Zero Order Reactions

A reaction is said to be of zero order if its rate is independent of the concentration of the reactants, i.e., the rate is proportional to the zeroth power of the concentration of the reactants.

For the reaction

A → products

to be of zero order,

-dx/dt = k[A]⁰ = k

Some photochemical reaction and a few heterogeneous reactions are zero-order reactants. Such reactions are not common.

Example:

1. Photochemical reaction between hydrogen and chlorine:

H₂(g) Cl₂(g) → 2HCl(g)

This photochemical reaction is zero-order reaction. The reaction is studied by placing H₂ and Cl₂ gases over water. the rate of reaction is studied by nothing the rate at which water rises in the vessel due to dissociation of HCl formed. the rate of rise of water is the same as the rate of disappearance of H₂ and Cl₂, i.e., the concentration of the gases phase will not change with time, although the quantities will change.

2. Decomposition of N₂O on hot platinum surface:

N₂O → N₂ + 1/2 O₂

Rate [N₂O]⁰ = k[N₂O]⁰ = k

d[N₂ O]/dt = k

3. Decomposition of NH₃ in presence of molybdenum or tungsten is a zero-order reaction.

[Mo]

2NH₃→ N₂ + 3H₂

The surface of the catalyst is almost completely covered by NH₃ molecules. The adsorption of gas on the surface cannot change by increasing the pressure or concentration of NH₃. Thus, the concentration of gas phase remains constant although the product is formed. Therefore, this reaction zero order kinetics.

Other examples of zero order are:

4. Decomposition of NI on the gold surface.

5. Iodations of acetone in presence of H⁺ ions.

H⁺

CH₃COCH + I₂ → ICH₂COCH₃ + HI

The rate equation of this reaction does not include [I₂] factor, i.e.,

-dx/dt = k[CH₃ COCH₃ ][H⁺]