Reaction Order and Adsorption in Kinetics of Electro-Organic Processes

Reaction Order and Adsorption in Kinetics of Electro-Organic Processes 

In examination of mechanisms of organic electrochemical reactions, it is useful to apply procedures which are employed for ordinary chemical reactions in addition to the methods which arise because the reaction is electrochemical. 

the reaction order in the reactant itself is useful to evaluate. If the reactant is likely to be adsorbed significantly, then the reaction order must be interpreted, as in other types of heterogeneous reactions, with due attention to the type of adsorption isotherm and the extent of coverage. Since the kinetics of electrode reactions depend in a primary way on the potential (see Section 4), the reaction order must be specified and evaluated with respect to a constant electrode potential. Usually a supporting (electrochemically inactive) electrolyte is used so that effects of ion distribution and potential in the double layer remain approximately constant as other quantities are varied in the experimental analysis, viz., pH, reactant concentration. 

When adsorption is significant in an electro-organic reaction, interpretations of reaction order in terms of mechanisms must be made with care. Consider a case where the current density at a given constant potential E is 

The quotient (In S)i/d In c) is the derivative of the adsorption isotherm Si = f c) at constant potential and may be obtained from experimental evaluation of the isotherm. Usually it is convenient to express d lnS)i/d lnc) as dSjdc c/Su since the latter derivative is usually easier to obtain, particularly in cases of isotherms having linear and transcendental terms. 

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