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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.
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.
Description
The Galvanic cell's metals dissolve in the electrolyte at two different rates, leaving some electrons in the rest of the metal, which charges it negative with respect to the electrolyte. Each metal undergoes a different half-reaction, giving different dissolving rates, which causes an unequal number of electrons in the two metals. This results in a different electrode potential between the electrolyte and each metal. If an electrical connection, such as a wire or direct contact, is formed between the two, an electric current appears in the metal. At the same time, ions of the more active metal, which forms the anode, are transferred through the electrolyte 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 away, the 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.
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.
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)
See also
- Electrode potential
- Galvanic series
- Concentration cell
- Alessandro Volta
- Voltaic pile
- Volt
- 1800
- Battery (electricity)
External links
- "Galvanic (Voltaic) Cells and Electrode Potential". Chemistry 115B, Sonoma.edu.
- "Making and testing a simple galvanic cell". Woodrow Wilson Leadership Program in Chemistry, The Woodrow Wilson National Fellowship Foundation.
Electrochemistry | Electric batteries | Corrosion
Galvanický článek | Galvanische Zelle | Galvana pilo | Celda galvánica | Galvanische cel | Ogniwo galwaniczne | Célula electroquímica | Гальванический элемент | Chemical cell | Galvanický článok | Galvanisk cell
"Galvanic cell"