The decomposition of phosphine (PH3) on tungsten at low pressure is a first-order reaction. It is because the
- rate is proportional to the surface coverage
- rate is inversely proportional to the surface coverage
- rate is independent of the surface coverage
- rate of decomposition is very slow.
Answer
(1) rate is proportional to the surface coverage
Solution
In a surface-catalysed decomposition, rate ∝ fraction of the surface covered by the gas.
At low pressure the surface coverage is small and is proportional to the pressure (concentration) of PH3:
Rate ∝ surface coverage ∝ pPH3 first order.
(At high pressure the surface is fully covered and the rate becomes independent of pressure, i.e. zero order.)
Answer: (1)