(1) $\ce{PH3(g) <=>[W]PH3(adsorption)->[W]P(s) + 3/2 H2(g)}$

The decomposition of phosphine on tungsten surface at low pressure is a pseudo first order reaction. This is because the rate of the reaction depends on the partial pressure of phosphine and which depends on the surface area of tungsten covered by phosphine.

$\ce{r= k.(P_{PH3})= k'*(covered surface area of tungsten)}$

But I am not able to understand the pseudo term used for the order of this reaction.

At high pressure, this reaction follows zero order kinetics due to complete coverage of surface area of tungsten.

Rate at high pressure= k

(2) The formation of any gas at low pressure on the surface of tungsten due to adsorption is zero order reaction.

At low pressure, according to Freundlich adsorption isotherm, the extent of adsorption varies linearly with external pressure as:

$\ce{x/m = k*p^{1/n}}$, where $\ce{x}$ and $\ce{m}$ are the masses of adsorbate and adsorbent.

Thus, the extent of adsorption is independent of the concentration of gas. Which means this is a zero order reaction.

Do in anyway (1) and (2) contradict?

(3) Is chemisorption a pseudo second order reaction because of involvement of chemical reaction in the rate determining step? as mentioned here



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