5 Steps to a 5 AP Chemistry

are flowing through the wire they can do useful work, like powering a calculator or a pace-
maker. Electrochemical cellsuse indirect electron transfer to produce electricity by a redox
reaction, or they use electricity to produce a desired redox reaction.

Galvanic (Voltaic) Cells

Galvanic (voltaic) cellsproduce electricity by using a redox reaction. Let’s take that zinc/copper
redox reaction that we studied before (the direct electron transfer reaction) and make it a gal-
vanic cell by separating the oxidation and reduction half-reactions. (See Figure 16.1.)
Instead of one container, as before, two will be used. A piece of zinc metal will be placed
in one, a piece of copper metal in another. A solution of aqueous zinc sulfate will be added
to the beaker containing the zinc electrode and an aqueous solution of copper(II) sulfate
will be added to the beaker containing the copper metal. The zinc and copper metals will
form the electrodesof the cell, the solid portion of the cell that conducts the electrons
involved in the redox reaction. The solutions in which the electrodes are immersed are
called the electrode compartments. The electrodes are connected by a wire and... noth-
ing happens. If the redox reactions were to proceed, the beaker containing the zinc metal
would build up a positive charge due to the zinc cations being produced in the oxidation
half-reaction. The beaker containing the copper would build up a negative charge due to
the loss of the copper(II) ions. The solutions (compartments) must maintain electrical neu-
trality. To accomplish this, a salt bridge will be used. A salt bridgeis often an inverted
U-tube that holds a gel containing a concentrated electrolyte solution, such as KNO 3 in
this example. Any electrolyte could be used as long as it does not interfere with the redox
reaction. The anions in the salt bridge will migrate through the gel into the beaker contain-
ing the zinc metal, and the salt-bridge cations will migrate in the opposite direction. In this
way electrical neutrality is maintained. In electrical terms, the circuit has been completed
and the redox reaction can occur. The zinc electrode is being oxidized in one beaker, and
the copper(II) ions in the other beaker are being reduced to copper metal. The same redox
reaction is happening in this indirect electron transfer as happened in the direct one:

The difference is that the electrons are now flowing through a wire from the oxidation half-
reaction to the reduction half-reaction. And electrons flowing through a wire is electricity,

Zn(s) Cu (aq)+→ +^22 ++Zn (aq) Cu(s)

Electrochemistry  243

Wire

Salt bridge

NO 3 − K+

Zn2+

SO 42 − SO 42 −

Cu2+

Zn(s)→ Zn2+ + 2 e− Cu2+ + 2 e− → Cu(s)

Zn (–) Cu (+)

Anode

(oxidation)

Cathode

(reduction)

Cathode

compartment

e−

e−

Anode

compartment

Figure 16.1 A galvanic cell.