CH 4 (10/9) (PG 1-5)

Chemistry and Chemical Reactivity: Stoichiometry

Definition of Stoichiometry

  • Stoichiometry: The study of amount relationships in chemical reactions.

    • Provides quantitative information about chemical reactions.

    • Based on the law of conservation of matter: Mass is conserved in chemical reactions.

    • Formula: Total mass of reactants = Total mass of products.

Balancing Chemical Equations

  • Chemical equations must be balanced for mass:

    • Condition: Numbers of atoms on the reactant side must equal the numbers on the product side.

Stoichiometric Coefficients

  • Stoichiometric coefficients: Numbers in front of chemical formulas or symbols.

    • Indicate the number of atoms, molecules, and ions involved in a reaction.

    • Provide information about moles of pure substances (or species).

    • Example Equation: CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(l)

    • Interpreted as:

      • 1 molecule of CH₄ reacts with 2 molecules of O₂ to yield 1 molecule of CO₂ and 2 molecules of H₂O.

    • Equivalence in moles: 1 mole of CH₄ reacts with 2 moles of O₂ yielding 1 mole of CO₂ and 2 moles of H₂O.

    • Inference from the above: One stoichiometric coefficient multiplied by Avogadro’s number gives moles.

Steps to Solve Stoichiometry Problems

  1. Mass (Reactant) → # of Moles (Reactant) → # of Moles (Product) → Mass (Product).

  2. Use:

    • Molar Mass of Reactant

    • Molar Ratio of Product to Reactant

    • Molar Mass of Product

  3. A balanced chemical equation is needed to determine molar ratios of substances involved in the reaction.

Mole/Mass Relationships

  • Mass cannot directly relate in reactions without converting through moles unless the law of conservation applies.

Reactions Involving a Limiting Reactant

  • Insufficient amounts of reactants cause a situation where:

    • The limiting reactant is the one that limits the quantity of product that can be formed.

Limiting Reactants: Nuts and Bolts Analogy

  • Example: You have 32 nuts and 28 bolts.

    • If a set has 1 nut and 1 bolt, then:

    • 28 sets can be made (as bolts are limiting).

    • If a set has 2 nuts and 1 bolt, then:

    • 16 sets can be made (as nuts are limiting).

Reaction Yields

  • Theoretical yield: The maximum possible product yield based on reactant masses and stoichiometry.

  • Actual yield: The experimentally measured quantity of products resulted upon completion of the reaction.

  • Percent yield: Ratio of actual yield to theoretical yield expressed as a percentage.

    • Formula: (actual yield (in grams or moles))/(theoretical yield (in grams or moles)) x 100%