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 _____.

Zero order reaction

The correct answer is Option (3) → Zero order reaction

The order of a reaction describes how the rate of reaction depends on the concentration of reactants.

The general rate law is:

Rate = k[A]ⁿ

where

k = rate constant

[A] = concentration of reactant

n = order of the reaction

In a zero order reaction, the rate of reaction is independent of the concentration of the reactant.

Rate law:

Rate = k

Thus, even if the concentration of the reactant changes, the rate remains constant.

Zero order reactions commonly occur in surface catalyzed reactions when the catalyst surface becomes fully saturated with reactant molecules. Under such conditions, increasing reactant concentration does not increase the reaction rate.

In the given case:

• Decomposition of ammonia occurs on a hot platinum surface.
• The surface becomes saturated with ammonia molecules at high pressure.
• Doubling the concentration of ammonia does not change the reaction rate.

This behavior is characteristic of a zero order reaction.

Option-wise Explanation

Option 1: Second order reaction

In a second order reaction, the rate depends on the square of the reactant concentration. Doubling the concentration would increase the rate four times. Since the rate does not change here, this option is incorrect.

Option 2: First order reaction

In a first order reaction, the rate is directly proportional to the concentration of the reactant. Doubling the concentration would double the rate. Since the rate remains unchanged, this option is incorrect.

Option 3: Zero order reaction

In a zero order reaction, the rate is independent of the concentration of the reactant. This situation often occurs in surface catalyzed reactions when the catalyst surface is saturated. Therefore this option is correct.

Option 4: Third order reaction

In a third order reaction, the rate depends on the cube of the concentration of reactant. Doubling the concentration would increase the rate eight times. Since no change occurs in the rate here, this option is incorrect.