36.4.2 |
Bimolecular reactions on surfaces |
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A bimolecular reaction between two molecules A and B on a surface may occur through different alternative steps out of which following two are important |
(i) |
Langmuir – Hinshelwood mechanism |
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The two reacting molecules A and B react after being adsorbed on neighbouring sites on the surface of the catalyst. This mechanism is called as Langmuir - Hinshelwood mechanism. Reaction rate  , for such reaction may be written as follows: |
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Putting the values of
 from equations (36.2.15) and (36.2.16), we get |
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Above equation could be subjected to two special cases as follows: |
1. |
If the pressures of A and B species, pA and pB are both sufficiently low so that KA pA and KB pB may be neglected in comparison with unity, the rate equation becomes, |
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This would mean reaction to be second order. This is a frequently observed behaviour in Langmuir –Hinshelwood mechanism. |
2. |
If a reactant A is very weakly adsorbed, kA pA in the denominator of equation (36.4.11) may be neglected, and the rate equation become, |
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Reaction of hydrogen with ethylene on copper follows a nearly similar rate law as follows: |
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If reactant B is adsorbed very strongly such that KB pB >>1, equation (36.4.13) becomes |
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The rate is now inversely proportional to PB. Such behaviour is observed in reaction between carbon monoxide and oxygen on quartz and on platinum. In these cases, rate in inversely proportional to the pressure of carbon monoxide, which must be strongly adsorbed. |
(ii) |
Langmuir - Rideal mechanism |
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If reaction is due to collision of gaseous molecules A with adsorbed B molecules, the mechanism is known as Langmuir – Rideal mechanism and the rate law in this case will be, |
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The reaction of ethylene and H2 on copper surface presents a case of Langmuir – Rideal mechanism. C2H4 gets adsorbed much strongly on the surface. Rate equation for the reaction of C2H4 and H2 on copper surface follows as below: |
