 amu 13 C % amu 14 C Trace amount (<0.001%) 25

26 Naturally occurring bromine is composed of two isotopes which have atomic masses of and amu. Isotope 1 is present at an abundance of 50.69% while isotope 2 is present at an abundance of 49.31%. Use this information to determine the average atomic mass of naturally occurring bromine 26

27 The atomic weights provided in a Periodic Table are the average atomic weights of the naturally occurring elements 27

28 Formula mass Atoms have atomic masses (atomic weights) Compounds have formula masses (formula weights) To calculate l formula mass, sum the atomic masses of all atoms in the molecule l 28

29 Molecular mass Recall that the formulas for ionic compounds are always written as empirical formulas because there is no discrete molecular formula The masses of molecular compounds with defined formulas may be referred to as either formula masses or molecular masses (molecular weights) The masses of empirical compounds are always referred to as formula masses (formula weights) NaCl, FW = C 6 H 6, FW = C 2 H 2, FW =

30 What is the formula weight of ammonium sulfate? 1 H (NH 4 ) 2 SO 4 6 N O S

31 Determine the molecular mass of ethanol, C 2 H 6 O 1 H C O A.About 29 amu B.About 40 amu C.About 58 amu D.About 46 amu 31

32 Percent composition: the percentage, by mass, contributed by each element in a substance ( )( ) Atoms of Element AW % Element e = 100 FW of Compound 32

33 What are the mass percentages of each element in ammonium sulfate? ( Atoms of Element)( AW) % Element = 100 FW of Compound (NH 4 4) 2 SO N FW = O S H

34 How do we bridge the microscopic world of amu with the macroscopic world of grams? 34

35 This week in chemistry Chapters 1 and 2 graded quizzes,chapter 3 graded quiz available Wednesday Recitations meet, graded, bring iclickers. Grading: two points participation, two points maximum for correct answers. I wrote this week s questions Office hours: Mon. and Wed., 2:30-4 PM, other times by appt. Supplemental instruction: M, T, W, R Read materials for chapters 3 and 4- overheads and text work problems in chapters 1-3 self tests, watch self test tutorials to help in developing problem-solving skills. Work old exam problems. 35

36 The mole (mol) The S.I unit of amount of a substance A collection of a very large number of particles The amount of a substance that contains as many objects (atoms, molecules, or other particles) as the number of atoms in exactly 12 grams of the 12 C isotope 36

37 1 mol = x particles Avogadro s number 602,200,000,000,000,000,000,000 How big a quantity is the mol???? 37

38 One 12 C atom weighs 12 amu 602x C atoms weigh 12 grams One mol of 12 C atoms weigh 12 grams 38

39 Molar mass: the mass, in grams, of 1 mol (6.02 x particles) of a substance The more a particular particle weighs, the higher it s molar mass One mol of pennies has more mass than one mol of carbon atoms 39

40 1 molecule of H 2O: 1 dozen molecules of H 2O: 1 mole oeof H 2 2O molecules: oecues mass = 18.0 amu = 2.99 x mass = 216 amu = 3.59 x mass = 1.08 x amu = g g 18.0 g Mass = 18.0 amu/molecule Mass = 18.0 g/mole 40

41 Review some terms Atomic mass: mass of a single atom of a single isotope amu Average atomic mass: average mass of a collection of isotopes (as found in nature) amu Formula mass: the sum of the average atomic masses of the elements in a compound amu Molecular mass: the same as formula weight, applied to covalent (molecular) compounds amu Molar mass: the mass of a mol of particles g/mol 41

42 Interconversion of mass, mols, and number of particles Molar Avogadro s mass mass moles number molecules 42

43 Using molar mass and Avogadro s number as conversion factors g C atoms g H 2 O molecules 1 mol C atoms 1 mol H 2 O molecules 1 mol C atoms g C atoms 1molCatoms 6.02 x C atoms g C atoms 6.02 x C atoms 1molHO 2 molecules g H 2 O molecules 1 mol H 2 O molecules 6.02 x H 2 O molecules 6.02 x H 2 O molecules 602x Catoms 1 mol H 2O molecules g C atoms 43

