http://www.ck12.org Chapter 23. Electrochemistry
FIGURE 23.7
An electrolytic cell uses an external power
source (a battery) to drive a nonspon-
taneous reaction. The copper half-cell
undergoes oxidation, while the zinc half-
cell undergoes reduction.Recall that in the last section, this same pair of half-cells was used as an example of a voltaic cell. In the spontaneous
direction, Zn metal is oxidized to Zn^2 +ions while Cu^2 +ions are reduced to Cu metal. In a voltaic cell, the zinc
electrode would be the anode, and the copper electrode would be the cathode. However, when the same half-cells are
connected to a battery via an external wire, the reaction is forced to run in the opposite direction. The zinc electrode
is now the cathode and the copper electrode is the anode.
Oxidation (anode): Cu(s)→Cu^2 +(aq)+2e− E^0 =− 0 .34 V
Reduction (cathode): Zn^2 +(aq)+2e−→Zn(s) E^0 =− 0 .76 V
Overall reaction: Cu(s)+Zn^2 +(aq)→Cu^2 +(aq)+Zn(s) E^0 cell=− 1 .10 VThe standard cell potential is negative, indicating a nonspontaneous reaction. The battery must be capable of
delivering at least 1.10 V of direct current in order for the reaction to occur. Another difference between a voltaic
cell and an electrolytic cell is the signs that are commonly given to the electrodes. In a voltaic cell, the anode is
negative and the cathode is positive. In an electrolytic cell, the anode is positive because it is connected to the
positive terminal of the battery. The cathode is negative. Electrons still flow through the cell from the anode to the
cathode.
Examples of Electrolysis Reactions
Several electrolysis reactions are commonly performed on a large scale for the commercial production of certain
substances. In this section, we will examine three examples of electrolysis.