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Enthalpy is a thermodynamic property of a substance given by H = U + pV, where U is the internal energy, p is the pressure, and V is the volume.

For any process that occurs at constant pressure the heat absorbed or evolved is equal to the enthalpy change if the only work is pressure/volume work, i.e. ΔH = ΔU + pΔV for a process of constant pressure. In chemical reactions the enthalpy change (ΔH) is related to the changes in Gibbs free energyG) and entropyS) by the equation ΔG = ΔH - TΔS.

Enthalpy is usually expressed in units of kilojoules per mole.

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