Which of the following is an example of a first-order reaction?

The correct answer is Option (1) → Hydrogenation of ethene, \( \ce{C2H4(g) + H2(g) -> C2H6(g)} \)

The rate of the hydrogenation of ethene depends on the first power of the concentration of ethene, so it is a first-order reaction, although two molecules appear on the left of the equation:

    \( \displaystyle \text{Rate} = k[\ce{C2H4}] \)

Why the other options are incorrect

Option (2) Decomposition of gaseous ammonia on a hot platinum surface at high pressure — Incorrect. At high pressure the platinum surface is saturated with gas molecules, so the rate does not depend on [NH₃]: a zero-order reaction.

Option (3) Thermal decomposition of HI on a gold surface — Incorrect. This is another zero-order reaction on a metal surface.

Option (4) Reaction of nitric oxide with oxygen, \( \ce{2NO(g) + O2(g) -> 2NO2(g)} \) — Incorrect. Its rate law is Rate = k[NO]²[O₂], so it is a third-order reaction.

NCERT (2026-27 edition):
3.3.2 First Order Reactions – “Hydrogenation of ethene is an example of first order reaction. C₂H₄(g) + H₂ (g) → C₂H₆(g) Rate = k [C₂H₄]”