Chemical Quantities
Section 1: Information Given by Chemical Equations
- A balanced chemical equation gives relative numbers (or moles) of reactant and product molecules that participate in a chemical reaction.
- The coefficients of a balanced equation give the relative numbers of molecules.
- Ex. C2H5OH + 3O2 → 2CO2 + 3H2O
- The equation is balanced.
- All atoms present in the reactants are accounted for in the products.
- 1 molecule of ethanol reacts with 3 molecules of oxygen to produce 2 molecules of carbon dioxide and 3 molecules of water.
- 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water.
Section 2: Mole-Mole Relationships
- A balanced equation can predict the moles of product that a given number of moles of reactants will yield.
- 2 mol of H2O yields 2 mol of H2 and 1 mol of O2.
- 4 mol of H2O yields 4 mol of H2 and 2 mol of O2.
- The mole ratio allows us to convert from moles of one substance in a balanced equation to moles of a second substance in the equation.
Section 3: Mass Calculations
- Stoichiometry is the process of using a balanced chemical equation to determine the relative masses of reactants and products involved in a reaction.
Steps for Calculating the Masses of Reactants and Products in Chemical Reactions
- Balance the equation for the reaction.
- Convert the masses of reactants or products to moles.
- Use the balanced equation to set up the appropriate mole ratio(s).
- Use the mole ratio(s) to calculate the number of moles of the desired reactant or product.
- Convert from moles back to masses.
Section 4: The Concept of Limiting Reactants
- Contains the relative amounts of reactants that matches the numbers in the balanced equation.
Section 5: Calculations Involving a Limiting Reactant
Steps for Solving Stoichiometry Problems Involving Limiting Reactants
- Write and balance the equation for the reaction.
- Convert known masses of reactants to moles.
- Using the numbers of moles of reactants and the appropriate mole ratios, determine which reactant is limiting.
- Using the amount of the limiting reactant and the appropriate mole ratios, compute the number of moles of the desired product.
- Convert from moles of product to grams of product, using the molar mass (if this is required by the problem).
Section 6: Percent Yield
- theoretical yield: the maximum amount of a given product that can be formed when the limiting reactant is completely consumed
- The actual yield (amount actually produced) of a reaction is usually less than the maximum expected (theoretical yield).