PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 74

3.2.5 Molar thermodynamic functions

All the thermodynamic functions defined above are extensive quantities [see 1.3.1]. In chem-

istry we work most often with intensive, molar quantities, which are denoted using the subscript

m. It holds, e.g., that

Um≡U/n, Sm≡S/n, Cpm≡Cp/n.

Example

A thermodynamic process took place in a system containing 5 mol of nitrogen during which the

Gibbs energy increased by 100 J. What was the change in the Gibbs molar energy?

Solution

∆Gm=

∆G

n

=

100

5

= 20 J mol−^1.

3.2.6 Fugacity

Fugacity is defined by the relation

f=pstexp

(

Gm(T, p)−G◦m(T, pst)

RT

)

, (3.21)

wherepstis the standard pressure (usuallypst= 101 325 Pa), andGm(T, p) is the molar Gibbs
energy at temperatureTand pressurep, andG◦(T, pst) is the molar Gibbs energy of a system
in an ideal gaseous state at temperatureTand pressurepst.
U Main unit:Pa.
S Symbols:SymbolX◦denotes thermodynamic quantities of an ideal gas at the chosen standard
pressure [see also6.1.2].
Fugacity is a thermodynamic quantity which is useful when solving phase and chemical
equilibria.

Note:For an ideal gas, fugacity equals pressure [see3.5.8].