Illustrated Glossary of Organic Chemistry

Enthalpy (H): The total energy of a system. In introductory organic chemistry, a useful simplification is to think of enthalpy as the total energy contained within chemical bonds. So a change in enthalpy (ΔH) can be thought of as a change in bond energies as bonds are formed and broken:  ΔH = H (bonds gained in products) - H (bonds lost in reactants).

The enthalpy of hydrogenation (ΔH of hydrogenation; also called heat of hydrogenation) for this catalytic hydrogenation reaction can be estimated from (strength of two C-H σ bonds gained) - [(strength of carbon-carbon π bond lost) + (strength of H-H σ bond lost)]. In this example the bonds lost are shown in red, and the bonds gained in blue. The empirical (observed) ΔH is ~ -30 kcal mol^-1.