PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 89

Solution

From the conditiondF= 0 at[T, V]we obtain by integration

∆F= 0, [reversible process, T, V]

Since at[T, V]the Helmholtz energy has its minimum at equilibrium, it holds that during all

irreversible processes occurring at constant temperature and volume, the Helmhotz energy of the

system decreases

∆F < 0 , [irreversible process, T, V].

Analogous relations can be obtained for other thermodynamic functions.

The criteria of thermodynamic equilibrium are applied in calculating phase and chemical
equilibria. The choice of criterion depends on the conditions of equilibrium. Equilibria at
constant temperature and pressure are studied most often, in which case the criterion dG= 0,
[T, p] is applied [see (7.1)].