Fundamentals of Thermochemistry

Oct 1, 2024

Thermochemistry Lecture Notes

Introduction to Thermochemistry

  • Focus on thermochemistry, key equations, and internal energy.

Internal Energy

  • Change in Internal Energy (ΔE) Equation: ΔE = Q + W
    • Q: Heat energy entering or leaving the system.
    • W: Work done on/by the system.

Heat (Q)

  • Direction of Heat Flow: Heat flows from hot to cold.
    • System Example: If system is 100°C and surroundings are 50°C, heat flows out of the system.
    • Sign of Q:
      • Exothermic Process (Heat released): Q is negative.
      • Endothermic Process (Heat absorbed): Q is positive.
  • Units:
    • 1 kJ = 1,000 joules (J).
    • 1 calorie (cal) = 4.184 J.
    • 1 Calorie (Cal) = 1,000 cal.

Work (W)

  • Equation: W = PΔV
    • Work done on system: W is positive.
    • Work done by system: W is negative.
  • Gas Expansion/Compression:
    • Gas Expands: Work is negative, system does work.
    • Gas Compressed: Work is positive, work is done on system.
  • Conversion: 101.3 joules = 1 L·atm

Example Problem

  • Condition: 300 J of heat absorbed, gas expands from 2L to 3L at 5 atm.
  • Calculate ΔE:
    1. Calculate W: W = PΔV = 5 atm × (3L - 2L) = 5 L·atm.
    2. Convert to J: 5 L·atm × 101.3 J/L·atm = 506.5 J (negative sign due to expansion).
    3. Use ΔE = Q + W: ΔE = 300 J - 506.5 J = -206.5 J (System loses energy).

Heat Calculation

  • Equation: Q = mCΔT
    • m: Mass
    • C: Specific heat capacity (water: 4.184 J/g°C)
    • ΔT: Change in temperature
  • Example: Heat 50g of water from 25°C to 75°C:
    • Q = 50g × 4.184 J/g°C × 50°C = 10,460 J

Phase Change Calculation

  • Equation: Q = mΔH or Q = nΔH
    • ΔH: Enthalpy of fusion/vaporization
  • Example: Melt 54g ice at 0°C
    • Heat of fusion (water): 6 kJ/mol
    • Convert: 54g to moles (54g/18g/mol) = 3 mol
    • Q = 3 mol × 6 kJ/mol = 18 kJ

Thermochemical Equations

  • Combustion of propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
    • Heat released: 12200 kJ
  • Example: Calculate released energy for 64g O₂
    • Convert 64g to moles, then use stoichiometry to find energy.

Enthalpy of Reaction

  • Use enthalpies of formation:
    • ΔH = ΣΔH(products) - ΣΔH(reactants)
  • Example: CH₄ + 2O₂ → CO₂ + 2H₂O
    • Use given ΔHᶠ values to find reaction enthalpy.

Hess's Law

  • Combine reactions to estimate overall enthalpy.
  • Example: Use two given reactions to find enthalpy of a third reaction.

Conclusion

  • Review concepts and equations for thermochemistry.
  • Understanding of energy transfer, enthalpy calculations, and Hess’s Law.