Electrode Reactions

Electrode Reactions A typical electrode reaction involves the transfer of charge between an electrode and a species in solution. The electrode reaction usually referred to as electrolysis, typically involves a series of steps: * Reactant (O) moves to the interface: this is termed mass transport * Electron transfer can then occur via quantum mechanical tunnelling between the electrode and reactant close to the electrode (typical tunnelling distances are less than 2 nm) * The product (R) moves away from the electrode to allow fresh reactant to the surface Figure 1 Animation of a simple electrode reaction The above electrode steps can also be complicated by: * The applied voltage on the electrode * The reactivity of the species * The nature of the electrode surface * The structure of the interfacial region over which the electron transfer occurs Before we begin to look at the above processes in detail, let us consider a few examples of electrode reactions. Perhaps the 'simplest' is a single electron transfer Figure 2 A single electron transfer reaction Here the reactant Fe 3+ moves to the interface where it undergoes a one electron reduction to form Fe 2+. The electron is supplied via the electrode which is part of a more elaborate electrical circuit. For every Fe 3+ reduced a single electron must flow. By keeping track of the number of electrons flowing (ie the current) it is possible to say exactly how many Fe 3+ molecules have been reduced. A further example of electrochemistry in action is metal deposition Figure 3 Copper deposition at a Cu electrode In this case the electrode reaction results in the fomation of a thin film on the original surface. It is possible to build up multiple layers of thin metal films simply by passing current through appropriate reactant solutions. A final example of electrode reactions is shown in figure 4 Figure 4 Electron transfer followed by chemical reaction In this case an organic molecule is reduced at the electrode forming the radical anion. This species however is unstable and undergoes further electrode and chemical reactions. We will see later how electrochemistry can be employed to study and initiate a whole range of chemical processes. Equilibrium electrochemistry The above are all examples of electrolysis reactions, where an electron is forced in or out of the electrode. In this situation a current flows for as long as the reaction continues. …