Page 65 - MODERN ELECTROCHEMISTRY

10 CHAPTER 1

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