4.9.1 Enthalpy and Reversibility

Before leaving the discussion of the enthalpy, let us return to the defining differential equation for H and try to deal with an irreversible process. In that case:

$$dH = dq - p_{ext}dV + pdV + Vdp = dq + (p - p_{ext})dV + Vdp$$ (4.9.45)

At constant pressure this is:

$$dH = dq + (p - p_{ext})dV$$ (4.9.46)

In the case of a gas at constant pressure the term in parentheses generally is not zero and $\Delta H$ is not equal to q.

However, if the system is open to the atmosphere (think of a reaction in a beaker) the pressure in the system is the same as the pressure outside the system. In that case the term in parentheses vanishes even if the process is not reversible. One exception to this is the case of materials with considerable structural strength such as iron. In that case there may well be internal stresses that render p not equal to p_ext. However such energies are usually, but not always, very small and can often be neglected.

[Note: discussion of this point is usually neglected in most physical chemistry and thermodynamics text books. Indeed, the enthalpy is often not mentioned by physicists as they have little use for the concept. Of the current physical chemistry books with which I am familiar only L. M. Raff's Principles of Physical Chemistry (Prentice-Hall, 2001) pays any attention to it (see page 77).]