Question

The dissociation of HI is independent of pressure, while dissociation of PCL5 depends upon the pressure applied. why?

Answer 1

The dissociation of HI is given below :

H2(g) + I2(g)  2HI (g) 

Number of moles of reactant = 1 mole of H2 + 1 mole of I2 = 2

Number of moles of product = 2

As we can see that there is no change in the number of moles on both side of the reaction i.e. reactant side and product side. So according to Le Chatelier's principle pressure has no effect on the dissociation of HI as the reaction proceeds with no change in the number of moles.

Number of moles of reactant = 1 mole

Number of moles of product = 1 mole of PCl​3 + 1 mole of Cl2​ = 2

Since the number of moles on product side is greater than on reactant side so dissociation of PCl5 depends upon the pressure.

The above reaction is favoured by low pressure as reaction proceeds with increase in number of mole.