The rate of decomposition of gaseous ammonia on a hot platinum surface at a high pressure does not change when the initial concentration of ammonia is doubled. This is an example of _____.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Zero order reaction
For a reaction of order n, rate \( = k[\mathrm{A}]^{n} \). If the concentration is doubled, the rate is multiplied by \( 2^{n} \). A rate that does not change when the concentration is doubled means \( 2^{n} = 1 \), so n = 0: a zero order reaction.
Why the other options are incorrect
Option (1) Second order reaction — Incorrect. Doubling the concentration would make the rate \( 2^{2} = 4 \) times.
Option (2) First order reaction — Incorrect. The rate would double. (At low pressure, when the surface is not saturated, this decomposition does behave as first order.)
Option (4) Third order reaction — Incorrect. The rate would become \( 2^{3} = 8 \) times.
NCERT (2026-27 edition):
Zero Order Reactions – …
