Question

A system is composed of nitrogen gas and gasoline (liquid +vapour). According to the gibbs phase rule how many independent properties are required for phase equilibrium?Please show your workings and the steps you took for the calculation. The answer given to me is 2.

(By the way can you share with me when do you use generalised and when do we use condensed gibbs phase rule for solving?) I need it for a test.

Answer 1

The Gibb’s Phase Rule is

F = C + 2 –P

where F = number of independent variables that can be changed without changing the number of phases present at equilibrium. This is also called as degrees of freedom of the system;

C is the minimum number of chemical components required to constitute all phases of the system and P is the number of phases in the system.

For our system, we have C=2 (since two chemical components are required to define the two phases) and P=2 (two phases are present, a liquid phase and a vapour phase). Hence, we have,

F = 2 + 2 – 2 = 2

So, number of degrees of freedom for this system will be 2.

The condensed phase rule is

F = C + 1 – P

and this condensed phase rule is only applicable for a binary system in the sphere of solid state chemistry. In this case, the pressure of the system is envisioned to remain constant, so there will be no pressure variations leading to change in phase at equilibrium. However, this phase rule is applicable only for problems in metallurgy.