PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 75

3.2.7 Fugacity coefficient

The fugacity/pressure ratio is called thefugacity coefficientφ

φ=

f

p

. (3.22)

U Main unit:dimensionless quantity.

Note:For an ideal gasφ= 1.

Example

Knowing the values of the molar Gibbs energy of carbon dioxideGm(T, p) =−53 183J/mol at

T = 350 K andp= 10 MPa, andG◦m(T, pst) =−65 675. 14 J/mol, wherepst= 101.325 kPa,

calculate its fugacity and the fugacity coefficient at 350 K and 10 MPa.

Solution

By substituting into the definition (3.21) we obtain

f= 0. 101325 ×exp

(

− 53183 −(− 65675 .14)

8. 314 × 350

)

= 7.415 MPa, φ=

7. 415

10

= 0. 7415.

3.2.8 Absolute and relative thermodynamic quantities

If a numerical value can be unambiguously assigned to a thermodynamic quantity of a system in

a given state, we say that the quantity is absolute. If a numerical value can be assigned only to

a change in a thermodynamic quantity during the system’s transition from one thermodynamic

state to another, we say that the quantity is relative.

Temperature, pressure, volume, amount of substance, fugacity, and heat capacities represent

typical absolute quantities. For these quantities there exist natural and universal (independent

of the substance) referential states. Such referential states are temperature 0 K, volume 0

m^3 , pressure or fugacity 0 Pa. Internal energy, enthalpy, the Helmholtz energy and the Gibbs

energy represent typical relative quantities. There do not exist any universal referential states

for these quantities.