Which one of the following characteristics of the transition metals is associated with their catalytic property/activity ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → variable oxidation states
Transition metals can change their oxidation state easily, so they take up and give back electrons and offer a lower-energy path for a reaction. In the iodide–persulphate reaction, \( \ce{Fe^3+} \) is reduced and then oxidised back:
\( \displaystyle \ce{2Fe^3+ + 2I- -> 2Fe^2+ + I2} \)
\( \displaystyle \ce{2Fe^2+ + S2O8^2- -> 2Fe^3+ + 2SO4^2-} \)
Why the other options are incorrect
Option (1) colour of hydrated ion — Incorrect. Colour comes from d–d transitions and has no part in catalysis.
Option (3) high enthalpy of atomisation — Incorrect. This explains the hardness and high melting points of the metals, not catalysis.
Option (4) paramagnetic behaviour — Incorrect. Paramagnetism comes from unpaired electrons and is a magnetic property, not the cause of catalytic activity.
NCERT (2026-27 edition):
Catalytic Properties – “The transition metals and their compounds are known for their catalytic activity. This activity is ascribed to their ability to adopt multiple oxidation states and to form complexes. Vanadium(V) oxide (in Contact Process), finely divided iron (in Haber’s Process), and nickel (in Catalytic Hydrogenation) are some of the examples. … Also because the transition metal ions can change their oxidation states, they become more effective as catalysts. For example, iron(III) catalyses the reaction between iodide and persulphate ions.”