Which of the following is a zero order reaction?

The correct answer is Option (2) → Decomposition of \(NH_3\)

In a zero order reaction the rate does not depend on the concentration of the reactant: Rate = \( k[\mathrm{R}]^{0} = k \). Such reactions are uncommon; they occur mainly on metal surfaces and in some enzyme-catalysed reactions, where the surface (or the enzyme) is the limiting factor.

The decomposition of gaseous ammonia on a hot platinum surface at high pressure is the standard example. Ammonia covers the whole platinum surface; adding more ammonia cannot raise the amount adsorbed, so the rate stays constant:

    \( \displaystyle \ce{2NH3(g)} \xrightarrow[\text{1130 K}]{\text{Pt catalyst}} \ce{N2(g) + 3H2(g)} \)

    \( \displaystyle \text{Rate} = k[\ce{NH3}]^{0} = k \)

So the decomposition of ammonia is the zero order reaction.

Why the other options are incorrect

• Option (1) Decomposition of \(N_2O_5\) — Incorrect. \( \ce{2N2O5 -> 4NO2 + O2} \) is first order in \( \ce{N2O5} \).

• Option (3) Decomposition of \(N_2O\) — Incorrect. The decomposition of \( \ce{N2O} \) (\( \ce{2N2O -> 2N2 + O2} \)) is not a surface-saturated reaction of this kind; its rate depends on the concentration of \( \ce{N2O} \), so it is not zero order.

• Option (4) Radioactive decay of unstable nuclei — Incorrect. All radioactive decay follows first order kinetics; the rate is proportional to the number of nuclei present.

NCERT (2026-27 edition):
Zero Order Reactions – …

Note: The same/similar question has been asked in different years by NTA as per the details given below: