Project ForgeA vapour phase catalytic reaction (Q+R→S) follows Rideal mechanism (R and S are not adsorbed). Initially, the mixture contains only the reactants in equimolar ratio. The surface reaction step is rate controlling. With constants a and b, the initial rate of reaction (−r0) in terms of total pressure (PT) is given by
- A−r0 = aPT/(1+bPT)
- B−r0 = aPT/(1+bPT²)
- −r0 = aPT²/(1+bPT)
- D−r0 = aPT²/(1+bPT)²
Explanation
In the Rideal mechanism, only Q adsorbs: rate=k·θQ·PR, with θQ=K·PQ/(1+K·PQ) (Langmuir isotherm for the single adsorbed species). Since the mixture is equimolar, PQ=PR=PT/2 initially, so PQ and PR are both proportional to PT. The numerator k·θQ·PR then scales as PT² (via PQ·PR in the numerator once θQ's denominator is cleared), while the denominator (1+K·PQ) scales as (1+bPT) — giving −r0=aPT²/(1+bPT).
Source: GATE CH 2014 (2014)
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