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Galvanic cell
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The Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells. It is sometimes called a Voltaic cell.
Common usage of the word "battery" has evolved to include a single Galvanic cell, but the first batteries had many Galvanic cells. The Galvanic cell should not be confused with the electrolytic cell.
780, 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.
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Encyclopedia
The Galvanic cell, named after Luigi Galvani, is a part of a battery consisting of an electrochemical cell with two different metals connected by a salt bridge or a porous disk between the individual half-cells. It is sometimes called a Voltaic cell.
Common usage of the word "battery" has evolved to include a single Galvanic cell, but the first batteries had many Galvanic cells. The Galvanic cell 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
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 voltmeter), 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, from the Copper Sulfate, 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. Electrons will flow from the Anode to the Cathode.
An essential piece of a Galvanic Cell is the salt bridge. The salt bridge serves the vital role of allowing each half-cell's charge to be balanced while preventing the solutions from mixing with each other. Should the solutions be mixed, the reaction will be purely chemical and will not require electrons to flow through the wire -- thus, no electricity can be harbored from the cell. Should the charges not be balanced, the anode will have an abundance of positive ions, and the cathode will have an abundance of negative ions. Electrons will not flow with this imbalance. The salt in the salt bridge, however, will ionize and neutralize the charges in each half-cell, allowing the reaction to proceed.
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)
An alternate notation for this cell would be:
Zn(s) | Zn+2(aq) || Cu+2(aq) | Cu(s)
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 which the junction potential is near zero, such as a salt bridge.
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
The potential of a cell can be determined by use of a standard potential table for the two half cells involved. An oxidation potential table could also be used, but the reduction table is more common. The calculation assumes that the cell operates at zero current flowing through the circuit.
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 overall reaction that is going on is
- Cu2+ + Zn ? Cu + Zn2+.
The electric potential is then +0.34 V - -0.76 V = 1.10 V under standard conditions and when no current flows in the cell.
If the cell is operated under non-standard conditions, the potentials must be adapted using the Nernst equation. If a current is allowed to flow in the circuit, the potential is going to shift towards zero in comparison with that predicted by the Nernst equation.
When measurements of the cell potential are performed, then the positive terminal of the electrometer needs to be be connected to the right-hand half-cell. This is because the potential of the cell is defined as:
- ECell = ERight - ELeft
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
- . Chemistry 115B, Sonoma.edu.
- . Woodrow Wilson Leadership Program in Chemistry, The Woodrow Wilson National Fellowship Foundation.
- An animation.
- . Chemical Education Research Group, Iowa State University.
- . Chemistry 30, Saskatchewan Evergreen Curriculum.
- . Spark Notes, by Barnes & Noble. Sparknotes.com.
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