CHEMICAL EQUILIBRIUM DOUBTS

Question

4 moles of PCl5 are heated in a closed container of 4 dm³ container at 400 k.

At equilibrium, 50% of PCl5 is dissociated.

What is the value of K_c for the dissociation of PCl5 into PCl3 and Cl2 at 400k ?

Answer 1

The reaction is as follows:

          PCl₅ (s)     PCl₃ (l) +  Cl₂ (g)

Initial      4             0              0

Equilib     2             2             2

 

K_C = [PCl₃] * [Cl₂]

              [PCl₅]   

In this equation at 400K, PCl5 (mp 439.9K) is a solid and the others are gases. At the high pressure in the container (32 atm), PCL3 (bp 349.2 K, mp 179.6 K) will be a liquid. We ignore the denominator and PCl3 in the equation, as K_C is for gas phase reactions. The concentration for Cl2 is 2 (mol)/ 4(dm³), or 0.5M. dm is decimeter and dm³ is liter (L).

If we put the value in the equation, we get K_C = 0.5 mol dm^-3.