When two chemical equations are added together to build a target equation, their enthalpy changes

Core Concept

Enthalpy is a state function, so it depends only on the initial and final states, never on the route between them. When steps are summed, intermediate species cancel algebraically and the ΔH values accumulate to the overall value. This additivity is the operational core of Hess's law. It lets chemists combine tabulated reactions to obtain enthalpies that cannot be measured directly.