Choose the examples of first order reactions.
(A) Artificial radioactive decay of unstable nuclei
(B) Hydrogenation of ethylene
(C) Thermal decomposition of HI on gold surface
(D) Decomposition of \( N_2O \)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (B) and (D) only
In a first order reaction the rate is proportional to the first power of the concentration of one reactant, Rate = k[R], and the half-life does not depend on the initial concentration. In a zero order reaction the rate does not depend on concentration at all; this happens in some reactions on a metal surface, once the surface is fully covered by the gas.
(A) Artificial radioactive decay of unstable nuclei — Correct. Radioactive decay, natural or artificial, follows first order kinetics.
(B) Hydrogenation of ethylene — Correct. Its rate depends on the first power of the ethene concentration.
(C) Thermal decomposition of HI on gold surface — Incorrect. \( \ce{2HI(g)} \xrightarrow{\ce{Au}} \ce{H2(g) + I2(g)} \) is a zero order reaction: the gold surface is saturated with HI, so changing the concentration of HI does not change the rate.
(D) Decomposition of \(N_2O\) — Correct. \( \ce{2N2O(g) -> 2N2(g) + O2(g)} \) is first order, like the decomposition of \( \ce{N2O5} \).
NCERT (2026-27 edition):
First Order Reactions – …
Zero Order Reactions – “… The decomposition of gaseous ammonia on a hot platinum surface is a zero order reaction at high pressure.
\( \displaystyle \ce{2NH3(g)} \xrightarrow[\text{Pt catalyst}]{1130\ \mathrm{K}} \ce{N2(g) + 3H2(g)} \)
In this reaction, platinum metal acts as a catalyst. At high pressure, the metal surface gets saturated with gas molecules. So, a further change in reaction conditions is unable to alter the amount of ammonia on the surface of the catalyst making rate of the reaction independent of its concentration. The thermal decomposition of HI on gold surface is another example of zero order reaction.”