For the equilibrium Ni(CO)4 (g) ⇌ Ni(s) + 4 CO(g), what is the effect of increasing total pressure on the position of equilibrium?

• A. Shift toward Ni(CO)4 (g) (the reactant side)
• B. Shift toward Ni(s) (the solid) side
• C. No shift occurs
• D. Equilibrium constant changes

Answer: (A)

Increasing total pressure shifts the equilibrium toward the side with fewer gas molecules. Here, the reactant side has one mole of gas (Ni(CO)4), while the product side has four moles of gas (4 CO) plus a solid Ni(s). Raising the pressure favors reducing the number of gas molecules, so the system moves toward Ni(CO)4(g). The equilibrium constant Kp stays the same at a given temperature; only the distribution of species changes to accommodate the new pressure, and the solid phase is unaffected.