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Hess's Law

Hess's Law states that the total enthalpy change of a reaction is the same, no matter how many steps the reaction takes, as long as the initial and final conditions are the same.

Key Concepts​

  • State Function: Enthalpy depends only on initial and final states, not the path
  • Reaction Manipulation: Reactions can be reversed or multiplied and their ΔH adjusted accordingly
  • Sum of Steps: ΔH_total = Σ ΔH_steps

Using Hess's Law​

  1. Break the target reaction into known reactions
  2. Adjust each reaction (reverse or multiply) to match the target
  3. Sum ΔH values to find the overall enthalpy change

Example​

C(s) + O₂(g) → CO₂(g)

  • Can be calculated using formation or combustion reactions of CO and CO₂
  • ΔH_total = ΔH_CO + ΔH_CO₂ − ΔH_CO

Importance​

  • Allows determination of enthalpy changes that are difficult to measure directly
  • Widely used in thermodynamics, chemical engineering, and materials science
  • Essential for energy calculations and reaction design

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