PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 243

8.2.8 Reactions in the solid phase

• Choosing as the standard state a pure substance at the temperature and pressure of the
  system, the activity of solids which do not form solid solutions equals 1.
  For a reaction between immiscible solids we have

∆rG= ∆rG• [T, p] (8.17)

and the reaction proceeds in the direction of decrease in the Gibbs energy until at least one
of the reacting substances disappears from the mixture. The reactants are in equilibrium with
the products only if ∆rG•= 0.

• Solid solutions (e.g. amalgams) are processed in the same way as liquids [see the
  preceding section].

Example

Say whether red (1) and white (2) phosphor can be in equilibrium at 298 K and 101.325 kPa

if you know that under these conditions the molar Gibbs energies of the pure substances are

G•m, 1 =− 6. 804 kJ mol−^1 andG•m, 2 = 5. 2 kJ mol−^1.

Solution

For the reaction

0 = P(red,s)−P(white,s)

we have∆rG•=G•m, 1 −G•m, 2 =− 12. 004 kJ mol−^1. For the sake of the unit activities of both

solid substances it follows

∆rG= ∆rG•< 0

and therefore equilibrium cannot be attained, but the reaction proceeds until all white phosphor

transforms into red phosphor. In the same way it can be shown that

G= (n◦ 1 +ξ)G• 1 + (n◦ 2 −ξ)G• 2

will linearly decrease with increasingξ.