Chapter9 stoichiometry-100707061730-phpapp01

• 1.Chapter 9Stoichiometry

2. Introduction to StoichiometrySection 1 3. Much knowledge of chemistry based onquantitative analysis of substances inchemical reactions Composition stoichiometry dealswith mass relationships of elements incompounds Reaction stoichiometry deals withmass relationships between reactantsand products in a chemical reaction 4. Based on chemical equations and law of conservation of matter All calculations start with balanced chemical equation This gives numbers of moles of reactants and products 5. Reaction-Stoichiometry Problems Can be classified according toinformation given in problem and infoyou are expected to find, the unknown May both be reactants, may both beproducts, or both Masses usually expressed in grams Solved by using ratios to convert givenquantities by following methods: 6. Problem Type 1: Given and unknown quantities are amounts in moles General plan:Amt of given (mol) amt of unknown(mol) 7. Problem Type 2: Given is in moles and unknown is a mass in gramsAmt given (mol) amt unknown (mol) amt unknown (g) 8. Problem Type 3: Given is mass (g) and unknown is in molesMass given (g) amt given (mol) amount unknown (mol) 9. Problem Type 4: Given is mass (g) and unknown is mass (g)Mass given amt given (mol) amt unknown (mol) mass unknown 10. Mole Ratio Solving any reaction-stoichiometry problem requires use of mole ratio to convert from moles to grams Mole ratio conversion factor that relates amounts in moles of any two substances involved in a chemical reaction Get directly from balanced equation 11. Example: 2Al2O3(l) 4Al(s) + 3O2(g) Moles ratios: 12. 2Al2O3(l) 4Al(s) + 3O2(g) Use mole ratios to convert from amountin moles of one substance to amount inmoles of another Ex. 13.0 mol Al2O3 13. Molar Mass Mass of one mole of a substance It is the conversion factor that relatesmass of substance to amount in moles To solve stoichiometry problems, youneed to determine molar mass ofsubstances 14. Molar masses of substances: Al2O3 = 101.96 g/mol O2 = 32.00 g/mol Al = 26.98 g/mol Can use these as conversion factors 15. Find number of grams of Al equal to 26.0 mol of Al 16. Ideal Stoichiometric CalculationsSection 2 17. Ideal Stoichiometric Calculations Equations are very important becauseyou get the mole ratio directly from it First thing to solving these types ofproblems is balancing the equation Chemical equations help makepredictions about reactions withouthaving to run experiments (and wasteresources) in lab 18. Calculations in this book are theoretical They tell amounts of reactants and products under ideal conditions (where all reactants completely converted into products) Hardly ever happens in real life 19. Conversions of Quantities inMoles If asked for moles of product made from specific number of moles of reactant: 20. Plan requires 1 conversion factor Mole ratio 21. Sample Problem 22. 1. Analyze Given: Amount of CO2 = 20 mol Unknown: Amount of LiOH in moles 23. 2. Plan Amount CO2 (mol) amount LiOH (mol) Requires a mole to mole ratio between CO2 and LiOH 24. 3. Compute 25. How many moles of sodium will react with water to produce 4.0 mol of hydrogen in the following reaction? 2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g) 8.0 mol Na 26. How many moles of lithium chloride will be formed by the reaction of chlorine with 0.046 mol of lithium bromide in the following reaction?2LiBr(aq) + Cl2(g) 2LiCl(aq) + Br2(l) 0.046 mol LiCl 27. Conversions of Amounts in Moles toMass Usually asked to find mass in grams of product formed 28. Practice Problem In photosynthesis, plants use energy from the sun to produce glucose, C6H12O6, and oxygen from the reaction of carbon dioxide and water. What mass, in grams, is produced when 3.00 mol of water react with carbon dioxide? 29. 1. Analyze Given: Amount H2O = 3.00 mol Unknown: Mass of glucose produced (in g) 30. 2. Plan Chemical equation is Need 2 conversion factors: Mole ratio of CO2 to H2O Molar mass of CO2 31. 3. Compute 32. Practice Problem 1 Phosphorous burns in air to produce a phosphorous oxide in the following reaction.4P(s) + 5O2(g) P4O10(s)What mass of phosphorous will be needed to produce 3.25 mol P4O10? 403 g 33. Practice Problem 2 Hydrogen peroxide breaks down, releasing oxygen, in the following reaction.2H2O2(aq) 2H2O(l) + O2(g)What mass of oxygen is produced when 1.840 mol of H2O2 decompose? 29.44 g 34. Conversions of Mass to Amounts inMoles In this type of problem you are starting with massof some substance Plan: 35. Sample Problem 36. 