Hess' Law

In 1840, Germain Henri Hess published his paper that laid out his theory of adding the energies of reaction. Hess's Law states that,

"The heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps."

Simply put, this means that enthalpies for reaction stages can be added together to determine net enthalpy change.

In other words, the enthalpy changes (ΔH's) for each stage of a multi-stage reaction can be summed to get the overall enthalpy change.