We can carry out a similar set of operations on the enthalpy, H = U + pV. The quantities
U, p, and V are all state functions; therefore H is also a state function and dH is an
exact differential. It turns out that H is a useful thermodynamic function when the
pressure is under our control: we saw a sign of that in the relation ΔH= qp (eqn 2.19b).
We shall therefore regard H as a function of p and T, and adapt the argument in
Section 2.11 to find an expression for the variation of H with temperature at constant
volume. As explained in the following Justification, we find that for a closed system of
constant composition