What is the equilibrium expression for Ni(CO)4 (g) ⇌ Ni (s) + 4 CO (g)?

• A. Kc = [CO]^4 / [Ni(CO)4]
• B. Kc = [CO]^4 / a(Ni(CO)4)
• C. Kp = (P_CO)^4 / (P_Ni(CO)4)
• D. Kc = [Ni(CO)4] / [CO]^4

Answer: (A)

In a heterogeneous equilibrium, pure solids and liquids do not appear in the equilibrium expression because their activities are effectively 1. Here, nickel is a pure solid, so it drops out of the expression. The gaseous species are the ones that set the constant, and the product side has four moles of CO formed, so its concentration appears to the fourth power in the numerator. The reactant is the gas Ni(CO)4, so its concentration goes in the denominator. Using concentrations, the equilibrium constant is Kc = [CO]^4 / [Ni(CO)4]. If you were using pressures instead of concentrations, you would write Kp = (P_CO)^4 / P_Ni(CO)4, but the question, asking for Kc, corresponds to the first form.