Chapter 12 Stoichiometry

The Arithmetic of Equations 12.1

How do chemists use balanced chemical equations? Using Equations A balanced chemical equation tells you the amounts of reactants to mix and what amount of product to expect Key Question How do chemists use balanced chemical equations? Chemists use balanced chemical equations as a basis to calculate how much reactant is needed or how much product will be formed in a reaction.

The coefficients in a balanced equation indicate both the relative numbers of molecules (or formula units) in the reaction and the relative numbers of moles.

The calculation of quantities in chemical reactions is a subject of chemistry called stoichiometry Calculations using balanced equations are called stoichiometric calculations Chemists can track reactants and products in a reaction by stoichiometry It allows chemists to tally the amounts of reactants and products using ratios of moles or representative particles derived from chemical equations

Chemical Equations Key Question In terms of what quantities can you interpret a balanced chemical equation? In terms of different quantities, including numbers of atoms, molecules, or moles; mass; and volume.

REMEMBER Representative particles use Avogadro’s number Moles to mass use molar mass Moles to volume (at STP) use molar volume Can relate molar mass to molar volume to calculate density

Chemical Calculations 12.2

Writing and Using Mole Ratios Mole ratio or (MOLE METHOD)is a conversion factor derived from the coefficients of a balanced chemical equation interpreted in terms of moles Key Question How are mole rations used in chemical calculations? Mole ratios are used to convert between a given number of moles of a reactant or product to moles of a different reactant or product.

Interconverting Procedure g  m  m  g g  m  m  rp L  m  m  L

Other Stoichiometric Calculations From mole ratios you can calculate any measurement unit that is related to the mole The given quantity can be expressed in numbers of representative particles, units of mass, or volumes of gases at STP Key Question What is the general procedure for solving a stoichiometric problem? The given quantity is first converted to moles Use mole method to calculate the moles of the wanted substance Moles of wanted substance is converted to any other unit desired

Limiting Reagent and Percent Yield 12.3

Limiting and Excess Reagents Key Question How is the amount of product in a reaction affected by an insufficient quantity of any of the reactants? In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of product that forms.

Limiting reagent is the reactant that determines the amount of product that can be formed by a reaction The reactant that is not completely used up in a reaction is called the excess reagent May be called limiting reactant and excess reactant!

There are no restrictions on the starting amounts of the reactants in any reaction Many reactions are carried out using an excess of one reagent The quantities of reactants consumed and the quantities of the products formed, are restricted by the quantity of the limiting reactant Example: The car stops because you run out of the limiting reactant in the combustion reaction, the fuel.

Percent Yield Key Question What does the percent yield of a reaction measure? The efficiency of a reaction carries out in the laboratory

Theoretical yield is the maximum amount of product that could be formed from given amounts of reactants The amount of product that actually forms when the reaction is carried out in the laboratory is called the actual yield Percent yield is the ratio of the actual yield to the theoretical yield expressed as a percent

There are many reasons for this difference: The actual yield is almost always less than (and can never be greater than the theoretical yield There are many reasons for this difference: Part of the reactants may not react The reactants may react in a different way than that desired (side reactions) It is not always possible to recover all of the product from the reaction mixture

Percent yield = actual yield x 100% theoretical yield

Practice: Percent Yield Page 406 Numbers 30 and 31