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Batteries work by converting stored chemical energy into electrical energy. Both automobile batteries as well as 9-V alkaline batteries use the chemical energy stored in different chemical substances to produce electrical energy.
Automobile batteries are created by joining in series 6 lead-acid type galvanic cells. Each of the individual cells provides a potential difference of 2.1 V and as potential difference is added when the cells are joined in series, 6 cells provide an approximate 12 V that is required for most automobiles. Car batteries are rechargeable and while they are discharged when used to start the engine and for lighting etc., they are recharged through the alternator of the car as it moves.
The reactions that take place in a car battery while it is discharging are:
Pb(s) + HSO4-(aq) --> PbSO4(s) + 2e− at the cathode
PbO2(s) + HSO4−(aq) + 3H+(aq) + 2e− --> PbSO4(s) + 2H2O(l) at the anode
The reverse reactions take place while charging.
In a 9-V alkaline battery, there are 6 1.5 V cells linked in series. The cells consist of a zinc anode and a manganese dioxide cathode. The electrolyte in the cells is the alkali potassium hydroxide. The reactions that take place in an alkaline battery while it discharges are:
Zn(s) + 2OH−(aq) --> ZnO(s) + H2O(l) + 2e− at the cathode, and
2MnO2(s) + H2O(l) + 2e− -->Mn2O3(s) + 2OH−(aq) at the anode
Most 9-V alkaline batteries cannot be recharged unlike a car battery.
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