the zinc metal has reacted to colorless Zn^2 ions. The spontaneous redox re-
action is
Zn (s) Cu^2 →Zn^2 Cu (s) E° 1.104 V
However, in this example, the reaction occurred spontaneously and we were
not able to extract any useful work out of the reaction.
Suppose we were to set up the same reaction, but with the oxidation and re-
duction half-reactionsphysically separated, as in Figure 8.3. On the left side,
zinc metal can be oxidized to zinc ions, and on the right side the copper ions
are reduced to copper metal. The two half-reactions aren’t completely sepa-
rated. A salt bridgeconnects them to maintain an overall charge balance. The
salt bridge allows positive ions to flow into the reduction side of the system,
and negative ions to flow into the oxidation side of the system. In both cases,
this acts to preserve the electrical neutrality of each side.* Some conducting
medium, usually a wire, connects the two metal electrodes.If we attach some
electrical device such as a voltmeter or a lightbulb to the wire, we can operate
the device: we can extract work from the spontaneous electrochemical reaction,
as shown in Figure 8.3. By separating the individual half-reactions, we can get
energy in terms of electrical work from the spontaneous chemical reaction.214 CHAPTER 8 Electrochemistry and Ionic Solutions
*Other methods besides salt bridges are also used to maintain charge balance.Figure 8.3 The same redox reaction as in Figure 8.2 is shown, but now each half-reaction is
physically separate from the other. As this redox reaction occurs, we can extract useful work from
the transfer of electrons, as shown.© Richard Magna/Fundamental Photographs