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) .
The total reaction being:
- Zn(s) + Cu2+(aq) → Zn2+(aq) + 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 anode 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 anions move from the cathode to the anode via the salt bridge and the Zn2+ cations move in the opposite direction to maintain neutrality.
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