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:

(A), (B) and (D) only

The correct answer is Option (1) → (A), (B) and (D) only

First order reactions are those in which the rate depends on the concentration of one reactant.

A first order reaction is one in which the rate of reaction is proportional to the concentration of the reactant. To put it another way, doubling the concentration doubles the reaction rate. A first-order reaction can have one or two reactants, as in the case of the decomposition reaction.

Nuclear decay is an excellent example of a first order process. The rate of decay is simply proportional to the amount of the radioactive isotope. This is due to the fact that any isotope has the same chance of decaying at any given time.

A classic example is the thermal decomposition of nitrogen pentoxide. Other examples include radioactive decay, decomposition, and hydrolysis of aspirin.

Detailed Explanation

Option A: Artificial radioactive decay of unstable nuclei

Radioactive decay rate depends only on the number of undecayed nuclei present.

Rate ∝ N

Hence it follows first order kinetics.

Option B: Hydrogenation of ethylene

In presence of catalyst and excess hydrogen, rate depends mainly on ethylene concentration. Thus it behaves as pseudo first order reaction. Hence included.

Option C: Thermal decomposition of HI on gold surface

This is a surface catalyzed reaction. Once surface becomes saturated, rate becomes independent of HI concentration. Hence it behaves as zero order, not first order.

Option D: Decomposition of $N_2O$

Decomposition of nitrous oxide follows first order kinetics. Rate depends on concentration of $N_2O$.