CAPF AC 2022 Q38: Science & Technology – Chemistry
Hydrogenation of alkenes can be carried out in the presence of
- (a) copper
- (b) zinc
- (c) aluminium
- (d) nickel
Answer: (d) nickel
Explanation
Catalyst for hydrogenation
How to think: hydrogenation means adding hydrogen to an unsaturated compound.
An alkene has a double bond; hydrogen adds across it to give an alkane.
This needs a metal catalyst: nickel, palladium or platinum.
The reaction: alkene + H2 → alkane, in the presence of nickel.
| Fact | Detail |
|---|---|
| Catalysts | Nickel, palladium, platinum |
| Change | Unsaturated → saturated |
| Everyday use | Turning vegetable oils into solid vanaspati ghee |
| Health note | Hydrogenation can produce harmful trans fats |
Answer: (d) nickel.
Why each option is right or wrong
- (a) Incorrect. Copper is not the catalyst used for adding hydrogen to alkenes.
- (b) Incorrect. Zinc does not catalyse hydrogenation.
- (c) Incorrect. Aluminium does not catalyse hydrogenation.
- (d) CORRECT. Hydrogenation of alkenes is carried out with a nickel catalyst.
Key point
Hydrogenation (addition reaction): unsaturated hydrocarbon + H2 → saturated hydrocarbon, catalyst Ni, Pd or Pt. Used to make vanaspati from vegetable oil. Animal fats are saturated; vegetable oils are unsaturated.
Remember
Hydrogenation needs nickel (or Pd, Pt).
Common mistakes
- Forgetting that a catalyst is needed at all for this addition reaction.
- Marking zinc or copper from memory of other reactions with acids.
- Mixing up hydrogenation (adding hydrogen) with hydration (adding water).
Exam tip
Educated guess: NCERT writes the reaction with Nickel catalyst above the arrow and mentions vanaspati ghee right after it.
Sources
- NCERT Class X Science — Carbon and its Compounds (addition reaction)
- NCERT Class XI Chemistry — Hydrocarbons (alkenes)