Ask A Scientist Chemistry Archive

(Created prior to 1993)

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