In a gas-phase equilibrium where the total number of moles of gas is greater on the left side than on the right, what is the effect of increasing external pressure?

• A. The reaction shifts toward the side with more moles of gas.
• B. The reaction shifts toward the side with fewer moles of gas.
• C. The temperature of the system rises.
• D. The equilibrium constant changes.

Answer: (B)

When pressure is increased in a gas-phase equilibrium, the system shifts to reduce the pressure. It does this by favoring the side with fewer moles of gas, thereby decreasing the total amount of gas present and the pressure it exerts.

In this scenario, the left side has more moles of gas than the right, so the shift goes toward the right. This is why the equilibrium moves toward the side with fewer moles of gas. The temperature is not necessarily changed by this pressure change, and the equilibrium constant remains determined by temperature, not by pressure.