Reaction on surfaces

Usually it is more convenient to use limiting cases as follows:

Case I

k₂, the rate of decomposition is very large compared with the rate of adsorption and desorption. In this case,
k₂>> (k₁C_a + k_-1) and hence equation (36.4.4) reduces to,

v = k₁ C_a

Physically, the assumption that k₂ is large implies that an adsorbed molecule decomposes immediately after coming in contact with the surface. This situation is found for the decomposition of Hl on Pt and N₂O on gold.

Case II

k₂ is very small (negligible) in comparison to (k₁C_a + k_-1) and hence equation (36.4.4) reduces to :


at low concentration of the species A, KC_a<< 1

Where as at high concentration KC_a >> 1 or 1

Sub-Case of I

A diatomic molecule A₂ dissociates upon adsorption to the surface.
This reaction can be written as

Because two surface sites are involved in the adsorption and desorption process, the rates of adsorption,_a, and desorption _d, are

Where, C_s = total surface sites/cm², = fraction of the surface covered and p represents the pressure of the molecules A₂.

At equilibrium, these rates are equal, and so