Which of the following metals cannot be used to absorb \(H_2\) gas on its surface?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Iron
Platinum, palladium and nickel take up large volumes of hydrogen on their surface. Palladium can take up several hundred times its own volume of \( \ce{H2} \). This is why finely divided Ni, Pt and Pd are the catalysts used in hydrogenation reactions. Iron is not used to take up hydrogen in this way.
Why the other options are incorrect
Option (2) Platinum — Incorrect. Adsorbs \( \ce{H2} \) strongly; Pt is a standard hydrogenation catalyst.
Option (3) Palladium — Incorrect. Takes up the largest volume of \( \ce{H2} \) of all (occlusion).
Option (4) Nickel — Incorrect. Adsorbs \( \ce{H2} \); finely divided Ni is used for the hydrogenation of oils.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Types of Adsorption – “… Sometimes these two processes occur simultaneously and it is not easy to ascertain the type of adsorption. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. For example, dihydrogen is first adsorbed on nickel by van der Waals’ forces. Molecules of hydrogen then dissociate to form hydrogen atoms which are held on the surface by chemisorption.”