Enthalpy

In thermodynamics, enthalpy is the sum of the internal energy U of a system and the product of pressure p and V of the system,

$H=U+pV$

Enthalpy used to be called "heat content", which is why it is conventionally indicated by H.

The work term pV has dimension energy, in SI units joule, and H has the same dimension. Enthalpy is a state function, a property of the state of the thermodynamic system and its value is determined entirely by the temperature T, pressure p, and composition N_A, N_B, ... (molar quantities of A, B, ...) of the system and not by its history.

Often one considers a system with thermal conducting walls, so that (small) amounts of heat ΔQ can go through the wall in either direction: if ΔQ > 0, heat enters the system and if ΔQ < 0 heat leaves the system. Also one usually considers one manner of performing (small) amount of work ΔW by or on the system:

$\Delta W=pdV$

If dV > 0 the volume of the system increases and work is performed by the system, if dV <0, work is performed on the system, hence the internal energy increase of the system by the work term obtains a minus sign:

$dU=\Delta Q-\Delta W$

To be continued