Notes on Thermochemistry and Thermochemical Equations

Thermochemistry

Thermochemical Equations

  • Definition: A thermochemical equation is a chemical equation that includes the energy associated with a chemical reaction.
    • Key Point: Every reaction has an energy change associated with it.

Learning Objectives

  • Define a thermochemical equation and utilize its information to calculate energy changes in a reaction.
  • Explain what ΔH (delta H) represents in a reaction and identify methods to find it.

Key Concepts

Enthalpy (H)
  • Definition: Enthalpy is the heat content of a substance measured at constant pressure.
  • Heat of Reaction: Refers to the heat absorbed or released during a chemical reaction.
  • ΔH: Represents the change in enthalpy during a reaction.
Reaction Types
Exothermic Reactions
  • Characteristics: Release energy to the surroundings.
  • Equation Format:
    1. CH₄(g) + 2O₂(g) ⟶ CO₂(g) + 2H₂O(g) ΔH = -890.4 kJ (Energy produced as a product)
    2. CH₄(g) + 2O₂(g) ⟶ CO₂(g) + 2H₂O(g) + 890.4 kJ (Shows energy as a product)
  • Sign of ΔH: Negative (indicates energy is released).
Endothermic Reactions
  • Characteristics: Absorb energy from the surroundings.
  • Equation Format:
    1. C(s) + 2S(s) ⟶ CS₂(l) ΔH = +89.3 kJ (Energy required as a reactant)
    2. C(s) + 2S(s) + 89.3 kJ ⟶ CS₂(l) (Shows energy as a reactant)
  • Sign of ΔH: Positive (indicates energy is absorbed).

Heat of Reaction (ΔH)

  • It reflects the amount of energy involved when the coefficients of the reactants/products equal the number of moles in the reaction.
  • Conversion Factor: Coefficient of any substance corresponds to ΔH.
Example Calculations
Example 1: Energy Required
  • Problem: How much energy is needed when 1.27 moles of sulfur react with excess carbon?
    • Equation: C(s) + 2S(s) ⟶ CS₂(l) ΔH = +89.3 kJ
    • Calculation:
    • 1.27 mol S x (89.3 kJ / 2 mol S) = 56.7 kJ
Example 2: Moles of Water Produced
  • Problem: How many moles of water would be produced if this reaction gave off 347.1 kJ of energy?
    • Equation: CH₄(g) + 2O₂(g) ⟶ CO₂(g) + 2H₂O(g) + 890.4 kJ
    • Calculation:
    • -347.1 kJ x (2 mol H₂O / -890.4 kJ) = 0.7796 mol H₂O
Example 3: Energy Produced from Oxygen
  • Problem: How much energy is produced when 75.0g of oxygen reacts with excess ethanol (C₂H₅OH)?
    • Equation: C₂H₅OH(l) + 3O₂(g) ⟶ 2CO₂(l) + 3H₂O(g) + 1235 kJ
    • Calculation:
    • 75.0 g O₂ x (1 mol O₂ / 32.00 g O₂) x (1235 kJ / 3 mol O₂) = -965 kJ

Summary

  • Understand thermochemical equations and their implications for energy changes in reactions.
  • Distinguish between exothermic and endothermic reactions based on the sign of ΔH.
  • Apply calculations to determine energy changes and related quantities in chemical reactions.

Conclusion

  • Review and be prepared for Worksheet #3 to solidify understanding of thermochemistry concepts and calculations.