Hess’s Law states that the enthalpy change for the conversion of reactants to products is the same whether the conversion occurs in one step or several steps.

Example:

One Step Reaction

N2 (g) + 2 O2 (g) > 2 NO2 (g)

Two Step Reaction

N2 (g) + O2 (g) > 2 NO

Hess’s Law allows us to study energy changes in chemical reactions without actually performing the reaction in question

Two Rules for Finding ΔH Using Hess’s Law

1. If you reverse a chemical reaction, you must also reverse the sign of ΔH

2. If the coefficients in a balanced equation are multiplied by a factor, the ΔH must also be multiplied by the same factor

∴ 2 C (s) + 2 O2 (g) .→ 2 CO2 (g) ΔH = -786 kJ