The decomposition of phosphine (PH3) on tungsten at low pressure is a first-order reaction. It is because the
(a) rate is proportional to the surface coverage
(b) rate is inversely proportional to the surface coverage
(c) rate is independent of the surface coverage
(d) rate of decomposition is very slow
(a) PH3 P +3/2. H2
This is an example of surface catalysed unimolecular decomposition.
For the above reaction, rate is given as
Rate = k/1+
where, = partial pressure of absorbing substrate.
At low pressure,