What is the role of the metal catalyst (such as nickel) in catalytic hydrogenation?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → H2 and the other reactant bond to atoms of the metal surface, which weakens their bonds and lowers the activation energy
Nickel acts as a catalyst at a solid surface: \( \ce{H2} \) and the compound being hydrogenated (for example an alkene) bond to its surface atoms, which weakens their bonds, such as the H–H bond, and gives a path of lower activation energy.
Why the other options are incorrect
Option (1) The metal itself reduces the reactant — Incorrect. The hydrogen is the reducing agent. A catalyst is not used up: the nickel is left unchanged at the end and only speeds up the reaction.
Option (3) The metal only breaks the H–H bond and takes no other part in the reaction — Incorrect. The other reactant also bonds to the surface; this raises the concentration of the reactants at the surface and weakens their bonds too, so the metal does more than break the H–H bond.
NCERT (2026-27 edition):
4.3.12 Catalytic Properties – “… Vanadium(V) oxide (in Contact Process), finely divided iron (in Haber’s Process), and nickel (in Catalytic Hydrogenation) are some of the examples. Catalysts at a solid surface involve the formation of bonds between reactant molecules and atoms of the surface of the catalyst (first row transition metals utilise 3d and 4s electrons for bonding). This has the effect of increasing the concentration of the reactants at the catalyst surface and also weakening of the bonds in the reacting molecules (the activation energy is lowering).”