The standard enthalpy of formation, ΔH°f, of a compound is the enthalpy change when

Core Concept

The definition fixes three things: exactly one mole of product, elements as the only reactants, and every element in its standard state, such as graphite for carbon and O2(g) for oxygen. This shared baseline lets tabulated ΔH°f values combine to give the enthalpy of any reaction. Burning in oxygen instead defines the enthalpy of combustion, and building from free atoms defines atomization, both different quantities.