An electrochemical cell allows a spontaneous reaction to be carried out without bringing the reactants into actual contact. The basic principle is that, since redox reactions occur via exchange of electrons, an electrical circuit can be devised that permits electrons to flow between reactants (which then gives rise to products) through the circuit, thus avoiding the need to bring the reactants into physical contact.
Suppose we wish to produce the reaction:
The reaction could be carried out by immersing a strip of solid copper
in a solution of Cu(NO 
) 
and a solid silver strip in a solution of
AgNO . The Cu(NO ) solution contains Cu 
ions and NO 
ions while the AgNO solution contains Ag 
and NO ions.
The solid strips or electrodes are connected by a wire, while the
two solutions are connected by a salt bridge. A salt bridge is simply
a tube that contains a salt solution, such as NaNO . The ends of the
tube a stopped by porous plugs which prevent the solutions from mixing but
allow ions to pass through. This has the effect of maintaining charge neutrality
in the two solutions.
Since the reaction is spontaneous, the copper electrode will be oxidized,
allowing copper ions to enter the Cu(NO ) solution. The electrons
given off by the oxidation flow through the circuit to the silver electrode.
These electrons are picked up by Ag ions which are attracted to the electrode
from the solution and ``plate out'' there.
The current that flows through the wire can be measured by placing an ammeter in the circuit. By convention, the electrode at which oxidation occurs is called the anode, and the electrode at which reduction occurs is called the cathode. Thus, in the above example, the copper electrode is the anode and the silver electrode is the cathode. The reactions that occur at the electrodes are the half reactions discussed above:
The notation for such an electrochemical cell is the following:
Cu|Cu ||Ag |Ag
which signifies that there is a copper electrode at the anode, and a
solution that contains Cu ions. The salt bridge is
designated as ||. Then there is a solution containing Ag ions and
a silver electrode at the cathode.
it Galvanic cells, electrochemical cells that produce useful electrical work via spontaneous redox reactions, can be used, for example, to produce current that could light a light bulb or run a clock or drive an electrical motor. The electrical energy is produced solely via the chemical reaction. By contrast, electrochemical cells which consume work and effect processes which would not otherwise occur spontaneously are called electrolytic cells. We will see an example of an electrolytic cell in the next lecture.
Electric charge will flow from one point to another when an
electrical potential difference 
exists between the
two points. This is also sometimes called an electromotive
force or emf. This can be measured by use of a voltmeter.
In addition, if a variable external voltage source is placed in the
circuit with a voltage 
, then the voltage
can also be measured by tuning the external voltage until
it equals the cell voltage . The net voltage read by the
voltmeter will be
If the external voltage source is kept just below , then the
reaction will occur very slowly, essentially reversibly, giving the
largest output of useful work.