Enthalpy is defined as
H = U + P·V
where U is the internal energy (Joules), P is the external pressure (Pascals), and V is the volume (cubic meters). The internal energy (U) and pressure (P) are extensive quantities.
Enthalpy is both a thermodynamic function of state and an extensive quantity. The total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead.
For an exothermic reaction, the change in enthalpy of the system is equal to the energy released in the reaction (including the energy retained in the system and that lost through expansion against the surroundings). Similarly, for an endothermic reaction, the change in enthalpy of the system is equal to the energy absorbed in the reaction (including the energy lost by the system and that gained through expansion against the surroundings).
If the Gibbs energy is known, enthalpy can be derived as follows:
H = G - T·(∂G/∂T)P,n
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