Hess' law of constant heat of summation underpins the Born-Haber cycle. According to Hess' rules, whether a chemical reaction occurs in one or more steps, the overall heat of the reaction remains constant. For example, the heat of reaction (H) = +Q for a chemical reaction given by

A → B.

Alternatively, if the reaction occurs in multiple steps:

A → C ΔH ₁ = q ₁

C → D ΔH ₂ = q ₂

D → B ΔH ₃ = q ₃

Then +Q = q ₁ + q ₂ + q ₃ according to Hess' law. It should be emphasised that this law also applies to cyclic processes.