Energetics

Key point: A catalyst increases the rates of processes by introducing new pathways with lower Gibbs energies of activation; the reaction profile contains no high peaks and no deep troughs. A catalyst increases the rates of processes by introducing new pathways with lower Gibbs energies of activation, ∆‡G. We need to focus on the Gibbs energy profile of a catalytic reaction, not just the enthalpy or energy profile, because the new elementary steps that occur in the catalysed process are likely to have quite different entropies of activation. A catalyst does not affect the Gibbs energy of the overall reaction, ∆_rG^O, because G is a state function.1 The difference is illustrated in Fig. 26.1, where the overall reaction Gibbs energy is the same in both energy profiles. Reactions that are thermodynamically unfavourable cannot be made favourable by a catalyst. Figure 26.1 also shows that the Gibbs energy profile of a catalysed reaction contains no high peaks and no deep troughs. The new pathway introduced by the catalyst changes the mechanism of the reaction to one with a very different shape and with lower maxima. However, an equally important point is that stable or nonlabile catalytic intermediates do not occur in the cycle. Similarly, the product must be released in a thermodynamically favourable step. If, as shown by the blue line in Fig. 26.1, a stable complex were formed with the catalyst, it would turn out to be the product of the reaction and the cycle would terminate. Similarly, impurities may suppress catalysis by coordinating strongly to catalytically active sites and act as catalyst poisons.

Figure 26.1 Schematic representation of the energetics of a catalytic cycle. The uncatalysed reaction (a) has a higher ∆‡G than a step in the catalysed reaction (b). The Gibbs energy of the overall reaction, ∆rG^O, is the same for routes (a) and (b). The curve (c) shows the profile for a reaction mechanism with an intermediate that is more stable than the product.

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