|MadSci Network: Engineering|
Dear Juliana; I assume you mean the common Zinc [Zn] - Carbon batteries, sometimes called the LeClanche cell. This contains the following major components: a Zn metal can, which is the anode, in the can is a mix of Manganese Dioxide [MnO2], Carbon Black, Ammonium Chloride, and water [this is the electrolyte]. Upright in the middle of this mix is the cathode which is a porous Carbon rod, with a metal cap. On the inner surface of the Zn can, the metal is oxidized. Zn = Zn [+2] + 2e [2 electrons] At the carbon cathode, MnO2 + H2O + e = MnO[OH] = OH- When the battery is used, these electrons leave the Zn metal, travel through the external circuit [flashlight bulb and wires for example], and enter the carbon electrode. When the battery is not in use, other reactions can also happen. Zn is an active metal and will slowly react with the water and ammonium chloride. That is, it corrodes to form Zn chloride and/or Zn hydroxide and some hydrogen gas is liberated. If this gas cannot leave the can through the porous carbon cathode, this will generate pressure and the can will swell. When the Zn corrodes it becomes thinner and finally it is perforated. Some of the liquid electrolyte will leak out, adsorb water from the air and the Zn will corrode rapidly. This is a simple explanation of this common problem. If you need more details, find a good book on basic electrochemistry. Regards; Charlie Crutchfield
Try the links in the MadSci Library for more information on Engineering.