Hess's Law Lecture Notes

Introduction to Hess's Law

• Concept: Enthalpy change (ΔH) is the same whether the process occurs in one step or multiple steps.
• Daily Life Example: Ice Cube Sublimation
  • Ice absorbs ΔH = 300 kJ energy to sublime directly to gas.
  • Alternative path:
    • Ice absorbs ΔH1 = 44 kJ to melt into water.
    • Water absorbs ΔH2 = 256 kJ to evaporate to gas.
  • Combined: ΔH1 + ΔH2 = ΔH (44 kJ + 256 kJ = 300 kJ).
  • Conclusion: Enthalpy change is path-independent.

Hess's Law Explanation

• First Statement:

  • Enthalpy change during a reaction is the same regardless of the number of steps.
  • Example:
    • Single step: A → D, ΔH = 24 kJ.
    • Multiple steps: A → B → C → D, where ΔH = ΔH1 + ΔH2 + ΔH3 (12 kJ + 8 kJ + 4 kJ = 24 kJ).

• Second Statement:

  • Total enthalpy change in a cyclic process is zero.
  • Mathematically, ∑ΔH = 0.

Important Example: Formation of Carbon Dioxide

• One-Step Reaction:
  • C (s) + O2 (g) → CO2 (g)
    • ΔH = -393.7 kJ/mole (exothermic).
• Two-Step Reaction:
  • Step 1: C (s) + 1/2 O2 (g) → CO (g), ΔH1 = -111.052 kJ/mole.
  • Step 2: CO (g) + 1/2 O2 (g) → CO2 (g), ΔH2 = -283 kJ/mole.
  • Combined: ΔH = ΔH1 + ΔH2 = -393.052 kJ/mole.

Importance of Studying Hess's Law

• Used for reactions where enthalpy change cannot be measured directly with a calorimeter (e.g., formation of CCl4).

Fast Method for Numerical Problems in Hess's Law

1. Standard Enthalpy of Formation of Methanol (CH3OH)

   • Components: C (s), H2 (g), O2 (g).
   • Balanced Reaction: C + 2H2 + 1/2 O2 → CH3OH.
   • Reverse reactions as needed, changing enthalpy signs.
   • Combine equations, canceling spectator molecules, and calculate ΔH.
   • Result: ΔH = -239 kJ.

2. Difficult Numerical Problem

   • Required reaction: Fe2O3 + 3CO → 2Fe + 3CO2.
   • Use enthalpy changes from given reactions, modifying as needed (e.g., reversing, multiplying).
   • Result: ΔH = +6 kJ.

Conclusion

• Hess's Law provides a systematic approach for calculating enthalpy changes for reactions through various pathways, reinforcing the principle of energy conservation within chemical processes.