44 An experiment calls for 0.36 mol of Al. What mass of Al is needed?? amu or g/mol Al

45 How many moles of silicon correspond to 454 g Si?? amu or g/mol Si

46 You have a graduated cylinder containing 25.4 ml Hg. How many moles of Hg are in the cylinder? How many atoms of Hg are in the cylinder? d Hg = g/ml 80 Hg

47 Stoichiometric relations of atoms in a molecule Sucrose is C 12 H 22 O 11 1 molecule of sucrose contains 12 atoms of C 11molecule l of sucrose contains 22 atoms of fh 1 molecule of sucrose contains 11 atoms of O 1 mol of sucrose contains 12 mols of C 11 mol of sucrose contains 22 mols of H 1 mol of sucrose contains 11 mols of O Wit Write these relations as conversion factors 47

48 Which conversion factor is incorrect for sucrose, C 12 H 22 O 11 : A. 12 atoms C B. 1moleculesucrose 1moleculesucrose l 22 atoms H C. 1moleculesucrose 22 mol H D. 1mol sucrose 11mol O 48

49 Interconversion of mass, mols, and number of particles Molar Stoich. Avog. mass relation # mass moles a moles b molecules l 49

50 How many C atoms are present in 46.0 grams of sucrose, C 12 H 22 O 11? 1 6 H C O

51 You have 23.6 grams of ethanol, C 2 H 6 O. How many moles of ethanol do you have? How many molecules of ethanol do you have? How many atoms of carbon do you have? What is the mass of one molecule of ethanol in grams? 51

52 You have 23.6 grams of ethanol, C 2 H 6 O. How many moles of ethanol do you have? Molar Stoich. Avog. mass relation # molecules mass moles a moles b or atoms How many molecules of ethanol do you have? How many atoms of carbon do you have? What is the mass of one molecule of ethanol in grams? 52

53 Problem solving strategy for stoichiometric calculations Molar Stoich. Avog. mass relation # molecules mass moles a moles b or atoms Stoich. relation molecules or atoms Molar mass is a conversion factor (g/mol) For a compound, the relationship between atoms and molecules is a conversion factor For a compound, the relation between numbers of mols is a conversion factor Avogradro s number can be used as a converion foactor to convert mols and molecules (or atoms) 53

54 Example to illustrate problem solving strategy for stoichiometric calculations By just observing this bike, no conversion factors can be deduced equating the mass of the bike, the bike frame, the wheels and pedals By just observing the bike, we can deduce conversion factors relating the numbers of the components of the bike Scale these relationships up to a collection of parts equal to a mol 54

55 Empirical formula from experimental analysis 1. Base calculation on 100 grams of compound. 2. Determine moles of each element in 100 grams of compound. 3. Divide each value of moles by the smallest of the values. 4. Multiply each number by an integer to obtain all whole numbers. 55

56 The compound hydrazine (FW = 32) consists of fn and dh. The mass percentages of fn and H in hydrazine are 87.42% and 12.58%, respectively. el What is the empirical formula for hydrazine? What is the molecular formula for hydrazine? 56

57 The compound hydrazine (FW = 32) consists of N and H. The mass percentages of N and H in hydrazine are 87.42% and 12.58%, respectively. What is the empirical formula for hydrazine? What is the molecular formula for hydrazine? 1. Base calculation on 100 grams of compound. 2. Determine moles of each element in 100 grams of compound. 3. Divide each value of moles by the smallest of the values. 4. Multiply each number by an integer to obtain all whole numbers. 57

58 What is the molecular formula for hydrazine? Determine true molar mass of hydrazine experimentally Compare true molar mass to the molar mass based on empirical formula The true molar mass will be the empirical mass multiplied by some number n Multiply n by the empirical formula to get the molecular formula 58

59 Mothballs are made of napthalene, a C and H containing compound. The mass % of C in naphthalene is 93.71%, while the mass % of H is 6.29%. Naphthalene has a true molar mass of 128 amu. What is the empirical formula for naphthalene? What is the molecular formula for naphthalene? 59

