Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 64 / d

For competing reactant and product species on shared equivalent sites, add each $b_j{C}_j$ term to the denominator:Rapid product desorption makes product coverages small, reducing competitive site blocking and simplifying the rate to reactant coverage alone. Without that assumption, product adsorption can inhibit conversion and make the apparent rate evolve as products accumulate.

$0.1\mathrm{M}=100\mathrm{mol}{\mathrm{m}}^{-3}=10^5\mathrm{mmol}{\mathrm{m}}^{-3}$. ThereforeThe surface is almost saturated, so the rate approaches the supplied limiting constant. This assumes rapid adsorption equilibrium, negligible product coverage, no competing species and unchanged substrate area/photochemical conditions.

For simultaneous competitive adsorption, use the butyric-acid constant inferred in part (c), $b_b\simeq 1.21{\mathrm{m}}^3\mathrm{mmo}{\mathrm{l}}^{-1}$, and $C_b=10^6\mathrm{mmol}{\mathrm{m}}^{-3}$:The propionic-acid rate is therefore approximatelyThe abundant, more strongly adsorbing butyric acid occupies most common sites, reducing propionic conversion by roughly a factor of $35$. This calculation follows the requested competitive Langmuir model despite the physical concern about the fitted saturation density noted earlier.