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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 Edit

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;

$H = E + pV,\,$

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:

$H = U + pV,\,$

where (all units given in SI)

EngineersEdit

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