Since                                   
And                                     
So                                        
Therefore

At                                                           ε = 0, X₃ = 0, X₄ = 0

and at                                                      ε = 1, X₁ = 0, X₂ = 0
                                                              
At                                               
                                                         

  where                                          Σv = v₃ + v₄ + v₁ + v₂
                                    
If                                                     p = 1 atm, T = 298 K
                                                     
The slope  at  is called the “affinity” of the reaction, and it is equal to ΔG⁰ at the standard reference state. The magnitude of the slope at (Fig.3.2) indicates the direction in which the reaction will proceed.

Fig. 3.2 Plot of G against ε at constants T and p

For water vapour reaction, is a large positive number, which indicates the equilibrium point is far to the left of and therefore, ε_ε is very small. Again for the reaction,  is a large negative value, which shows that the equilibrium point is far to the right of  and so ε_ε is close to unity.