Enthalpy: definition in chemistry
Enthalpy: A thermodynamic quantity, H, that describes the heat content of a system; its change, ΔH, equals the heat absorbed or released by a reaction at constant pressure.
Because absolute enthalpies cannot be measured, chemists work with enthalpy changes, usually in kilojoules per mole (kJ/mol). A negative ΔH means the reaction is exothermic, and a positive ΔH means it is endothermic. Enthalpy changes can be measured with a calorimeter or calculated from tables of standard values using Hess's law, which says the total change is the same whatever route the reaction takes.
Examples
- Burning methane — ΔH = −890 kJ/mol
- Forming liquid water from H₂ and O₂ — ΔH = −286 kJ/mol
- Melting ice — ΔH = +6.01 kJ/mol
- Neutralizing a strong acid with a strong base — about −57 kJ per mole of water
On the periodic table
Elements in their standard states, such as oxygen gas, graphite and solid iron, are given a standard enthalpy of formation of zero, which makes them the reference point for the values of all compounds.