Laws of Thermochemistry

There are two laws of thermochemistry: The Lavoisiter–Laplace law and the Hess’s Law of Constant Heat Summation.

Lavoisier - Laplace Law

When a chemical equation is reversed, the sign of Δ_rH is changed. For example,

N₂ (g) + O₂ (g) → 2NO (g); Δ_rH = 180.5 kJ

2NO (g) → N₂ (g) + O₂ (g); Δ_rH = –180.5 kJ

Hess’s Law of constant Heat Summation

Hess’s law states that the enthalpy of reaction is independent of the number and the nature of the intermediate steps. 

Since ΔH is a state function, it is not dependent on the way the reactions are carried out. The result of Hess’s law is that thermo-chemical equations can be added and subtracted just like algebraic equations to get the desired reaction.