Stoichiometry and Chemical Reactions

Writing Balanced Chemical Equations

Balanced chemical equations are essential for representing chemical reactions accurately. They show the reactants and products, as well as the correct stoichiometric coefficients to satisfy the law of conservation of mass.

• Balanced Equation: An equation in which the number of atoms for each element is the same on both sides of the reaction arrow.

• Example: The rusting of iron can be represented as:

• Fermentation of Sugar: The fermentation of glucose by yeast produces ethanol and carbon dioxide:

• Combustion of Ethanol: Combustion reactions involve a substance reacting with oxygen to produce oxides and energy. For ethanol:

• Reaction of Metals with Nonmetals: For example, the reaction between iron and chlorine gas:

Stoichiometry: Mole-to-Mole Relationships

Stoichiometry involves the calculation of reactants and products in chemical reactions using balanced equations. It allows chemists to predict the amounts of substances consumed and produced.

• Mole Ratio: The coefficients in a balanced equation indicate the ratio in which substances react and are produced.

• Example: For the reaction , the mole ratio between and is 3:4.

• Calculating Moles of Product: Use the mole ratio to convert moles of reactant to moles of product.

  • Given: 2.6 mol Find: Moles of

Mass-to-Mole and Mass-to-Mass Calculations

Stoichiometry also allows conversion between mass and moles using molar mass.

• Molar Mass: The mass of one mole of a substance, usually in grams per mole (g/mol).

• Example: To find the mass of produced from a given mass of , first convert grams to moles, use the mole ratio, then convert back to grams if needed.

Limiting Reactant and Theoretical Yield

In reactions with more than one reactant, the limiting reactant is the one that is completely consumed first, thus limiting the amount of product formed.

• Limiting Reactant: The reactant that determines the maximum amount of product that can be formed.

• Theoretical Yield: The maximum amount of product that can be produced from the limiting reactant.

• Example: For the reaction , if you have 16 g of Al and 10 mol of , calculate which is the limiting reactant by converting both to moles and comparing the required stoichiometric amounts.

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine to form an insoluble solid, called a precipitate.

• Example: When is added to a solution containing , precipitates: Note: The actual reaction with chromate is:

• Steps to Solve:

  1. Write the balanced equation.

  2. Calculate moles of each reactant.

  3. Determine the limiting reactant.

  4. Calculate the theoretical yield of the precipitate.

Sample Table: Limiting Reactant Comparison

The following table summarizes how to determine the limiting reactant in a reaction:

Additional info: Actual limiting reactant is determined by dividing the available moles by the stoichiometric coefficient and comparing values; the smallest value indicates the limiting reactant.

Summary of Key Terms

• Balanced Equation: Chemical equation with equal numbers of each atom on both sides.

• Stoichiometry: The calculation of reactants and products in chemical reactions.

• Limiting Reactant: The reactant that is completely consumed first in a reaction.

• Theoretical Yield: The maximum amount of product possible from given reactants.

• Precipitate: An insoluble solid formed from a reaction in solution.