Question

Initially, 0.50 atm each of PCl3(g) and Cl2(g) is added to a reaction vessel. Given the balanced chemical equation and KP, what is true about the equilibrium concentrations of reactants and products?
PCl5(g) ⇌ PCl3(g) + Cl2(g) KP = 1.21 × 10–2
The equilibrium lies to the _____, so the concentrations of PCl3 and Cl2 will be significantly _____ than the concentration of PCl5.
A. right; higher
B. right; lower
✓C. left; lower
D. left; higher

Can you explain why the answer is C ?

Answer 1

PCl5 (g) <==> PCl3 (g) + Cl2 (g)

	PCl5	PCl3	Cl2
	0	0.5	0.5
	+x	-x	-x
	+x	0.5-x	0.5-x

Kp = [PCl3][Cl2]/[PCl5]

1.21 *10^-2=(0.5-x) ^2/ x

x = 0.435

Equilibrium Cocentration ofPCL5 = 0.435 M, of PCl3 = 0.5-0.435 = 0.065 M and of Cl2 = 0.065 M

Reaction quotient = [PCl3][Cl2]/[PCl5] = 0.065 M * 0.065 M/0.435 = 9.7 *10^-3

As reaction quotient is less than Kp, so the equilibrium lies to the left. Also, concentration of PCl3 (0.065 M) and Cl2(0.065 M) is significantly less than that of PCl5 (0.435).