Chemistry · May / June 2022 · Paper 11 · Question 20

Sulfur dioxide reacts with oxygen in the air. 2SO2(g) + O2(g) ⇌ 2SO3(g) ΔH = –196 kJ / mol The reaction is very slow if no catalyst is present. Which statement explains this?

1. AAir contains only 21% oxygen so there is not enough oxygen for all the sulfur dioxide to react.
2. BOnly a small proportion of the sulfur dioxide and oxygen molecules have enough energy to react, even at high temperatures.
3. CThe reaction is exothermic and so at high temperatures the equilibrium shifts to the left.
4. DThe reaction is reversible and so products turn back to reactants; this happens more quickly at high temperatures.

As printed in the paper