Activation Energy Barrier

Steps:

• Reaction rate depends on collisions between SO₂ and O₂ molecules with energy exceeding the activation energy (E_a).
• At ambient temperatures, only a small fraction of molecules possess this minimum E_a, leading to few successful collisions.
• Without a catalyst to lower E_a, the reaction proceeds slowly despite being thermodynamically favorable (exothermic).
• Catalysts increase the proportion of energetic molecules, but here none is present, explaining the slowness.

Why B is correct:

• Collision theory states that reaction rate is proportional to the fraction of molecules with energy ≥ E_a; B directly describes this low fraction without a catalyst.

Why the others are wrong:

• A: Oxygen concentration (21%) is sufficient; slowness is kinetic, not due to reactant shortage.
• C: Equilibrium position (Le Chatelier's principle) affects yield, not initial rate or catalyst absence.
• D: Reversibility impacts equilibrium, but slowness stems from high forward E_a, not temperature-driven reversal.

Final answer: B