Increasing pressure of reacting gases affects the rate of reaction by which effect?

• A. Increases
• B. Decreases
• C. No effect
• D. Reverses

Answer: (A)

Increasing the pressure of reacting gases packs more gas molecules into the same volume, so their concentration goes up. The rate of a gas-phase reaction depends on how often reactant molecules collide; more molecules in a given space means more collisions per second, and thus a faster rate of reaction. With temperature held constant, these collisions are just as energetic on average, so the higher collision frequency translates into a higher rate rather than no change or a decrease. Reversing isn’t caused by pressure changes alone, so the rate doesn’t flip due to pressure.