The Continuous Flow of Electrons across an Interface Electrochemical Reactions

It has been argued in the preceding section that all surfaces carry an excess electric charge, i.e., that surfaces in contact with ionic solutions are electrified. However, the argument was made by considering an isolated piece of material unconnected to a source or sink of electrons.

Suppose now that the metal, an electronic conductor, is connected to a power supply,4 i.e., to a source of electrons so large in capacity that, say, 1019 to 1020 electrons5 drawn from the source leave it unaffected in any significant way. To make the discussion specific, assume that the electronic conductor is a platinum plate and the ionically conducting phase is an aqueous solution of HI.

Then, by connecting the electrical power source to the platinum plate, it becomes possible for electrons to flow from the source to the surface of the plate. Before this was done, the electrified platinum-solution interface was in equilibrium. Under these equilibrium conditions, the platinum plate had a net surface electric charge, and the ionically conducting solution had an equal excess electric charge, though opposite in sign. Furthermore, the passage of electrons across the interface, which is associated with electron-transfer reactions, is occurring at an equal rate in the two directions. What happens when a disequilibrating shower of extra charges from the power source arrives at the surface of the platinum? The details of what happens, the mechanism, is a long story, told partly in the following chapters. However, the essence of it is that the new electrons overflow, as it were, the metal plate and cross the metal-solution interface to strike and neutralize ions in the layer of solution in contact with the metal, e.g., hydrogen ions produced in solution from the ionization of HI in the solution phase. This process can proceed continuously because the power source supplying the electrons can be thought of as infinite in capacity and the ionic conductor also has an abundance of ions in it; these tend to migrate up to the metal surface to capture there some of the overflowing electrons.

What is being described here is an electrochemical reaction; i.e., it is a chemical transformation involving the transfer of electrons across an interface, and it can be written in familiar style as

The hydrogen ions are "discharged" (neutralized) on the electrode and there is an evolution of hydrogen outside the solution, as a gas.

4Actually, a power supply has two terminals and one must also consider how the metal-electrolyte interface is connected to the other terminal; however, this consideration is postponed until the next section.

5An Avogadro number (-1024) of electrons deposited from ions in solution produces 1 gram-equivalent (g-eq) of metal passed across an interface between metal and solution; hence 10l9to 102H electrons produce 10"5to 10"4 g-eq of material.

Fig. 1.4 The electrochemical reactor.

The simplicity of such a formulation should not obscure the fact that what has been described is a remarkable and distinctive part of chemistry. An electric current, a controllable electron stream, has been made to react in a controlled way with a chemical substance and produce another new chemical substance. That is what a good deal of electrochemistry is about—it is about the electrical path for producing chemical transformations. Much of electrochemistry is also connected with the other side of this coin, namely, the production of electric currents and therefore electric power directly from changes in chemical substances. This is the method of producing electrical energy without moving parts (see fuel cells, Chapter 13 in vol II).

1.4.3. Electrochemical and Chemical Reactions

There is another aspect of the electrochemical reaction that has just been described. It concerns the effect on the iodide ions of hydrogen iodide, which must also have been present in the HI solution in water. Where do they go while the hydrogen ions are being turned into hydrogen molecules?

The r ions have not yet appeared because only half of the picture has been shown. In a real situation, one immerses another electronic conductor in the same solution (Fig. 1.4). Electrical sources have two terminals. The assumption of a power source pumping electrons into a platinum plate in contact with an ionic solution is essentially a thought experiment. In the real situation, one immerses another electronic conductor in the same solution and connects this second electronic conductor to the other terminal of the power source. Then, whereas electrons from the power source pour into the platinum plate, they would flow away from the second electronic conductor (made, e.g., of rhodium) and back to the power source. It is clear that, if we want a system that can operate for some time with hydrogen ions receiving electrons from the

6There is not much limit on the kind of chemical substance; for example, it does not have to be an ion. C2H4 + 4 H20 —» 2 C02 + 12H+ + 12eisas much an electrochemical reaction as is 2 H+ + 2 e —> H2.

platinum plate, then iodide ions have to give up electrons to the rhodium plate at the same rate as the platinum gives up electrons. Thus, the whole system can function smoothly without the loss of electroneutrality that would occur were the hydrogen ions to receive electrons from the platinum without a balancing event at the other plate. Such a process would be required to remove the negatively charged ions, which would become excess ones once the positively charged hydrogen ions had been removed from the solution.

An assembly, or system, consisting of one electronic conductor (usually a metal) that acts as an electron source for particles in an ionic conductor (the solution) and another electronic conductor acting as an electron sink receiving electrons from the ionic conductor is known as an electrochemical cell, or electrochemical system, or sometimes an electrochemical reactor.

We have seen that electron-transfer reactions can occur at one charged plate. What happens if one takes into account the second plate? There, the electron transfer is from the solution to the plate or electronic conductor. Thus, if we consider the two electronic conductor-ionic conductor interfaces (namely, the whole cell), there is no net electron transfer. The electron outflow from one electronic conductor equals the inflow to the other; that is, a purely chemical reaction (one not involving net electron transfer) can be carried out in an electrochemical cell. Such net reactions in an electrochemical cell turn out to be formally identical to the familiar thermally induced reactions of ordinary chemistry in which molecules collide with each other and form new species with new bonds. There are, however, fundamental differences between the ordinary chemical way of effecting a reaction and the less familiar electrical or electrochemical way, in which the reactants collide not with each other but with separated "charge-transfer catalysts," as the two plates which serve as electron-exchange areas might well be called. One of the differences, of course, pertains to the facility with which the rate of a reaction in an electrochemical cell can be controlled; all one has to do is electronically to control the power source. This ease of control arises because the electrochemical reaction rate is the rate at which the power source pushes out and receives back electrons after their journey around the circuit that includes (Figs. 1.4 and 1.5) the electrochemical cell.

Thus, one could write the electrochemical events as

2 HI

Continue reading here: Equivalent Conductivity Varies with Concentration

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