Consider the equilibrium 2SO2 (g) + O2 (g) ⇌ 2SO3 (g). The forward reaction rate is observed to increase when the pressure is raised. What is the expected effect on the forward rate and the equilibrium yield of SO3?

• A. a greater rate of reaction and a greater yield of SO3 at equilibrium
• B. a greater rate of reaction and a same yield of SO3 at equilibrium
• C. a lesser rate of reaction and a smaller yield of SO3 at equilibrium
• D. a lesser rate of reaction and a greater yield of SO3 at equilibrium

Answer: (A)

Increasing pressure raises the concentrations of all gaseous species, so collisions become more frequent and the forward rate, which depends on [SO2]^2[O2], increases. At the same time, the reaction produces fewer moles of gas (2 SO3 vs 3 moles of reactants), so the system shifts toward the right to relieve the pressure. This shift raises the equilibrium yield of SO3. In short, both the forward rate and the amount of SO3 at equilibrium increase.