cell, electrical

When electrical energy is produced from chemical energy using two metals acting as electrodes in a aqueous solution, it is sometimes known as a galvanic cell or voltaic cell. Here the two metals copper (+) and zinc (−) are immersed in dilute sulphuric acid, which acts as an electrolyte. If a light bulb is connected between the two, an electric current will flow with bubbles of gas being deposited on the electrodes in a process known as polarization.

Device in which chemical energy is converted into electrical energy; the popular name is ‘battery’, but this strictly refers to a collection of cells in one unit. The reactive chemicals of a primary cell cannot be replenished, whereas secondary cells - such as storage batteries - are rechargeable: their chemical reactions can be reversed and the original condition restored by applying an electric current. It is dangerous to attempt to recharge a primary cell.

Each cell contains two conducting electrodes immersed in an electrolyte, in a container. A spontaneous chemical reaction within the cell generates a negative charge (excess of electrons) on one electrode, and a positive charge (deficiency of electrons) on the other. The accumulation of these equal but opposite charges prevents the reaction from continuing unless an outer connection (external circuit) is made between the electrodes allowing the charges to dissipate. When this occurs, electrons escape from the cell's negative terminal and are replaced at the positive, causing a current to flow. After prolonged use, the cell will become flat (cease to supply current). The first cell was made by Alessandro Volta in 1800. Types of primary cells include the Daniell, Lalande, Leclanché, and so-called ‘dry’ cells; secondary cells include Planté, Faure, and Edison. Newer types include the Mallory (mercury depolarizer), which has a very stable discharge curve and can be made in very small units (for example, for hearing aids), and the Venner battery, which can be made substantially solid for some purposes. Rechargeable nickel-cadmium dry cells are available for household use.

The reactions that take place in a simple cell depend on the fact that some metals are more reactive than others. If two different metals are joined by an electrolyte and a wire, the more reactive metal loses electrons to form ions. The ions pass into solution in the electrolyte, while the electrons flow down the wire to the less reactive metal. At the less reactive metal the electrons are taken up by the positive ions in the electrolyte, which completes the circuit. If the two metals are zinc and copper and the electrolyte is dilute sulphuric acid, the following cell reactions occur.

The zinc atoms dissolve as they lose electrons (oxidation):

Zn −2e- → Zn2+

The two electrons travel down the wire and are taken up by the hydrogen ions in the electrolyte (reduction):

2H+ + 2e− → H2

The overall cell reaction is obtained by combining these two reactions; the zinc rod slowly dissolves and bubbles of hydrogen appear at the copper rod:

Zn + 2H+ → Zn2+ + H2

If each rod is immersed in an electrolyte containing ions of that metal, and the two electrolytes are joined by a salt bridge, metallic copper deposits on the copper rod as the zinc rod dissolves in a redox reaction, just as if zinc had been added to a copper salt solution:

Zn − 2e− → Zn2+

Cu2+ + 2e− → Cu

(overall: Zn + Cu2+ → Zn2+ + Cu)