The meaning of the once widely used term the Gibbs Free Energy in terms of availablework energy is perfectly illustrated for chemical reactions by the Van’t Hoff EquilibriumBox. Combining this with DeDonder’s extent of reaction variable and using thereaction of NH3 to H2 and N2 at 200◦C as an example shows the difference betweentotal work energy and available work energy, and in addition allows calculation of theequilibrium composition, demonstration of the minimum in the Gibbs energy curve,and the standard relationship between ΔrG◦ and lnK