60 Mothballs are made of napthalene, a C and H containing compound. The mass % of C in naphthalene is 93.71%, while the mass % of H is 6.29%. Naphthalene has a true molar mass of 128 amu. What is the empirical formula for naphthalene? What is the molecular formula for naphthalene? 60

61 Quantitative information from balanced chemical equations 2H 2 + O 2 2H 2 O

62 Quantitative information from balanced chemical equations 2H 2 + O 2 2H 2 O Stoichiometrically equivalent quantities are used as conversion factors Molecules relate to molecules as mols relate to mols How many mols of H 2 O are produced from the reaction of excess H 2 with 3.5 mols of O 2? 62

63 A camping propane cylinder contains one pound of propane (C 3 H 8 ). What mass of CO 2 is produced from the complete combustion of all the propane in the cylinder? 1 lb = 454 g Write a chemical equation for this reaction Balance the equation Use the proper stoichiometric relationships from the balanced equation Molar mass Stoich. relation Molar mass mass a moles a moles b mass b 63

64 A camping propane cylinder contains one pound of propane (C 3 H 8 ). What mass of CO 2 is produced from the complete combustion of all the propane in the cylinder? Molar mass Stoich. relation 1lb= 454g Molar mass mass a moles a moles b mass b 64

65 A camping propane cylinder contains one pound of propane (C 3 H 8 ). What mass of CO 2 is produced from the complete combustion of all the propane in the cylinder? Molar mass Stoich. relation 1lb= 454g Molar mass mass a moles a moles b mass b 65

66 Limiting reactants 2H 2 + O 2 2H 2 O No limiting reactant 66

67 Limiting reactants 2H 2 + O 2 2H 2 O No limiting reactant 67

68 Limiting reactants 2H 2 + O 2 2H 2 O O 2 is the limiting reactant (H 2 is in excess) 68

69 Limiting reactants 2H 2 + O 2 2H 2 O H 2 is the limiting reactant (O 2 is in excess) 69

70 Silicon is used in computer chips and solar cells. The final step in the manufacturing of silicon metal is SiCl 4(l) +2M 2Mg (s) Si (s) +2M 2MgCl 2(s) 225 g of SiCl 4 is mixed with 225 g Mg. Is one reagent present in a limiting amount, and if so, which one? What t mass of Si(s) is formed if the reaction goes to completion? What mass of the excess reagent is left over after the reaction goes to completion? Molar mass Stoich. relation Molar mass mass a moles a moles b mass b 70

71 SiCl 4(l) + 2Mg (s) Si (s) + 2MgCl 2(s) 225 g of SiCl 4 is mixed with 225 g Mg. Molar mass Is one reagent present in a limiting iti amount, and if so, which h one? What mass of Si(s) is formed if the reaction goes to completion? What mass of the excess reagent is left over after the reaction goes to completion? Stoich. relation Molar mass mass a moles a moles b mass b 71

72 SiCl 4(l) + 2Mg (s) Si (s) + 2MgCl 2(s) 225 g of SiCl 4 is mixed with 225 g Mg. Molar mass Is one reagent present in a limiting iti amount, and if so, which h one? What mass of Si(s) is formed if the reaction goes to completion? What mass of the excess reagent is left over after the reaction goes to completion? Stoich. relation Molar mass mass a moles a moles b mass b 72

73 Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO (g) + 2H 2(g) CH 3 OH (l) 365gofCOaremixedwith650gH 65.0 g 2. Which is the limiting reagent? What is the maximal mass of methanol that can be formed?? What mass of the excess reagent is left over after the reaction goes to completion? 73

74 Methanol can be synthesized from carbon monoxide and dihydrogen (a combination reaction): CO (g) + 2H 2(g) CH 3 OH (l) 365gofCOaremixedwith650gH 65.0 g 2. Which is the limiting reagent? What is the maximal mass of methanol that can be formed?? What mass of the excess reagent is left over after the reaction goes to completion? 74

75 Theoretical vs. actual yield Theoretical yield is the quantity of product calculated to form when all of the limiting reagent reacts The actual yield is the amount of product actually obtained experimentally Actual yield theoretical ti yield actual yield %ild %yield = x100 theoretical yield 75

76 Theoretical vs. actual yield actual yield %yield = x 100 theoreticalti yield ild 76