The **enthalpy** or **heat content** is a quotient or description of thermodynamic potential of a system, which can be used to calculate the "useful" work obtainable from a closed thermodynamic system under constant pressure and entropy.

### Original definition

This is the heat change which occurs when 1 mol of a substance reacts completely with oxygen to form products at 298 K and 1 atm.
The function *H* is defined in the following equation;

where *E* represents the energy of the system. In the absence of an external field, the enthalpy may be defined, as it is generally known, by:

where (all units given in SI)

*H*is the enthalpy (joules)*U*is the internal energy, (joules)*p*is the pressure of the system, (pascals)*V*is the volume, (cubic metres)

## Engineers

Engineers do steam calculations based on the adjacent diagram,(shown).

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