Overview
This lecture explains how to find the mass of excess reactant remaining after a chemical reaction by identifying the limiting reactant, calculating reactant consumption, and determining leftovers through solved examples.
Steps to Determine Excess Reactant Remaining
- Step 1: Identify the limiting reactant by comparing moles-per-coefficient ratios of reactants.
- Step 2: Calculate the grams of excess reactant consumed using stoichiometric conversion from limiting reactant's mass.
- Step 3: Find mass remaining by subtracting grams consumed from the initial amount of excess reactant.
Example 1: Aluminum and Sulfuric Acid Reaction
- Write and balance the equation: 2Al + 3H₂SO₄ → 3H₂ + Al₂(SO₄)₃.
- Calculate moles: Aluminum (80g ÷ 26.98 g/mol = 2.965 mol); Sulfuric acid (40g ÷ 98.086 g/mol = 0.408 mol).
- Divide by coefficients: Sulfuric acid (0.408/3), Aluminum (2.965/2); the smallest value identifies the limiting reactant (H₂SO₄).
- Convert limiting reactant's grams to excess reactant's grams (7.33g Al consumed).
- Remaining aluminum: 80g - 7.33g = 72.6g.
Example 2: Aluminum and Elemental Sulfur Reaction
- Write and balance equation: 16Al + 3S₈ → 8Al₂S₃.
- Calculate moles: Aluminum (50g ÷ 26.98 g/mol = 1.853 mol), Sulfur (70g ÷ 256.56 g/mol S₈ = 0.273 mol).
- Divide by coefficients: Aluminum (1.853/16), Sulfur (0.273/3); smallest value is limiting reactant (S₈).
- Convert limiting reactant's grams to excess reactant's grams (39.26g Al consumed).
- Remaining aluminum: 50g - 39.26g = 10.74g.
Key Terms & Definitions
- Limiting Reactant — The reactant that is consumed first, determining the maximum amount of product.
- Excess Reactant — The reactant left over after the reaction is complete.
- Stoichiometry — Calculation of reactants and products in chemical reactions.
Action Items / Next Steps
- Practice solving for limiting and excess reactants with new examples.
- Review balancing chemical equations and molar mass calculations.