Lecture - 5 (Gibbs phase rule)

Gibbs phase rule 

The Gibbs phase rule describe the possible number of degrees of freedom in a closed system at Equilibrium, in terms of number of seperate phases and the number of chemical constituents in the system. It was given by J.W Gibbs in 1870.

It's formula is -

P+F = C+N   or   P+F = C+2

Here, 

'P' represents number of phases (liquid, gas, solid)

'C' represents number of chemical constituents

'F' represents degrees of freedom. Degrees of freedom is defined as the minimum number of independent intensive variables are required to fix the state of the system

'N' represents number of non-compositional variables 

In Thermodynamics there are 2 non-compositional variables that are pressure and temperature. So value of 'N' = 2 in Thermodynamics always.

For example - 

1) Calculate 'F' of gaseous mixture of oxygen and nitrogen 

 P = 1 because only gaseous phase is there

 C = 2 because two gasses oxygen and nitrogen is there

 ∴P+F = C+2

 1+F = 2+2

  F = 3

2) calculate 'F' of water and water vapour mixture
P = 2 because solid and liquid two phases are there
C = 1 because only one component water is there
∴ P + F = C + 2
   2 + F = 1 + 2
    F = 1
3) calculate 'F' for tripple point of water
 P = 3 because at triple point all the three phases of water exist that is solid, liquid and gas
 C = 1 because only one component water is there
∴ P + F = C + 2
   3 + F = 1 + 2
     F = 0

Remember pressure at triple point of water is 0.61 kilo Pascal. Temperature at triple point of water is 273.16 Kelvin or 0.01 degree Celsius.

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