1.21.3: Thermodynamic Energy: Potential Function
The Master Equation states that the change in thermodynamic energy of a closed system is given by equation (a).
\[\mathrm{dU}=\mathrm{T} \, \mathrm{dS}-\mathrm{p} \, \mathrm{dV}-\mathrm{A} \, \mathrm{d} \xi ; \quad \mathrm{A} \, \mathrm{d} \xi \geq 0 \nonumber \]
At constant entropy (i.e. \(\mathrm{dS} = 0\)) and constant volume (i.e. \(\mathrm{dV} = 0\)), equation (a) leads to equation (b).
\[\mathrm{dU}=-\mathrm{A} \, \mathrm{d} \xi \nonumber \]
But
\[A \, d \xi \geq 0 \nonumber \]
Therefore all spontaneous processes at constant \(\mathrm{S}\) and constant \(\mathrm{V}\) take place in a direction for which the thermodynamic energy decreases.
The latter statement shows the power of thermodynamics in that it is quite general; we have not stated the nature of the spontaneous process. Of course chemists are interested in those cases where the spontaneous process is chemical reaction. Thus we have a signal of what happens to the energy of the system; the key word here is spontaneous.
In the context of most chemists interests, equation (b) is not terribly helpful. Chemists do not normally run their experiments at constant \(\mathrm{S}\) and constant \(\mathrm{V}\). In fact it is not obvious how one might do this. Nevertheless equation (b) is important finding its application when we turn to other thermodynamic variables which can be used as thermodynamic potentials; e.g. Gibbs energy.