Hess's Law
Enthalpy is a state function
Principle
ΔHtotal = ΔH₁ + ΔH₂ + ΔH₃ + ...
The total enthalpy change is independent of pathway; sum intermediate steps to find overall ΔH.
Example
Find ΔH for: C(s) + ½O₂(g) → CO(g)
Given reactions:
(1) C(s) + O₂(g) → CO₂(g); ΔH₁ = -393.5 kJ
(2) CO(g) + ½O₂(g) → CO₂(g); ΔH₂ = -283.0 kJ
Reverse reaction (2): CO₂(g) → CO(g) + ½O₂(g); ΔH = +283.0 kJ
Add to (1): C(s) + O₂(g) → CO₂(g) then CO₂(g) → CO(g) + ½O₂(g)
Net: C(s) + ½O₂(g) → CO(g)
ΔH = -393.5 + 283.0 = -110.5 kJ
Answer: ΔH = -110.5 kJ
Tips
- Reverse a reaction: change sign of ΔH.
- Multiply reaction by factor: multiply ΔH by same factor.
- Cancel species appearing on both sides when summing.