Determining an Equilibrium Constant Using Experimental Data

Question

A mixture of PCl₃ and Cl₂ is allowed to react at 506 K.

PCl₃(g) + Cl₂(g) PCl₅(g)

The initial concentration of the reactants are [PCl₃] = 0.2060 M and [Cl₂] = 0.2340 M. After the system reaches equilibrium, it is found that the Cl₂ concentration has decreased to 0.0666 M. Based on these data, determine the value of the equilibrium constant, K, for this reaction.

Answer 1

Set up an ICE table:

PCl₃(g) + Cl₂(g) <==> PCl₅(g)

0.2060......0.2340............0...........Initial

-x.............-x...................+x...........Change

0.2060-x.....0.2340-x.......x...........Equilibrium

We can find x from the information that @ equilibrium Cl₂ concentration decreased to 0.0666 M

So, 0.2340 - x = 0.0666

x = 0.1674 M

At equilibrium we will have the following concentrations:

[PCl₃] = 0.2060 - 0.1674 = 0.0386 M

[Cl₂] = 0.0666 M

[PCl₅] = 0.1674 M

Kc = [PCl₅] / [PCl₃][Cl₂] = 0.1674 / (0.0386)(0.0666)

Kc = 65.1