1. S. Lobo
ond C. A. Bernardo Unversidode de Lourenco Marques Mozambique
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Textbook Errors, 719
Adsorption Isotherms and Surface Reaction Kinetics
In elementary textbooks of heterogeneous catalysis,lJ as well as in more advanced ones,3 a detailed discussion is included of the kinetics of some simple surface reactions. In the particular case of the himolecular reaction A B X, a rate expression can be derived by using the Langmuir adsorption isotherm to relate the gas pressures PAand PBto the corresponding surface coverages HA and B"
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The derivation of this equation implies that the product, X, is not adsorbed, and no reverse reaction takes place. The reaction is said to follow a Langmuir-Hinshelwood mechanism in this case. It is usually noted that if the pressure of one of the reactants (say, P g ) is kept constant and the other varied, the rate first increases, passes through a maximum and then decreases. The maximum is attained, some authors state, when bAPA= ~ B Pand , thus an approximate ratio of the adsorption coefficients may be calculated. In a recent test we asked our students to actually prove that eqn. (1) has a maximum when bAPA= bnP". To our surprise the less rated students found some way to prove it, the average students were not able to solve the question, and the best students found instead, the condition bAPA= 1 + bnPH. This pmved to be correct as can easily be found by derivating eqn. (1) and will be discussed below. The Meaning of the Adsorption Terms
Eqn. (1) has various particular cases of interest. If A is weakly adsorbed OA '5 0 and bnPA
