Question

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

Answer 1

The correct option is A

Rate is proportional to the surface coverage

$P{H}_3\stackrel{W}{\to }P+\frac{3}{2}{H}_2$
This is an example of surface catalyzed unimolecular decomposition.
For the above reaction, rate is given as
Rate=$k\frac{\alpha p}{1+\alpha p}$
Where, p =partial pressure of absorbing substrate.
At low pressure, $\propto p$ << 1 or Rate =$k\propto p$
So, $(\propto p+1)$ can be neglected.
Thus, the decomposition is predicted to be first order.