In Ni(CO)4 (g) ⇌ Ni(s) + 4 CO (g), why is the activity of Ni(s) not included in the equilibrium expression?

• A. Solid species have unit activity and are omitted
• B. Solids have zero activity
• C. The solid Ni does not participate in equilibrium
• D. Concentrations of solids are always constant

Answer: (A)

The main idea is that in equilibrium expressions, the activity of a pure solid or pure liquid is taken as 1 and is not included. This happens because the chemical potential of a pure phase is fixed by its phase, not by how much of it is present. As long as the solid Ni exists, its activity stays effectively constant, so it does not influence the equilibrium position.

For this reaction, the solid nickel is a pure phase with unit activity, so it drops out of the expression. The equilibrium expression only involves species whose activities can vary with the reaction—namely the gaseous Ni(CO)4 and the CO gas. So the equilibrium constant (for gases, effectively Kp) depends on the partial pressures of those gases, not on the amount of solid nickel.