Kinetic Considerations Concerning Redox Reactions

A converse exists to the calculation of equilibrium constants from the halfreduction potentials It is the possibility to obtain the unknown redox potentials of some couples. In order to achieve it, a redox equilibrium between two couples is investigated. The equilibrium constant is determined, if the standard redox potential of one of both couples is already known. The value of the other (unknown) is immediately deduced. This strategy is, of course, of great importance in physical and analytical chemistries. It is in this way that the standard potentials of slow electrochemical systems (see electrochemistry), in particular, those of organic redox couples, have been determined. 

The preceding considerations were based only on thermodynamic bases. They only permitted us to know whether or not the reaction was possible. A negative answer is definitive. If the answer is positive, it is not definite, however, that the reaction can take place, this time for kinetic reasons. We will now somewhat develop this point. 

Redox reactions may indeed be slow in water, contrary to the acid— base reactions, which can be considered immediate. If the reaction is very sluggish, no change is detectable. We are apparently in an equilibrium state that is actually a false equilibrium state. In other cases, the equilibrium may require a very long time to be reached, even if the reaction proceeds perceptibly. 

Here are some examples of redox reactions that are considered to be slow  

It is an experimental fact that when the electron exchange is not accompanied by a stmctural change of the Ox and Red forms of the couple, the reaction is fast. For example, this is the case with the couples Ag+/Ag(s), Fe +/Fe +, and [Fe(CN)6]3 /[Fe(CN)6f-. 

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The question of whether the oscillator model is adequate to the real conditions of electron transfer in solution is still open for discussion. This concerns, in particular, the assumption of a fixed distance between the two ions during the electron transfer, which means neglecting the kinetic energy of relative motion of the two ions. There is as yet no experimental evidence for the reliability of the oscillator model of electron transfer in polar liquids. Some experiments concerning redox reactions at electrodes will be discussed later. A consideration of the oscillator model for proton transfer and its experimental proof will be given in the next section. 

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