The thermal decomposition of HI on gold surface is an example of

Zero order reaction

The correct answer is Option (2) → Zero order reaction

Reactions occurring on metal surfaces (heterogeneous catalysis) often become zero order when the surface is fully covered by reactant molecules.

Option 1: First order reaction In a first order reaction, rate depends on concentration of one reactant. In surface reactions like HI decomposition on gold, once the surface is saturated, the rate no longer depends on concentration, so it does not follow first order kinetics.

Option 2: Zero order reaction When HI molecules adsorb onto the gold surface, active sites become fully occupied. Once saturation occurs, the rate depends only on surface reaction and becomes independent of HI concentration. Hence the reaction follows zero order kinetics.

Option 3: Second order reaction Second order reactions depend on concentration of two reactants or square of one reactant. Surface reactions after saturation do not show such dependence.

Option 4: Pseudo first order reaction Pseudo first order occurs when one reactant is in large excess in solution. Here the reaction is controlled by surface saturation, not excess reactant concentration.