Equation 3.50 is an example of a Maxwell relation. However, apart from being
unexpected, it does not look particularly interesting. Nevertheless, it does suggest that
there may be other similar relations that are more useful. Indeed, we can use the fact
that H, G, and A are all state functions to derive three more Maxwell relations. The
argument to obtain them runs in the same way in each case: because H, G, and A are
state functions, the expressions for dH, dG, and dA satisfy relations like eqn 3.49. All
four relations are listed in Table 3.5 and we put them to work later in the chapter.