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 ΔH₁ = q₁

C → D ΔH₂ = q₂

D → B ΔH₃ = q₃

Then +Q = q₁ + q₂ + q₃ according to Hess' law. It should be emphasised that this law also applies to cyclic processes.