Thermochemical Equations and Stoichiometry

Introduction

• Discuss two ways to write thermochemical equations:
  1. Include energy in the equation.
  2. Write the equation and then indicate the change in enthalpy (ΔH) to the side.

Thermochemical Equations

• General format:
  • Exothermic Reaction: Reactants → Products + Heat (ΔH is negative)
  • Endothermic Reaction: Reactants + Heat → Products (ΔH is positive)

Example 1: Exothermic Reaction

• 2Al (s) + 3/2 O₂ (g) → Al₂O₃ (s) + 1776 kJ/mol
  • Heat written on product side.
  • ΔH is negative (exothermic).

Example 2: Endothermic Reaction

• 2Al (s) + 3/2 O₂ (g) → Al₂O₃ (s)
  • ΔH = -1776 kJ/mol
  • Heat written separately with ΔH indicating the enthalpy change.

Explanation for Fractional Coefficients

• Equation of aluminum oxidation:

  2.Al (s) + 3/2.O₂ (g) → Al₂O₃ (s)
  • ΔH = -1776 kJ/mol (per mole of reaction)
  • Coefficients reflect per mole of Al₂O₃ produced.

Balancing for Standard Enthalpy Change

• For standard balanced equation without fractions:
  • 4Al + 3O₂ → 2Al₂O₃
  • ΔH = -3552 kJ (double of -1776 kJ)
  • Each coefficient must be divided by 2 for 1 mole Al₂O₃.

Stoichiometry with Thermochemical Equations

Example 1: Heat Produced by Methane Combustion

• Given: 3.30 mol of methane (CH₄)
  • Thermochemical equation: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -891 kJ/mol
  • Calculation: – (3.30 mol CH₄) × (-891 kJ/mol) – Result: -2940 kJ (energy for 3.30 mol of CH₄)

Example 2: Heat Produced by Oxygen Reaction

• Given: 54.00 g O₂
  • Conversion to moles: – (54.00 g O₂) / (32.0 g/mol) = 1.6875 mol O₂
  • Calculation: – (1.6875 mol O₂ / 2 mol O₂) × (-891 kJ) – Result: -750.2 kJ

Example 3: Mass of Water Produced

• Given: Enthalpy change of -423 kJ
  • Thermochemical equation: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -891 kJ/mol
  • Calculation:
    • (-423 kJ) × (2 mol H₂O / -891 kJ) × (18.02 g H₂O / 1 mol H₂O)
    • Result: 17.1 g H₂O

Example 4: Enthalpy for Doubled Reaction

• Given: 2CH₄ + 4 O₂ → 2CO₂ + 4 H₂O
  • ΔH for reaction is doubled: 2 × -891 kJ
  • Result: -1782 kJ

Conclusion: Understanding and utilizing thermochemical equations and stoichiometry helps in calculating energy changes and product formation in chemical reactions.