a
\(\mathrm{PH}_{3} \stackrel{W}{\longrightarrow} \mathrm{P}+\frac{3}{2} \mathrm{H}_{2}\)

Rate \(=\mathrm{k}\left[\mathrm{PH}_{3}\right]\)

It is dependent on surface coverage because at low pressure, surface area covered is proportional to partial pressure

of \(\mathrm{PH}_{3}\) and it is first order w.t.t. \(\mathrm{PH}_{3}\)

\(\theta=\frac{k P}{1+k P}\)

\(\theta=\) fraction of surface area covered at low pressure \(\theta=\mathrm{k} \mathrm{P}\)