Le Chatelier's principle: what happens to the position of equilibrium if pressure is increased for a gaseous system when the forward reaction reduces the number of gas moles?

• A. The equilibrium shifts toward the side with more moles of gas.
• B. The equilibrium shifts toward the side with equal moles of gas on both sides.
• C. The equilibrium remains unchanged.
• D. The equilibrium shifts toward the side with fewer moles of gas.

Answer: (D)

When a gas-phase equilibrium experiences a pressure increase, the system shifts to oppose that change by favoring the side with fewer gas moles. If the forward reaction reduces the number of gas molecules, increasing pressure pushes the equilibrium toward the product side, which has fewer moles. This is a direct application of Le Chatelier’s principle: the system adjusts to lessen the effect of the pressure change. For example, in the synthesis of ammonia, the reaction N2 + 3H2 ⇌ 2 NH3 moves toward NH3 when pressure is raised because there are fewer moles of gas on the product side. Remember, the equilibrium constant remains set by temperature, but the position changes to reflect the new conditions; if both sides had the same number of moles, pressure changes would not shift the position.