1. Analyze Given: Mass of NH3 = 824 g Unknown: a. amount of NO produced (in mol) b. amount of H2O produced (in mol) 37. 2. Plan First write balanced equation 38. Mole ratios needed: 39. 3. Compute 40. Mass-Mass Calculations Since you can never measure moles directly, mass-mass calculations are more common 41. Sample Problem 42. 1. Analyze Given: Amount of HF = 30.00 g Unknown: Mass of SnF2 produced in grams 43. 2. Plan 44. 3. Compute 45. Practice Problem 1 Calculate the mass of silver bromide produced from 22.5 g of silver nitrate in the following reaction: 2AgNO3 + MgBr2 2AgBr + Mg(NO3)2 24.9 g AgBr 46. Practice Problem 2 What mass of acetylene, C2H2, will be produced from the reaction of 90. g of calcium carbide, CaC2, with water in the following reaction?CaC2(s) + 2H2O(l) C2H2(s) + Ca(OH)2(aq) 37 g 47. Limiting Reactants and PercentYieldSection 3 48. In experiments, a reaction is rarely ever done withexact amounts of reactants Usually one or more reactants is in excess (toomuch) Once one of the reactants is used up, thereaction stops Substance used up first is the limiting reactant 49. Limiting reactant the reactant that limits theamounts of the other reactants that can combine andthe amount of product that can form in a chemicalreaction Excess reactant the substance that is not used upcompletely in reaction Consider the following reactionC(s) + O2(g) CO2(g) According to equation, 1 mol C reacts with 1 moloxygen to form 1 mol carbon dioxide Supposed you could mix 5 mol C with 10 mol O2 50. Consider the following reactionC(s) + O2(g) CO2(g) According to equation, 1 mol C reacts with 1 moloxygen to form 1 mol carbon dioxide Supposed you could mix 5 mol C with 10 mol O2 51. C(s) + O2(g) CO2(g) 52. Problem Limit ReactantProblem LR :Aluminum oxidizes accordingto the following equation: 4Al + 3O2 2Al2O3Powdered Al (0.048 mol) is placed into acontainer containing 0.030 mol O2. What isthe limiting reactant?O2 53. 1. Analyze Given: Amount of HF = 2.0 mol Amount of SiO2 = 4.5 mol Unknown: Limiting reactant 54. 2. Plan The given amount of either reactant is used to calculate required amount of other reactant Calculated amount compared with amount you actually have Limiting reactant can be identified 55. Under ideal conditions, 2.0 mol HF requires 0.50mol SiO2 for complete reaction Because 4.5 mol SiO2 is available, that is morethan is required so. HF is limiting reactant 56. Practice Problem 1 Aluminum oxidizes according to the followingequation: 4Al + 3O2 2Al2O3 Powdered Al (0.048 mol) is placed into acontainer containing 0.030 mol O2. What isthe limiting reactant? O2 57. Practice Problem 2 Heating zinc sulfide in the presence of oxygen yields the following:ZnS + O2 ZnO + SO2 If 1.72 mol ZnS is heated in the presence of 3.04 mol O2, which reactant will be used up? ZnS 58. Practice Problem 3 Use the following equation for the oxidation of aluminum in the following problems:4Al + 3O2 2Al2O3 59. 4Al + 3O2 2Al2O3 Which reactant is limiting if 0.32 mol Al and0.26 mol O2 are available? Al How many moles of Al2O3 are formed from thereaction of 6.38 x 10-3 mol of O2 and 9.15 x 10-3mol of Al? 4.25 x 10-3 mol Al2O3 If 3.17 g Al and 2.55 g of O2 areavailable, which reactant is limiting? O2 60. Practice Problem 4ZrSiO4 + 2Cl2 ZrCl4 + SiO2 + O2 What mass of ZrCl4 can be produced if 862 g of ZrSiO4 and 950 g of Cl2 are available? 1.10 x 103 g 61. Percent Yield Amounts of products calculated in problems represent theoretical yields Theoretical yield maximum amount of product that can be made from a given amount of reactant In lab, amount of product is usually less than theoretical yield 62. Why? Some reactant may be used competing in sidereactions that reduce amount of product Once product is formed, usually collected inimpure form Some product lost during purification Actual yield measured amount of productgotten from reaction 63. Chemists usually interested in efficiency ofreaction Expressed by comparing actual and theoreticalyields Percent yield ratio of actual yield to theoretical yield multiplied by 100 64. Sample Problem 65. 1. Analyze Given: Mass C6H6 = 36.8 g Mass of Cl2 = excess Actual yield of C6H5Cl = 38.8 g Unknown: Percent yield of C6H5Cl 66. 2. Plan First do mass-mass calculation to find theoretical yield 67. Then percent yield can be found 68. 3. Compute 69. Practice Problem Calculate the percent yield in each of thefollowing cases T Yield = 50.0 g, A Yield = 41.9 g 83.8% T yield = 290 kg, A Yield = 270 kg 93% T Yield = 0.00192 g, A yield = 0.00089 g 46%