Spontaneous Redox Reactions
(Heath Chemistry Textbook, Pages 688 – 690)
Stop for a moment to think how strange and amazing it is that a chemical reaction
can light a flashlight or power a portable radio. The reactions that cause battery-operated
items to work are oxidation-reduction reactions. An electrochemical cell is a chemical
system in which an oxidation-reduction reaction can occur. When electrons flow from
the reducing agent to the oxidizing agent, an electric current is established in the cell.
Electrochemical cells in which the redox reaction occurs spontaneously are called
batteries or voltaic cells. The current produced by batteries can do useful electrical work,
such as running an electric motor, which converts electrical energy into mechanical
energy.
Today's society depends on voltaic cells. An automobile uses one type of voltaic
cell, a storage battery, to start its engine. A common battery, or dry cell, is another kind
of voltaic cell. Flashlights, toys, games, watches, and portable computers all operate on
power supplied by voltaic cells.
Simple Voltaic Cells
A voltaic cell can easily be constructed
so that it can do useful work, as shown to the
right. The beakers contain a solution copper(II)
sulfate, CuS04, and zinc sulfate, ZnS04. Both
copper(II) sulfate and zinc sulfate are
electrolytes, which, as you learned, are
substances whose water solutions conduct
electric current. A zinc strip is placed in the
zinc sulfate solution and a copper strip is placed
in the copper(II) sulfate solution. These metal
strips are electrodes, electrically conducting
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solids that are placed in contact with the electrolyte solutions. The salt bridge is
composed of a concentrated solution of a strong electrolyte, such as potassium nitrate.
In order to complete the circuit, a wire is connected electrode, and those wires are
attached to an LED (light diode). As soon as the circuit is complete, the LED lights up.
The voltaic cell is doing useful electrical work. However, in a minutes, the glow in the
LED begins to fade and will eventually go out.
Reactions that drive voltaic cells
A very simple observation helps to explain how the zinc-copper voltaic cell works.
When the mass of each electrode is measured before and after the reaction, it is found to
be different. The copper electrode has a greater mass after the reaction, and the zinc
electrode has less mass. As the cell works, the zinc electrode is oxidized, forming zinc
ions that are released into solution.
Zn -> Zn2+ + 2e-
The copper ions from the copper(II) sulfate solution are reduced at the copper
electrode.
Cu2+ + 2e- -> Cu
The zinc electrode loses electrons, while the copper electrode gains electrons.
The electrons flow through the wire from the zinc electrode to the copper electrode, as
shown in Figure 20-5. Sufficient current flows to cause the LED to light up.
What is the purpose of the salt bridge? If charges build up at either electrode,
they will repel like charges, and current will stop flowing. The salt bridge is necessary to
complete the circuit and prevent the buildup in each beaker of excess positive or negative
charges. As positively charged zinc ions, Zn2+, are formed in the left cell compartment,
negatively charged nitrate ions, NO3 -, are attracted from the salt bridge and enter the
beaker. At the same time, positively charged copper(II) ions, Cu2+, are removed from the
solution in the right cell compartment and potassium ions, K+, enter the beaker to take
their place. The contents of both beakers remain electrically neutral, even as the
concentrations of reactants and products change during the reaction. The current will
continue to flow in the cell until the cell reaches equilibrium.
Anodes and cathodes
Zinc metal is more easily oxidized than copper metal. Copper ions are more
easily reduced than zinc ions. The spontaneous reaction in the voltaic cell is expressed:
the sum of the equations for the two half-reactions.
Zn -> Zn2+ + 2eCu2+ + 2e- -> Cu
Cu2+ + Zn -> Zn2+ + Cu
In this example, the zinc electrode is called the anode, and the copper electrode is
called the cathode. The anode is the site of oxidation, and the cathode is the site of
reduction.