Galvanic cell

From Wikipedia, the free encyclopedia

You have new messages (last change).

The Galvanic cell, named after Luigi Galvani, consists of two metals connected by a salt bridge between the individual half-cells. It is also known as a voltaic cell and an electrochemical cell.

Contents

[edit] History

In 1780, Luigi Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this "animal electricity". The Voltaic pile, invented by Alessandro Volta in the 1800s, is a similar concept to the galvanic cell. These discoveries paved the way for all electrical batteries.

[edit] Description

Scheme of a galvanic cell
Enlarge
Scheme of a galvanic cell

The Galvanic cell's metals dissolve in the electrolyte at two different rates. Metals become positive ions upon dissolving, and leave electrons behind. As a result, the metal aquires a negative net charge while the electrolyte becomes equally positive. Each metal undergoes a different half-reaction, giving different dissolving rates, which builds up different electrode potentials between the electrolyte and each metal. If an electrical connection, such as a wire or direct contact, is formed between the two electrodes, an electric current appears in the metal. At the same time, an equal electric current composed of positive ions appears in the electrolyte. Ions of the more active metal which forms the anode are transferred to the electrolyte. Dissolved ions are also transferred to the less active metal, the cathode, and deposited there as a plating. In this way the anode is consumed or corroded. When the anode material corrodes entirely away, the cell's potential drops and the current halts. The metal may be regarded as the fuel which powers the device. A similar process is used in electroplating. The electric current in the electrolyte is equal to the current in the external circuit, so a complete circuit is formed with a path through the electrolyte.

There is a flow of electrons from the oxidized ion at the anode to the reduced atom (formerly an ion) at the cathode. It is this flow, due to this redox reaction which constitutes the current.

[edit] Electric potential of a Galvanic cell

The electrode potential of a cell can be easily determined by use of a standard potential table. An oxidation potential table could also be used, but the reduction table is more common. The first step is to identify the two metals reacting in the cell. Then one looks up the Eo (standard electrode potential, in volts) for each of the two half reactions. The electric potential for the cell is equal to the more positive Eo value minus the more negative Eo value.

For example, in the picture above the solutions are CuSO4 and ZnSO4. Each solution has a corresponding metal strip in it, and a salt bridge connecting the two solutions and allowing SO42− ions to flow freely between the copper and zinc solutions. In order to calculate the electric potential one looks up copper and zinc's half reactions and finds that:

Cu2+ + 2e− → Cu (E = +0.34 V)
Zn2+ + 2e− → Zn (E = −0.76 V)

Thus the reaction that is going on is really

Cu2+ + Zn → Cu + Zn2+

The electric potential is then +0.34 V −(−0.76 V) = 1.10 V

If the cell is operated under non-standard conditions, the potentials must be adapted using the Nernst equation.

[edit] Galvanic corrosion

Galvanic corrosion occurs when two dissimilar metals are placed in contact with each other in the presence of an electrolyte, such as salt water, resulting in the unintentional formation of a galvanic cell and concomitant chemical reaction of the metals involved. There are several ways of reducing and preventing this form of corrosion. One way is to electrically insulate the two metals from each other, for example by using plastic or fibre washers to separate steel water pipes from copper-based fittings or by using a coat of grease to separate aluminum and steel parts. Another way is to keep the metals dry and/or shielded from ionic compounds (salts, acids and bases), for example by encasing the protected metal in plastic or epoxy. Another method, called "cathodic protection", uses one or more sacrificial anodes made of a metal which is more active than the protected metal. Metals commonly used for sacrificial anodes include zinc, magnesium, and aluminum. Finally, an electrical power supply may be connected to oppose the corrosive galvanic current. (see Impressed-Current Cathodic Protection)

[edit] Cell types

[edit] See also

[edit] External links

Retrieved from "http://en.wikipedia.org../../../g/a/l/Galvanic_cell.html"