Stoichiometry Chemistry – Chapter 12

What is Stoichiometry? Chemical reactions, such as the combustion of a candle, stop when one of the reactants is used up Stoichiometry – study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction Stoichiometry and balanced chemical equations: Example: 4Fe + 3O2 2Fe2O3 4 atoms of Fe react w/ 3 molecules of O to produce 2 formula units of Fe2O3 4 mol of Fe react w/ 3 mol of O to produce 2 mol of Fe2O3

Mole ratio – ratio between the numbers of moles of any 2 substances in a balanced chemical equation Example: 2Al + 3Br2 2AlBr3 (See p. 356 for mole ratios)

Stoichiometric Calculations Steps in Stoichiometric Calculations: Write a balanced chemical equation Determine the moles of the given substance using a mass-to- mole conversion Determine the moles of the unknown substance from the moles of the given substance From the moles of the unknown substance, determine the mass of the unknown substance using a mole-to-mass conversion

Limiting Reactants Why Reactions Stop One or more reactants are in excess and the reaction proceeds until all of one reactant is used up Amount of product depends upon the reactant that is limited Limiting reactant – limits the extent of the reaction and determines the amount of product Excess reactants – the other reactants that remain after the reaction stops

Calculating the Product When a Reactant is Limited Determine which is the limiting reactant Find number of moles of each reactant Determine if the reactants are in the correct mole ratio as given in the balanced equation After the limiting reactant has been determined, the amount of product in moles can be calculated by multiplying the given number of moles of the limiting reactant by the mole ratio that relates the product and limiting reactant Calculate the mass of the product by multiplying by the molar mass of the product

Calculating the Excess Reactant Determine amount that actually reacted by calculating the mass of the non-limiting reactant needed to react completely with the number of moles of the limiting reactant using a mole-to-mass calculation Multiply the moles of the limiting reactant by the mole ratio of the reactants to obtain the number of moles of the non- limiting reactant. Obtain the mass of the non-limiting reactant Some reactions stop while portions of the reactants are still present in the reaction mixture. B/c this is inefficient, excess of one reactant is used (usually the least-expensive).

Percent Yield Many reactions stop before all of the reactants are used up, so the actual amount of product is less than expected Theoretical yield – maximum amount of product that can be produced form a given amount of reactant A reaction rarely produces this Actual yield – amount of product actually produced when the chemical reaction is carried out in an experiment Percent yield – ratio of the actual yield to the theoretical yield (expressed as %) Catalyst – substance that speeds a reaction but does not appear in the chemical equation