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Passage (Q1-Q4): Transition metals and their compounds are widely used as catalysts in industry and in biological systems. For instance, in the Contact process vanadium(V) species (V2O5 or VO3-) oxidise SO2 to SO3: SO2 + 1/2 O2 --V2O5--> SO3. The oxidation is believed to occur in two steps. First, V5+ together with oxide ions converts SO2 to SO3 while V5+ is reduced to V4+: 2 V5+ + O2- + SO2 -> 2 V4+ + SO3. Then V5+ is regenerated by oxygen: 2 V4+ + 1/2 O2 -> 2 V5+ + O2-. The sum of the two steps gives the overall reaction SO2 + 1/2 O2 -> SO3. Why are transition metals and their compounds effective catalysts for such reactions?

1. Because they possess a completely filled s-subshell
2. Because their atoms are of comparable size owing to poor d-subshell shielding
3. Because they provide an alternative reaction pathway with a lower activation energy
4. Because their oxidation states always differ by exactly two units

Correct answer: Because they provide an alternative reaction pathway with a lower activation energy

Solution

Transition metals act as catalysts mainly because their variable oxidation states let them offer an alternative mechanism with a lower activation energy, as shown by the V5+/V4+ cycling in the Contact process.

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