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  4. The decomposition of phosphine [PH3] on tungsten at low pressure is a first-order reaction. It is because the

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Q. The decomposition of phosphine $[PH_3]$ on tungsten at low pressure is a first-order reaction. It is because the

NEETNEET 2016Chemical Kinetics

Solution:

$PH_3 \rightarrow{W} P + \frac{3}{2} H_2$
Rate = $k[PH_3]$
It is dependent on surface coverage because at low pressure, surface area covered is proportional to partial pressure of $PH_3$ and it is first order w.r.t $PH_3$.
$\theta = \frac{kP}{1 + kP}$
$\theta $ = fraction of surface area covered at low pressure $\theta$ = k P