CDS (I) 2024 General Knowledge · Question 78 of 120

Correct — D, Nickel and H₂. Turning an unsaturated hydrocarbon into a saturated one means adding hydrogen across a double or triple bond, and NCERT names the catalysts for it precisely: dihydrogen gas adds to alkenes and alkynes in the presence of finely divided catalysts like platinum, palladium or nickel to form alkanes, a process called hydrogenation. The metal works by adsorbing dihydrogen on its surface and loosening the hydrogen–hydrogen bond so that the two atoms can be delivered to the carbon skeleton. Platinum and palladium do the job at room temperature; nickel needs a higher temperature and pressure, which is exactly why industry prefers it — it is far cheaper than the platinum-group metals and the extra heat costs less than the metal would. Nickel is the only one of the four metals offered that appears anywhere in that list.

• (a)Copper and H₂ — Copper is used for the opposite kind of step — heated copper dehydrogenates alcohols, removing hydrogen rather than adding it across a carbon-carbon bond. It is not a hydrogenation catalyst for alkenes.
• (b)Iron and H₂ — Iron is the classic catalyst of the Haber process, where nitrogen and hydrogen combine to give ammonia. Pairing iron with hydrogen is therefore very tempting, but that reaction has nothing to do with hydrocarbons.
• (c)Zinc and H₂ — Zinc appears in organic chemistry as a reducing agent in its own right, for instance zinc dust in the Clemmensen reduction, and zinc oxide with chromium oxide catalyses methanol synthesis. It is not used to hydrogenate a carbon-carbon double bond.

Alkenes and alkynes are unsaturated because they carry a double or a triple carbon-carbon bond; alkanes are saturated because every carbon is bonded to four separate atoms. Adding dihydrogen across the multiple bond converts one into the other. The reaction is thermodynamically favourable but extremely slow without help, because the hydrogen-hydrogen bond is strong. A transition metal surface solves that by adsorbing the hydrogen molecule and weakening its bond, letting the atoms transfer to the adsorbed hydrocarbon. It is a textbook case of heterogeneous catalysis, since the catalyst is a solid while the reactants are a gas and a liquid.

The everyday face of this reaction is the hardening of edible oils. Vegetable oils are liquid because their fatty-acid chains carry several double bonds that kink the molecule; hydrogenating some of those bonds straightens the chains, they pack better, and the oil sets into the semi-solid fat sold in India as vanaspati. That is nickel-catalysed hydrogenation at industrial scale. The chemistry has a nutritional tail worth knowing: when the reaction is only partial, some of the surviving double bonds are left in the trans arrangement rather than the natural cis one, and those trans fats are the reason partial hydrogenation has become a food-policy question rather than only a chemistry one. A related NCERT point worth carrying is that alkynes can be stopped halfway at the alkene stage using Lindlar's catalyst, which is palladium on charcoal deliberately poisoned so that it does not go all the way.

• Dihydrogen adds to alkenes and alkynes over finely divided platinum, palladium or nickel to give alkanes.
• The metal adsorbs dihydrogen on its surface and activates the hydrogen-hydrogen bond.
• Platinum and palladium work at room temperature; nickel needs higher temperature and pressure but is much cheaper.
• Hydrogenation of vegetable oils over nickel is how liquid oils are hardened into vanaspati.
• Lindlar's catalyst — palladised charcoal poisoned with sulphur compounds or quinoline — stops an alkyne at the alkene stage.

• Picking iron because the Haber process pairs iron with hydrogen — that reaction makes ammonia, not alkanes.
• Assuming any metal will do because all four options are paired with H₂; only the platinum-group metals and nickel catalyse this addition.
• Confusing hydrogenation with dehydrogenation, where copper removes hydrogen from an alcohol.

Asked as a one-line recall item where the hydrogen is held constant across all four options, so the whole decision rests on the metal.

• practice — not a real PYQ

  The conversion of liquid vegetable oil into vanaspati is an example of

  • (a)oxidation
  • (b)hydrogenation
  • (c)saponification
  • (d)esterification

  Answer(b) hydrogenation — hydrogen is added across the double bonds of the oil over a nickel catalyst, hardening it.

• practice — not a real PYQ

  Which catalyst is used in the Haber process for the manufacture of ammonia?

  • (a)Nickel
  • (b)Platinum
  • (c)Iron
  • (d)Vanadium pentoxide

  Answer(c) Iron — vanadium pentoxide belongs to the Contact process for sulphuric acid, and nickel and platinum to hydrogenation.