Page 202 - Instant notes

Physical chemistry 188

with

The Lindemann mechanism also explains another experimental feature of first order
reactions, that the value of k uni varies with pressure. Towards the high pressure limit
(where [M] is large and ) the value of k uni becomes independent of
pressure:

The physical basis for this is that at sufficiently high pressures the rate of collisional
activation of A to A* is sufficiently fast to always maintain equilibrium between the two
so that the rate determining step for overall reaction is the first order elementary
reaction from A* to P. Conversely, towards the low pressure limit (where
), the reaction effectively becomes bimolecular:

with the rate determining step being the rate at which the bimolecular collisions between
A and M yield activated A*.

Langmuir surface adsorption kinetics

Suppose a surface consists of a number of energetically equivalent adsorption sites to
which a gaseous reactant molecule A can bind reversibly:

and that θ denotes the fraction of all sites currently occupied. The rate of adsorption at
that instant is proportional to the pressure of A in the gas, p A, and to the fraction of sites
currently unoccupied (1–θ):
rate of adsorption=k 1 p A(1–θ)

where k 1 is the associated adsorption rate constant. The rate of desorption at the same
instant is proportional to the fraction of sites occupied:
rate of desorption=k −1θ