Substance A can exist in two solid phases at 298 K and 1 bar

Substance A can exist in two solid phases at 298 K and 1 bar: A(s₁) and A(s₂). The transformation

A(s₁) to A(s₂)

has a free energy less than 0 at 298K. The two phases evaporate to produce the same species in the vapor phase : A(g). Which solid phase do you expect will have a vapor pressure of 1 bar at the lower temperature, s₁ or s₂? Explain.

Solution

Substance A exists in two phases A(s1) and A(s2)

Transformation,

A(s1) ---> A(s2)

is thermodynamically favoured process with dG < 0.

So A would exists mostly in the A(s2) phase. The solid phase thus that will have a vapor pressure of 1 bar at lower temperature = A(s2) [Existence of thermodynamically more stable species]