Chapter12 Stoichiometry

I. What is Stoichiometry? The study of quantitative relationships between amounts of reactants used and products formed by a chemical reaction Based on the law of conservation of mass – matter is neither created not destroyed

A. Stoichiometry and the balanced chemical equation The coefficients in a balanced equation represent the numbers of moles of particles The mass of any substance can be determined by multiplying the number of moles of the substance by the conversion factor that relates mass and number of moles

B. Mole Ratios Ratios between the numbers of moles of any two substances in a balanced chemical equation With mole ratios you can calculate the amounts of reactants and products if you know at least one’s mass

II. Stoichiometric Calculations A. Stoichiometric mole-to-mole conversion When solving for an unknown, the mole-to- mole ratio should include the unknown as the numerator B. Stoichiometric mole-to-mass conversion C. Stoichiometric mass-to-mass conversion

D. Steps in Stoichiometric Calculation 1. Write a balanced chemical equation 2. Determine the moles of the given substance using a mass-to-mole conversion 3. Determine the moles of the unknown substance from the moles of the given substance 4. From the moles of the unknown substance, determine the mass of the unknown substance using a mole-to-mass conversion

III. Limiting Reactants A. Why do reactions stop? Rarely are reactants in a chemical reaction present in the exact ratios specified by the balanced equation Usually, one or more reactants are in excess, while one is limited The amount of product depends upon the reactant that is limited, or limiting reactant Left-over reactants are called excess reactants

B. Calculating the Product When a Reactant is Limited If masses of reactants are given, you can convert each to moles to find which is the limiting reactant and then you can calculate the mass of product expected C. Why use an excess of a reactant? Using an excess of one reactant drives the reaction to continue until all of the limiting reactant is used up

IV. Percent Yield Some reactants never succeed in producing the predicted amount of product Many reactions stop before all of the reactants are used up, so the actual amount of product is less than expected A theoretical yield of a reaction is the maximum amount of product that can be produced from a given amount of reactant based on a balanced chemical reaction

The actual yield of a reaction is the amount of product actually produced when the chemical reaction is carried out in an experiment and its mass is measured The percent yield of product measures how efficient a chemical reaction was by determining the ratio of the actual yield to the theoretical yield Actual yield__ Percent Yield = Theoretical yield X100