Kinetic and thermodynamic effects

The interplay between thermodynamic and kinetic effects is very important, and as such it is possible to spend vast amounts of time and effort on it. This inevitably leads to complexity, which can be rather daunting and overbearing at first sight. However, thankfully, only a few basic ideas are actually needed to guide you through most of the situations that will arise at this level. 

The principal guideline is that reversible reactions are governed by thermodynamic considerations, while irreversible reactions are controlled by kinetic factors. After we have looked at a few general principles, we will examine these two topics in a little more detail. Finally, in this chapter, we will make a few general comments on catalysts, with particular reference to acid/base catalysis. 

In any general reaction between A and B to give C and D, the reagent A must physically approach the substrate B so that the electrons within each molecule may interact this results in some bonds being broken and re-formed so as to produce the new molecules C and D. This process may be represented on a diagram called a reaction co-ordinate diagram. 

On such a diagram, the ordinate, they-axis, represents the total free energy of the system the abscissa, the x-axis, represents how far the reaction has proceeded from the starting materials, A and B, to give the products, C and D. For a simple, one-step, reaction in which the products are more stable than the starting materials, the curve that represents the reaction pathway finishes lower than it started, i.e. the right hand side is lower than the left. 

For a two-step reaction, the reaction co-ordinate diagram is as follows. In this case, there is an energy well, i.e. there is a peak either side of it. The species that is caught in the energy well is called an intermediate and may be isolated. The presence of the peaks on either side of the intermediate indicates that energy is required for both the forward or the reverse reaction to proceed from this intermediate species, i.e. there is an activation energy barrier that the intermediate must overcome before it may revert either to the starting materials or proceed to the final products. This is indicative of a real species, which can be isolated. This is in contrast to a transition state, which occupies the cusp of the 

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The reaction of eq. 16.9 will regenerate the antioxidant Arj-OH at the expense of the antioxidant At2-OH. Despite the fact that such regeneration reactions are not simple electron transfer reactions, the rate of reactions like that of eq. 16.9 has been correlated with the E values for the respective Ar-0. Thermodynamic and kinetic effects have not been clearly separated for such hierarchies, but for a number of flavonoids the following pecking order was established in dimethyl formamid (DMF) by a combination of electrolysis for generating the a-tocopherol and the flavonoid phenoxyl radicals and electron spin resonance (ESR) spectroscopy for detection of these radicals (Jorgensen et al, 1999) ... 

Miodownik et al. 1979, Watkin 1979). Irradiation can cause void-swelling, suppression of a formation in stainless steels and non-equilibrium precipitation of silicides. These phenomena are complex and occur by a combination of thermodynamic and kinetic effects. However, it was shown by Miodownik et al. (1979) that a thermodynamic analysis could be used to good effect to rationalise the effect of radiation on silicide formation. Although the work was done for a simple alloy system, it demonstrates how thermodynamics can be used in unusual cirounstances. 

Even in the simplest situation for which a = a2 = 0.5, the global behavior of the response depends upon three parameters, the difference between the formal potentials AEf, and the rate constants of both steps k(j and k. Thus, the observed current-potential curves are the result of the interaction of thermodynamic and kinetic effects so the appearance of two or one waves would not be due solely to thermodynamic stability or instability of the intermediate species but also to a kinetic stabilization or destabilization of the same [4, 31]. This can be seen in Fig. 3.19 in which the current-potential curves of an EE process with AE = 0 mV taking place at a planar electrode with a reversible first step... 

Thermodynamic and Kinetic Effects on the Feasible Products of Reactive Distillation A-zeo-tropes and A-rheo-tropes... 

Complexes in which apparently genuine internal effects are observed or which contain ligands such as SO " and alkyl groups that are known to show marked thermodynamic and kinetic effects, are listed in Tables II and III (corrinoids). 

In the next chapter, some thermodynamic and kinetic effects will be considered. We have already seen an example of a thermodynamic effect, namely when the difference between intra- and inter-molecular hydrogen bonding was discussed. In the next chapter the reason why intramolecular hydrogen bonding is favoured over intermolecular hydrogen bonding will be examined. 

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