PHYSICAL CHEMISTRY IN BRIEF

CHAP. 3: FUNDAMENTALS OF THERMODYNAMICS [CONTENTS] 104

3.5.8.1 Ideal gas.

For an ideal gas, fugacity is equal to pressure

f=p , [ideal gas]. (3.102)

3.5.8.2 Changes at phase transitions

At reversible phase transitions, fugacity does not change. If, e.g., the liquid phase is in equi-
librium with the gaseous phase, we have

f(l)=f(g), [T, p]. (3.103)

Note:According to the theorem of corresponding states [see2.2.9], the fugacity coefficient

φ=f/pis the function of reduced temperatureTrand reduced pressurepr. To estimate

fugacity, we use generalized diagrams of the fugacity coefficient as a function ofTrandpr.

Example

Calculate the fugacity of ethane at temperature 270 K and pressure 1000 kPa. For ethane at this

temperature and pressure under 1200 kPa, the equation of statez= 1− 1. 1359 · 10 −^4 papplies,

with pressure given in kPa.

Solution

We substitute for the compressibility factorzfrom equation (3.100) and integrate

f=pexp(− 1. 1359 × 10 −^4 ×p) = 1000×exp(− 0 .11359) = 892.7 kPa.

3.5.9 Changes of thermodynamic quantities during irreversible processes.

Some formulas presented in this chapter cannot be directly applied to irreversible processes as
they apply only for reversible processes. On the other hand, a change in a thermodynamic
quantity depends solely on the initial and final state of the system, but it does not depend