The enthalpy change (ΔH) in a reaction is equal to the heat input under conditions of constant pressure and temperature. It is not equal to the total energy change, as work may be done by expansion against the external pressure. The corrections are usually small, and enthalpy is used as a measure of the energies involved in chemical reactions. Endothermic reactions (positive ΔH) are ones need a heat input, and exothermic reactions (negative ΔH) give a heat output.

Hess' Law states that ΔH does not rely on the pathway taken between initial and final states, and is a consequence of the First Law of Thermodynamics, which asserts the conservation of total energy. It describes a schematic thermodynamic cycle where the ΔH can be expressed as the sum of the values for individual steps: