JP

Chemistry Lecture on Stoichiometry and Limiting Reactants

Empirical vs. Molecular Formulas

  • Empirical formulas are considered less useful by themselves but serve as a foundation to derive molecular formulas.
  • Molecular formulas can be determined from percent composition.

Chemical Equations and Balancing

  • Importance of balancing chemical equations:
    • Allows prediction of how much reactant B will react with reactant A.
    • Essential for stoichiometry, which involves calculations based on balanced equations.

Stoichiometry

  • Pull coefficients from balanced equation to establish stoichiometric ratios.
  • Mole conversions:
    • When switching between substances (e.g., A and B, A and C, C and B, etc.), conversion through moles is necessary.
    • Regardless of the measurement (grams, liters, milliliters), conversion must start from the mole.
  • Conversion steps:
    1. If given mass (grams, kilograms, micrograms), convert to grams.
    2. Use the periodic table to convert grams to moles.
    3. Use the stoichiometric ratios from the balanced equation to make calculations.

Example: Combustion of Propane (C₃H₈)

  • Complete combustion of hydrocarbons requires oxygen, producing carbon dioxide (CO₂) and water (H₂O).
  • The balanced equation for propane:
    • C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
  • Required to balance before solving stoichiometry problems:
    • Count carbons, hydrogens, and oxygens in the products and reactants.

Problem-Solving Approaches

  • Conversion of grams of propane to moles:

    1. Calculate molar mass of propane (C₃ = 123 + H₈ = 18 = 44.1 g/mol).
    2. Use the given grams (13.42 g of propane) to find moles:
      ext{Moles of propane} = rac{13.42g}{44.1g/mol}.

  • Determine moles of CO₂ produced using the stoichiometric ratio (3 moles CO₂ produced per 1 mole of propane).

  • Final calculation includes converting moles of CO₂ back to grams for the result.

Limiting Reactants

  • Defined as the reactant that limits the amount of product formed in a chemical reaction.
  • Example analogy using hot dogs and buns:
    • If having 6 hot dogs and 20 buns, the limiting factor is the hot dogs (6 servings possible).
  • The theoretical yield is the maximum amount of product that can be formed from the limiting reactant.
  • Steps to identify the limiting reactant:
    1. Calculate moles of both reactants based