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Physical chemistry 128

A redox reaction is any reaction between two species where electrons are transferred
from one species to another. The species losing electrons is said to be oxidized; as it is
giving electrons away and reducing the other reactant it is also called a reducing agent
or reductant. The species gaining electrons is reduced and acts as an oxidizing agent or
oxidant, as it causes oxidation. In electrochemistry, this transfer of electrons between
chemical species is accomplished via an external electrical circuit by using an
electrochemical cell. This consists of two half-cells connected together. The simplest
half-cell consists of a metal M (or electrode) dipping into an aqueous solution of its
z+
metal ion, M (the electrolyte), which has the half-cell reaction:

The electrons in this reaction are deposited on the metal and their concentration and
energy, controlled by the relative stability of the metal and metal ion, determines the
electrode potential. A salt bridge, usually consisting of a solution or gel containing
saturated KCl in a glass tube, connects the two half-cell solutions (Fig. 1a). An
alternative method of separating the two half-cell solutions is a porous glass frit or
porous ceramic (Fig. 1b), which also allows the passage of ions without mixing the
solutions and enables the half cells and solutions to be combined into one container (see
Topic E5). This is experimentally simpler, but introduces an extra cell voltage due to the
liquid junction in the frit, and is thus avoided when making thermodynamic
measurements.

Galvanic and electrolytic cells

The difference in potential of the two metals results in a potential difference (also called
a voltage or electromotive force, emf) between the two half-cells. This can be measured
by means of a high impedance voltmeter (Fig. 1b), which measures the voltage or driving
force for reaction without allowing current to flow, from which can be calculated
thermodynamic data (see Topic E4). Alternatively the reaction can be allowed to proceed
by connecting the two half-cells by an outside circuit (a wire or a resistor) and allowing
the current to flow. These are both examples of galvanic cells, where the spontaneous
chemical reaction occurs (see Topic B5). Electrons flow from the electrode with the most
negative potential (the anode, where oxidation occurs) to that with the most positive
potential (the cathode, where reduction occurs). The salt bridge (or porous glass frit)
allows ions to transfer into each half-cell. This flow counteracts the imbalance of charge
that would develop in each half-cell as electrons pass from one electrode to the other,
which would inhibit the reaction. The need for a salt bridge or frit is avoided if both half-
cells can share a common electrolyte. This is a special case, where all redox active ions in
the solution react specifically at one half-cell electrode only and therefore do not have to
be separated from the other electrode.