PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 103

3.5.7 Gibbs energy

3.5.7.1 Temperature and pressure dependence

The Gibbs energy is calculated from the definition (3.14), and from the dependence of enthalpy
(3.74) and entropy (3.83) onTandp

G(T, p) =G(T 1 , p 1 ) + [H(T, p)−T S(T, p)]−[H(T 1 , p 1 )−T 1 S(T 1 , p 1 )]. (3.96)
The change in the Gibbs energy with pressure at constant temperature can be calculated
by integrating equation (3.36) with respect to the general prescription (3.31)

G(T, p 2 ) =G(T, p 1 ) +

∫p 2

p 1

Vdp , [T]. (3.97)

3.5.7.2 Changes at phase transitions

The Gibbs energy does not change during reversible phase transitions

∆G= 0, [T, p, reversible phase transition]. (3.98)

For irreversible phase transitions, we have the inequality

∆G < 0 , [T, p, irreversible phase transition]. (3.99)

and the change in the Gibbs energy is calculated using the procedure described in3.5.9.

3.5.8 Fugacity

Dependence on state variables for a homogeneous system
The following equations follow from the definition (3.21)

f=pexp

[∫

p

0

z− 1

p

dp

]

, (3.100)

and

f=

RT

Vm

exp

[

z− 1 −

∫Vm

∞

z− 1

Vm

dVm

]

, (3.101)

wherez is the compressibility factor. Relation (3.100) and (3.101) in particular are used to
calculate fugacity from equations of state.