PHYSICAL CHEMISTRY IN BRIEF

CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 198

Obr.7.6:Isothermal diagram of a two-component system. The curves illustrate the dependence of
the dew and boiling pressures on the molar fraction of component 1 in the liquid and vapour phases.

Solution

Substituting into (7.24) gives

γ 1 =

y 1 p

x 1 ps 1

=

0. 552 × 62. 39

0. 252 × 72. 3

= 1. 890 ,

γ 2 =

y 2 p

x 2 ps 2

=

0. 448 × 62. 39

0. 748 × 31. 09

= 1. 202.

7.6.4 General solution of liquid-vapour equilibrium.

From the intensive criterion of equilibrium (7.3) it follows for each componenti, i= 1, 2 ,... k

p yiφi(T, p, y 1 , y 2 ,... , yk− 1 ) =γi(x 1 , x 2 ,... , xk− 1 , T, p)xiφsi(T, psi(T))psi(T), (7.25)

whereφiis the fugacity coefficient of componentiin a gaseous mixture of composition (y 1 , y 2 ,... yk− 1 )
at the temperature and pressure of the system [see(6.82)] andφsi(T, psi(T)) is its fugacity coef-
ficient at a state of pure saturated vapour at the system temperature.
Equation (7.25) can be written for each component of the mixture; however, numerical
methods always have to be used for the calculation.

Note: If the experimental pressure is higher than that calculated from Raoult’s law, we

say that the system exhibitspositive deviationsfrom Raoult’s law. If the experimental

pressure is lower than that following from Raoult’s law, we say that the system exhibits

negative deviationsfrom Raoult’s law, which is the case shown in Figure7.5.

7.6.5 Phase diagrams of two-component systems

• Isothermal diagram