Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 4 / b / ii / Solution

At coexistence of the two pure solid forms, their chemical potentials are equal and ${\Delta }_r{G}^{\circ }=0$. Treating the tabulated enthalpy and entropy differences as constant givesAbove this temperature the model favours the higher-entropy yellow form; below it the red form is favoured. This is a prediction from the given thermodynamic data, not a guarantee of an immediately visible colour change: kinetic trapping, particle-size effects and temperature-dependent heat capacities are not included. A ratio of masses of two coexisting pure solids is not the equilibrium constant, since their bulk solid activities are each unity.