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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