Daniell cell

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The Daniell cell (var. sp. Daniel cell), also called the gravity cell or crowfoot cell was invented in 1836 by John Frederic Daniell, who was a British chemist and meteorologist. The Daniell cell was a great improvement over the voltaic pile used in the early days of battery development. The Daniell cell's theoretical voltage is 1.1 volts and the chemical reaction is

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s).

1 Porous pot cell
2 Gravity cell
3 Chemistry
4 See also
5 External links

[edit] Porous pot cell

The Daniell cell proper consists of a central zinc anode dipping into a porous earthenware pot containing zinc sulfate solution. The porous pot is, in turn, immersed in a solution of copper sulfate contained in a copper can, which acts as the cell's cathode. The use of a porous barrier prevents the copper ions in the copper sulfate solution from reaching the zinc anode and undergoing reduction. This would render the cell ineffective by bringing the battery to equilibrium without driving a current.

[edit] Gravity cell

Early 20th-century engraving of a gravity cell. Note the distinctive crowfoot shape of the zinc anode.

In the 1860s, a Frenchman named Callaud invented a variant of the Daniell cell which dispensed with the porous barrier. Instead, a layer of zinc sulfate sat on top of a layer of copper sulfate, the two kept separate by their differing densities. The zinc anode was suspended in the top layer whilst the copper cathode sat in the bottom layer. A layer of oil was often added on top to prevent evaporation. This variant was known as the gravity cell, and sometimes the crowfoot cell due to the distinctive shape of the electrodes. This arrangement was less costly for large multicell batteries but could not be moved and was vulnerable to loss of integrity if too much electric current was drawn, which would cause the layers to mix. The crowfoot cell was also called the Exchange Telegraph Cell because it was used by the Exchange Telegraph Company.

[edit] Chemistry

In the Daniell cell, copper and zinc electrodes are immersed in a solution of copper (II) sulfate and zinc sulfate respectively. At the anode, zinc is oxidized per the following half reaction:

Zn_(s) → Zn²⁺_(aq) + 2e^- .

At the cathode, copper is reduced per the following reaction:

Cu²⁺_(aq) + 2e^- → Cu_(s) .

In the Daniell cell which, due to its simplicity, is often used in classroom demonstrations, a wire and light bulb may connect the two electrodes. Electrons that are “pulled” from the zinc travel through the wire, providing an electrical current that illuminates the bulb. In such a cell, the sulfate ions play an important role. Having a negative charge, these anions build up around the anode to maintain a neutral charge. Conversely, at the cathode the copper (II) cations accumulate to maintain this neutral charge. These two processes cause copper solid to accumulate at the cathode and the zinc electrode to "dissolve" into the solution.

Since neither half reaction will occur independently of the other, the two half cells must be connected in a way that will allow ions to move freely between them. A porous barrier or ceramic disk may be used to separate the two solutions while allowing ion flow. When the half cells are placed in two entirely different and separate containers, a salt bridge is often used to connect the two cells. In the above wet-cell, sulfate ions move from the cathode to the anode via the salt bridge and the Zn²⁺ cations move in the opposite direction to maintain neutrality.