In the gas-phase equilibrium 2BrCl(g) ⇌ Br2(g) + Cl2(g), what happens if the container volume is decreased (pressure increased) at constant temperature?

• A. Shift to left
• B. Shift to right
• C. No shift
• D. Kp changes

Answer: (C)

When a gas-phase equilibrium is subject to a change in volume at a constant temperature, the direction of shift depends on the change in the number of gas moles. If the total number of gas moles is the same on both sides, there’s no shift. In this reaction, there are 2 moles of gas on the left and 2 moles on the right, so Δn_gas = 0. If you decrease the volume (increase pressure), each gas’ partial pressure increases by the same factor, so the expression for the equilibrium constant Qp, which is (P_Br2 × P_Cl2)/(P_BrCl2)^2, remains unchanged. Because temperature is fixed, Kp stays the same as well. Therefore the system does not shift its position, even though the pressure is higher.