Page 63 - MODERN ELECTROCHEMISTRY

8 CHAPTER 1

1.4.2. 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
4
supply, i.e., to a source of electrons so large in capacity that, say, to
5
electrons 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.

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