Reactions that involve a change in oxidation number are called oxidation-reduction reactions. An element is oxidized if the oxidation numberhas become more positive in value. For example, in the equation:the oxidation number of zinc has changed from 0 to +2. The term reductiondescribes the opposite process, in which the oxidation number becomes morenegative in value. In the same equation, for example, the hydrogen is reduced.The oxidation number has changed from +1 to 0. If everything is countedthrough the entire equation, oxidation and reduction are equal and balance to0.BATTERY AND FUEL CELLSWhen electric energy is needed, batteries and fuel cells are one way toprovide it. A battery chemically stores and then releases energy. A fuel cellconverts energy produced by a chemical reaction directly into usable power.Batteries range in size from single-cell models smaller than coins tomulti-cell units that fill large rooms. Portable radios, pocket calculators, watches,and hearing aids are typical devices powered by batteries. Very large batteryinstallations supply standby energy for equipment such as that in telephoneexchanges.Alessandro Volta, an Italian professor, devised the first battery in 1800 toprovide steady electric current for study and practical use. Before that time,only static electricity--a novelty with no practical value--could be produced.Batteries are either primary or secondary. A primary battery produces itsenergy by consuming one of the chemicals it contains. When the chemical isgone, the battery no longer produces energy and must be replaced. Thecarbon-zinc batteries used in flashlights and tape recorders are primary.Secondary batteries, or storage batteries, obtain energy by transforming certainkinds of chemicals to other kinds. When the change is complete, the battery nolonger produces energy. It can be renewed, or recharged, however, by sendingcurrent from another source through it to restore the chemicals to their...
