More information about electrochemical cells
In an electrochemical cell, two half-cells are connected by
a salt bridge and a wire. Each half-cell consists of a metal (called
the electrode) and a salt solution of that metal. For example,
a half-cell might contain Zn(s) and ZnSO4(aq).
In the electrochemical cell, reduction takes place in the half cell that has a more positive
cell potential, and the oxidation occurs in other half cell. The electrode at which oxidation occurs is called
the anode, and the electrode at which reduction occurs
is called the cathode.
Electrons flow along the wire from the anode to the cathode
during the reaction. The salt bridge helps to equalize the build-up
of charge that would result if electrons simply flowed from one
compartment to the other. The salt bridge contains a solution
of a salt, such as KNO3, whose ions can flow freely
from the bridge into the solutions. In this manner, charge
does not build up, and electrons can continue to flow.
The example below shows a nickel/zinc cell. One compartment
contains ZnSO4(aq) and a zinc electrode, while the other
compartment contains Ni(NO3)2(aq) and a nickel
electrode. The compartments are connected by a KNO3 salt bridge
and a wire with a voltmeter.
A table of standard reduction potentials gives the following
Since Ni has a less negative reduction potential, Ni2+ is
reduced and and the zinc reaction is reversed to show that Zn is oxidized. The two half rections are:
According to the equations, the Zn half cell is producing electrons.
These electrons travel through the wire and the voltmeter to combine with
the Ni2+ in the Ni half cell. Because the elecrons flow from
the zinc cell to the nickel cell when Zn2+ is formed, the zinc
cell could begin to build up a positive charge and the nickel
cell a negative charge. However, the salt bridge alleviates any
charge build-up. If the zinc cell begins to acquire a positive
charge, the NO3- anions travel through the salt bridge
and into the zinc solution. Similarly, if the nickel cell begins to become
negatively charged, the K+ cations travel through the salt bridge
and into the nickel solution. Because net charges do not accumulate
in either compartment, electrons may continue to flow from anode
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