Question

Identify the equilibrium-constant expression for the following reaction: PCl5(g) ⇌ PCl3(l) + Cl2(g)?

1) Kc = [PCl3][Cl2]/[PCl5]
2) Kc = [PCl5]/[PCl3][Cl2]
3) Kc = [PCl3][Cl2]
4) Kc = [PCl5][PCl3][Cl2]

Answer 1

The correct equilibrium-constant expression for the reaction PCl5(g) ⇌ PCl3(l) + Cl2(g) is Kc = [PCl3][Cl2]/[PCl5], where the concentrations of gaseous products and reactants are used in the ratio.

Step-by-step explanation:

The equilibrium constant expression for the given reaction PCl5(g) ⇌ PCl3(l) + Cl2(g) involves only the gaseous and aqueous species concentrations or partial pressures since liquids and solids are omitted from the equilibrium constant expression. Therefore, the correct equilibrium-constant expression, denoted as Kc, for this reaction is the ratio of the concentration of the products to the reactants, where each is raised to the power of its stoichiometric coefficient in the balanced chemical equation. The reaction has one mole of PCl3 and one mole of Cl2 as products, and one mole of PCl5 as a reactant. As such, the equilibrium constant expression for Kc would be:

Kc = [PCl3][Cl2]/[PCl5]

The equilibrium constant expression represents how product and reactant concentrations relate at equilibrium. For gases, a similar expression can be written for Kp, involving partial pressures instead of concentrations.