Chapter 11: Stoichiometry

Particle and Mole Relationships Chemical reactions stop when one of the reactants is used up Stoichiometry: the study of quantitative relationships between the amounts of reactants used and amounts of products formed by a chemical reaction The amount of each reactant present at the start of a chemical reaction determines how much product can form Based on the law of conservation of mass Matter is neither created or destroyed in a chemical reaction Therefore, mass of reactants = mass of products

Evidence of the Law of Conservation of Mass

Mole Ratio Mole Ratio: ratio between the numbers of moles of any two substances in a balanced chemical reaction Can be used as a conversion factor in calculations The number of mole ratios that can be written for any equation is (n)(n-1) where n is the number of species in the chemical reaction The coefficients in a chemical equation indicate the relationship between mole of reactants and products

Mole Ratio ____K(s) + ____Br2 (l)  ____KBr(s) Find the ratio between bromine and potassium bromide How many mole ratios can be written for this reaction?

Stoichiometry Calculations ____Al(s) + ____O2 (g)  ____Al2O3 How many grams of Al are needed to completely react with 2.4 mol of O2?

Key points to remember: Must have a balanced reaction Coefficients represent moles Only go to the reaction once for a conversion

____VO + ____Fe2O3  ____FeO + ____V2O5 How many grams of Fe2O3 are needed to produce 90.5 g of FeO?

Limiting Reactant Reactions proceed until one of the reactants is used up and one is left in excess Limiting Reactant: limits the extent of the reaction and thereby, determine the amount of product formed Excess Reactant: all the leftover, unused reactants **Determining the limiting reactant is important because the amount of the product formed depends on this reactant

Determining the Limiting Reactant Method #1 Calculate the final mass of a product from both reactant quantities and determine which is the limiting reactant based on the least amount of product formed Solve any additional questions using the limiting reactant quantity as the starting point Method #2 Calculate the number of moles for each reactant Divide moles of reactant by the coefficient of the balanced equation. The substance with the smaller answer is the limiting reactant

___SO2 + ___CaCO3 + ___O2  ___CaSO4 + ___CO2 What is the limiting reactant when 95.0 g of SO2 and 100 g of CaCO3 are added to excess O2? How many g of CO2 are formed in the reaction? How many grams of excess remain?

Percent Yield Percent Yield = x 100 Theoretical Yield: the maximum amount of product that can be produced from a given amount of reactant Actual Yield: the amount of product produced when the chemical reaction is carried out in an experiment Percent Yield: the ratio of the actual yield to the theoretical yield expressed as a percent Actual yield Theoretical yield Percent Yield = x 100

Calculating Percent Yield You calculate the theoretical yield of a chemical reaction starting with 50.0g of reactant is 25.0g of product. What is the percent yield if the actual yield is 22.0g of product?

Why a percent is less than 100% Reactions do not always go to completion Impure reactants Some product can be lost in the recovery process

Percent Yield ____KClO3  ____KCl + ____O2 A 95.8 g sample of KClO3 is heated in the lab and 20.1 g of KCl is recovered. What is the % yield of this process?