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 information:

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 to cathode.