Galvanic cell

History

In 1780, Luigi Galvani discovered that when two different electrical batteries.

Description

 

A galvanic cell consists of two half-cells. Each half-cell has: (1) an 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 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 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 half reactions. The electric potential for the cell is equal to the more positive E^o value minus the more negative E^o 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

Main article: Galvanic corrosion

Galvanic corrosion is a process that degrades metals electrochemically. This 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

• Concentration cell
• Electrolytic cell
• Electrochemical cell
• Lasagna cell
• Lemon battery

See also

• Sacrificial anode
• Electrode potential
• Galvanic series
• Alessandro Volta
• Voltaic pile
• Volt
• Battery (electricity)
• Electrosynthesis

References

Galvanic cells v • d • e
Zinc-air battery
Lithium-ion polymer battery | Lithium-titanate battery | Vanadium-redox battery
Models:		Fuel cell
Parts:		Half-cell