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Date: Before 1993


Question:
What are some chemicals used in batteries? I am doing this for an advanced chem project.



Replies:
The common dry cell (also called the LeClanche cell) is composed of a carbon rod stuck into a damp paste made of NH4Cl, ZnCl2, and MnO2, in a zinc (Zn) canister. The anode reaction is the oxidation of Zn(s) to Zn2+, and the cathode reaction is the reduction of NH4+ to form NH3(g) and H2(g). (These gases are converted by the other chemicals to water and zinc salts). Then, there is the alkaline battery. It also uss a zinc anode, but the container is a steel case which serves as a conductor of electrons to the oxidizing agent, in this case, MnO2. So here the anode reaction is Zn(s) + OH -) ZnO(s) + H2O(l) + 2 e- and the cathode reaction is 2MNo@ + H2O(l) +2 e- -) Mn2O3(s) + 2OH-- No gases are formed in the alkaline battery, unlike the dry cell. Oh, I neglected to mention that the damp paste inside the alkaline battery is basic, or alkaline (in case you did not notice) which is why Zn 's oxidation chemistry is different in this cell. The mercur (watch)battery is similar to the alkaline battery, but the cathode is mercury oxide...there is also som NaOH or KOH in the paste. The cathode reaction: HgO(s) + H2O(l) +2e- -) Hg(l) + 2OH- All three of these batteries have a common limitation; when the chemicals inside all reach equilibrium, the battery is useless and cannot be readily recharged (so it has to be thrown away, or, better yet, recycled). That is why a second kind of battery, called a "storage battery," has also been developed.

STORAGE BATTERIES: The most common kind of storage battery is the lead storage battery used in your car. The anode is made of porous lead (Pb) and the cathode is made of compressed lead oxide (PbO2). The electrodes are arranged in an alternating array, hanging in a bath of sulfuric acid (H2SO4) and separated by thin fiberglass sheets. The anode reaction: Pb(s) + SO4(2-) (aq) -) PbSO4(s) +2e- . The cathode reaction is PbO2(s) + 4H+(aq) + SO4(aq) + 2 e- -) PbSO4(s) + 2H2O(l). The upshot is that as the cell is used to supply electricity, both electrodes get coated with a film of lead sulfate and the sulfuric acid is consumed. Adding together these reactions properly, we get the overall chemistry: Pb(s)+PbO2(s)+2H2SO4(aq) -) 2PbSO4(s) + 2H2O(l). By supplying electrical energy to the battery, this reaction can be reversed and the lead sulfate forced back into solution, and reconstituting the electrodes. This can only happen so many times, however, until the electrodes become so pitted and corroded that they cannot be reformed again. Then you go to Sears and buy a new one. Another storage battery is the rechargeable ni-cad batteries you buy at the store. "Ni-cad" is short for nickel-cadmium alkaline battery. The anode reaction: Cd(s) +2OH-(aq) -) Cd(OH)2(s) + 2e-. The cathode: NiOOH(s) + H2O(l) + e- -) Ni(OH)2(s) + OH- (aq). These are the most commonly used batteries and the chemistry by which they work. Please let me know if you need to know more about anodes, cathodes, oxidation, reduction, and so forth. I would like to mention that my source of information was the text "Chemistry and Chemical Reactivity" by Kotz and Purcell. It is a college-level chem text they use(d) at Yale when I taught there, and a wonderful source of general information.

Topper



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