Electrochemistry at surfaces on open circuit

In addition, the cell resistance may be calculated from the electrolyte conductivity and the cell dimensions. Hence, all the terms in the cell voltage equation are readily accessible to measurement. This topic is treated in more detail in Chapter 2, Section 2.3.7. 

So far in this chapter, only electrochemical cells where the two electrodes are clearly identifiable and separate have been considered. There is, however, another type of situation where electrolysis can occur and which is important in technology. This arises whenever a conducting surface stands in an electrolyte medium which contains appropriate electroactive species to support both an oxidation and a reduction on the surface, e.g. consider the electrode reactions occurring simultaneously on the same surface  

Clearly, reaction, (1.131) can only occur by any mechanism if it is thermodynamically favourable, i.e. there is a negative-free energy associated with it. Hence, the electrochemical mechanism will be of interest if this reaction is kinetically hindered in a homogeneous solution or if the material of the surface is one of the reactants. In the former case, the surface plays the role of a catalyst, allowing the overall chemical change to occur via a mechanism involving the transfer of electrons to and from the surface. Note that since the surface is on open circuit (there are no external electrical connections) and it cannot retain a net charge, it is essential that the rates of the two electron-transfer reactions (1.129) and (1.130) are equal. 

In all cases, the conversion of A + P to B + Q obviously leads to a change in the concentrations of A, B, P and Q and this leads to a shift in the equilibrium potentials and a change in exchange current densities. Hence, the figure is strictly only correct at one instant in the reaction. 

The morphology and the catalytic activity of the surface will vary, especially if the reaction involves metal deposition or dissolution and this will again lead to the need for modification of the figure. 

Qearly, reaction, (1J31) can onJy occur by any mechaniim if it is thermodynamically favourable, i.e. there is a negative free energy associated with it. 

t has been assumed that mass transport is never important. 

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Diagram, after U. R. Evans, a pioneer in corrosion electrochemistry. Any corroding metal should, under open-circuit conditions, exist at corroding at a rate which can be predicted ft-om the Evans Diagram. The value of will depend on the position of the curves. The anodic curve is a function of the type of metal and the charge it receives when it goes into solution. The cathodic curve is a function of the type of cathodic reaction, the type of surface on which that reaction takes place, the concentration of reacting species, and other factors, most of which are dependent on the environment in which the metal sits. 

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