Chapter 9.3 : Limiting Reactants and Percent Yield
- 1. Limiting reactants and percent yieldChapter 9.3
- 2. Objectives:Describe a method for determining which of two reactant is a limiting reactant.Calculate the amount in moles or mass in grams of a product, given the amounts in moles or masses in grams of two reactants, one of which is in excess.Distinguish between theoretical yield, actual yield, and percent yield. Calculate percent yield, given the actual yield and quantity of a reactant.
- 3. Limiting ReactantDefined as: reactant that limits the amounts of the other reactants that can combine and the amount of product that can form in a chemical reaction.Also referred to as limiting reagentRelate to airplane flightExcess reactant : Substance not used up completely in chemical reaction50 passengers in excess400 passengers350 seatsLimitingExcess
- 4. In Chemical ReactionsC(s) + O2(g) CO2(g)If you start with:5 mol10 mol5 mol5 carbon atoms10 oxygen moleculesUsed up first!!5 excess oxygen moleculesLimiting reactant
- 5. Before-During-After charts(B-D-A)C(s) + O2(g) CO2(g)Before (B)5 mol10 mol1During (D)5 mol5 mol15 mol0 molAfter (A)Excess ReactantLimiting ReactantStep 1:Use moles that are present before the reaction startsStep 3:Subtract to determine limiting reactant and excess.Step 2:Use coefficients to determine moles used during reactionIf negative number, start over!
- 6. Sample Problem 1Silicon dioxide (quartz) is usually quite unreactive but reacts readily with hydrogen fluoride according to the equation below. If 2.0 mol of HF are exposed to 4.5 mol of SiO2, which is the limiting reactant?SiO2(s) + 4HF(g) SiF4(g) + 2H2O(l)massmoles2.0 mol4.5 molB4.5 mol2.0 mol41Limiting Reactantis HF4.5 mol18.0 molD2.0 mol0.50 mol41A4.0 mol0 mol-16.o mol0 molStart over!!
- 7. Sample Problem 2The black oxide of iron, Fe3O4, occurs in nature as the mineral magnetite. This substance can also be made in the laboratory by the reaction between red-hot iron and steam according to the equation below. When 36.0 g of H2O is mixed with 167 g of Fe, which is the limiting reactant?What mass in grams of black iron oxide is produced?What mass in grams of excess reactant remains when the reaction is completed?3 Fe(s) + 4H2O(l) Fe3O4(s) + 4H2(g)mass36.0 g167 g116 g÷ 55.85 g/mol÷ 18.01 g/molx231.55 g/mol12.99 mol2.00 molmoles0.500 mol4B2.99 mol2.00 molx55.85 g1 mol31.49 mol= 83.2 g FeD2.00 mol1.50 mol4Excess FeLR is H2OA0 mol1.49 molExcess Fe
- 8. Percent YieldTheoretical Yield: Maximum amount of product that can be produced from a given amount of reactant.The amount you get using stoichiometryActual Yield:
- 9. Measured amount of product obtained from a reaction.
- 10. The amount you get in the laboratory
- 11. Percent Yield:
- 12. Ratio of the actual yield to the theoretical yield multiplied by 100.Actual YieldPercent Yield=x100Theoretical Yield
- 13. Sample Problem 3Chlorobenzene, C6H5Cl, is used in the production of many important chemicals, such as aspirin, dyes, and disinfectants. One industrial method of preparing chlorobenzene is to react benzene, C6H6, with chlorine, as represented by the equation below. When 36.8 g of C6H6 react with an excess of Cl2, the actual yield of C6H5Cl is 38.8 g. What is the percent yield of C6H5Cl?C6H6(l) + Cl2(g) C6H5Cl(s) + HCl(g)mass36.8 g53.0 g÷ 78.12 g/molx112.56 g/mol1moles0.471 mol0.471 mol1Actual Yield38.8 g73.2 %Percent Yield=x100=x100=Theoretical Yield53.0 g