Galvanic cell

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The Galvanic cell, named after Luigi Galvani, consists of two different metals connected by a salt bridge or a porous disk between the individual half-cells. It is also known as a voltaic cell or electrochemical cell. It should not be confused with the electrolytic 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 similar to the galvanic cell. These discoveries paved the way for electrical batteries.

Description

File:Galvanische Zelle.png
Schematic of Zn-Cu galvanic cell

A galvanic cell consists of two half-cells. Each half-cell has: (1) an electrode, which in the figure are the plates of Zn (zinc) and Cu (copper); and (2) an electrolyte, which in the figure are aqueous solutions of ZnSO4 and CuSO4. The metal of a metallic electrode tends to go into solution, thereby releasing positively charged metal ions into the electrolyte, and retaining negatively charged electrons on the electrode. Thus each half-cell has its own half-reaction. For the Daniell cell, depicted in the figure, the Zn atoms have a greater tendency to go into solution than do the Cu atoms. More precisely, the electrons on the Zn electrode have a higher energy than the electrons on the Cu electrode. Because the electrons have negative charge, to give electrons on it a higher energy the Zn electrode must have a more negative electrical potential than the Cu electrode. However, in the absence of an external connection between the electrodes, no current can flow.

When the electrodes are connected externally (as in the figure, with wire and a lightbulb), the electrons tend to flow from the more negative electrode (Zn) to the more positive electrode (Cu). Because the electrons have negative charge, this produces an electric current that is opposite the electron flow. At the same time, an equal ionic current flows through the electrolyte. For every two electrons that flow from the Zn electrode through the external connection to the Cu electrode, on the electrolyte side a Zn atom must go into solution as a Zn2+ ion, at the same time replacing the two electrons that have left the Zn electrode by the external connection. By definition, the anode is the electrode where oxidation (removal of electrons) takes place, so in this galvanic cell the Zn electrode is the anode. Because the Cu has gained two electrons from the external connection, it must release two electrons at the electrolyte side, where a Cu2+ ion plates onto the Cu electrode. By definition, the cathode is the electrode where reduction (gain of electrons) takes place, so the Cu electrode is the cathode.

A good way to remember which process takes place at which electrode is to remember that anode and oxidation both begin with vowels, while reduction and cathode both begin with consonants. Also, by taking the first three letters from reduction and cathode respectively, Red Cat can be formed. Similarly, by taking the first two letters from oxidation and anode a similar descriptor is found in An Ox.

Notation

The galvanic cells, as the one shown in the figure, are conventionally described using the following notation:

Zn(s) | ZnSO4(aq) || CuSO4(aq) | Cu(s)
(anode)........................(cathode)

where: (s) denotes solid; (aq) means aqueous solution; the vertical bar, |, denotes a phase boundary; and the double vertical bar, ||, denotes a liquid junction, for example a salt bridge, for which the junction potential is near zero [1].

Corrosion

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 that powers the device. A similar process is used in electroplating. The ionic 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.

As can be seen, electrons flow from the oxidized ion at the anode to the reduced atom (formerly an ion) at the cathode. The flow due to this redox reaction 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 or porous disk 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 is a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are placed in contact with each other in the presence of an electrolyte, such as salt water, forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.

Cell types

See also

External links

References

  1. Atkins, P., "Physical Chemistry", 6th edition, W.H. Freeman and Company, New York, 1997

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