Relationship To Heat
In order to discuss the relation between the enthalpy increase and heat supply we return to the first law for closed systems: dU = δQ - δW. We apply it to the special case that the pressure at the surface is uniform. In this case the work term can be split in two contributions, the so-called pV work, given by -pdV (where here p is the pressure at the surface, dV is the increase of the volume of the system) and other types of work δW ' such as by a shaft or by electromagnetic interaction. So we write δW = -pdV+δW '. In this case the first law reads
or
From this relation we see that the increase in enthalpy of a system is equal to the added heat
provided that the system is under constant pressure (dp = 0) and that the only work done by the system is expansion work (δW ' = 0)
Read more about this topic: Enthalpy
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