Electrochemical Cell

The practical problem when large numbers of sodium and chlorine atoms react is that the electrons are flowing in every direction—wherever a sodium atom can find a chlorine atom. We therefore cannot harness the electron flow to do useful electrical work. In order to use the electricity to light up a bulb, for example, we must make the electrons flow in a single direction through a wire; then we can put a bulb in their path and they will have to push through the filament to get from the sodium atoms to the chlorine atoms, lighting the filament up in the process. In other words, we must separate the sodium atoms from the chlorine atoms, so that they can only transfer their electrons on our terms: through the wire that we provide. Such an arrangement constitutes a voltaic or galvanic cell. It has the effect of converting chemical potential energy—a chemical push—into electrical potential energy—an electrical push: in other words, a voltage.

The sodium-plus-chlorine reaction is difficult to use in practice, because chlorine is a gas and sodium is a highly reactive metal that is nasty to handle. But many other chemical reactions can be used to make voltaic cells for generating electricity. All that is needed is a reaction between a substance (atoms, molecules, or ions) that wants to give up electrons and a substance that wants to grab onto electrons: in other words, an oxidation-reduction reaction. Then it is just a matter of arranging the substances so that the passing of electrons from one to the other must take place through an external wire. Strictly speaking, the resulting devices are voltaic cells, but people generally call them batteries.

As an illustration of how a voltaic cell works, we can choose the metallic elements silver (Ag) and copper (Cu) with their respective ions in solution, Ag⁺ and Cu⁺⁺. Because copper atoms are more eager to give up electrons than silver atoms are, the copper atoms will tend to force the Ag⁺ ions to take them. Or to say it the other way, Ag⁺ ions are more eager to grab electrons than Cu⁺⁺ ions are, so they will take them away from copper atoms to become neutral silver atoms. Thus, the spontaneous reaction that will take place when all four species are mixed together is

This equation says that a piece of copper metal dipped into a solution containing silver ions will dissolve and become copper ions, while at the same time silver ions "plate out" as metallic silver. (This is not how silver plating is done, however, because the silver comes out as a rough and non-adhering coating on the copper. The silver plating of dinnerware and jewelry is done in an electrolytic cell.) To make a useful voltaic cell out of the copper-silver system, we must put the Cu and Cu⁺⁺ in one container, the Ag and Ag⁺ in a separate container, and then connect them with a wire. Bars of copper and silver metal should be dipped into solutions of copper nitrate, Cu(NO₃)₂, and silver nitrate, AgNO₃, respectively. A salt bridge should be added between the two containers. It is a tube filled with an electrolyte—a solution of an ionic salt such as potassium nitrate KNO₃, which allows ions to flow through it. Without the salt bridge, electrons would tend to build up in the silver container and the reaction would stop because the negative charge has no place to go. The salt bridge allows the negative charge, this time in the form of NO₃<-b1.0001>- ions, to complete the circuit by crossing the bridge from the silver container back into the copper container. Now the circuit is complete and the reaction can proceed, producing a steady flow of electrons through the wire and keeping the bulb lit until something runs out—either the copper bar is all dissolved or the silver ions are all depleted: our "battery" is dead.

In principle, a voltaic cell can be made from the four constituents of any oxidation-reduction reaction: any two pairs of oxidizable and reducible atoms, ions, or molecules. For example, any two elements and their respective ions can be made into a voltaic cell. Examples: Ag/Ag⁺ with Cu/Cu⁺⁺ (as above), or Cu/Cu⁺⁺ with Zn/Zn⁺⁺ (zinc), or H /H⁺ (hydrogen) with Fe/Fe⁺⁺⁺ 2 (iron), or Ni/Ni⁺⁺ (nickel) with Cd/Cd⁺⁺ (cadmium). The last cell is the basis for the rechargeable nickel-cadmium (nicad) batteries that are used to power many electrical devices from razors to computers. When voltaic cells are used for portable purposes, they are "dry cells:" instead of a liquid solution, they contain a non-spillable paste. The lead storage battery in automobiles, however, does contain a liquid: a sulfuric acid solution.