ABSTRACT
The majority of chemical reactions, and almost all biochemical processes in υiυo, are performed under constant pressure conditions and involve small volume changes. When a process takes place under constant pressure, and assuming that no work other than pV work is involved, then the relationship between the heat changes and the internal energy of the system is given by:
dU=dq−pexdV (infinitesimal change) ∆U=q−pex∆V (finite change)
The enthalpy, H, is defined by the expression; H=U+pV, Hence for a finite change at constant pressure:
∆H=∆U+ pex∆V
Thus, when the only work done by the system is pV work, ∆H=q at constant pressure
Expressed in words, the heat exchanged by a system at constant pressure is equal to the sum of the internal energy change of that system and the work done by the system in expanding against the constant external pressure. The enthalpy change is the heat exchanged by the system under conditions of constant pressure.
