Wet cell

From Wikipedia, the free encyclopedia

A wet cell is a galvanic electrochemical cell with a liquid electrolyte. A dry cell, on the other hand, is a cell with a pasty electrolyte. Wet cells were a precursor to dry cells and are commonly used as a learning tool for electrochemistry. It is often built with common laboratory supplies, like beakers, for demonstrations of how electrochemical cells work. A particular type of wet cell known as a concentration cell is important in understanding corrosion.

Batteries are usually a collection of several galvanic cells that work together to produce a greater charge than a single cell could. Car batteries are wet cells and give a good example of the pros and cons of such systems. A standard 12-V car battery consists of 6 lead acid cells that each produce 2 volts. The most commonly used lead-acid battery consists of a lead metal anode and a lead oxide cathode, both of which are immersed in a solution of sulfuric acid. As seen in car batteries, a disadvantage of such a system is that it is extremely heavy. On the plus side, however, the redox reaction that occurs is readily reversible allowing it to have a long, reliable, and useful life. In a car battery, the cell is recharged by the car's alternator.

[edit] Daniell cell

The most famous wet cell is the Daniell cell, which is sometimes referred to as a crowfoot or gravity cell. The Daniell cell was developed in 1836 by the British chemist (and meteorologist) John Daniell as a reliable source of steady electrical current. 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:

Image:Dry-cell.JPG

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, which must be a non-reactive conductor, 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.