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Describe about lead stroage battery construction and its uses. |
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Answer» Solution :Lead STORAGE battery : (i) `{:("Anode",:,"Spongy lead"),("Cathode",:,"Lead plate bearing" PbO_2),("Electrolyte",:,"38% by mass of " H_2SO_4" with density 1.2 g//ml"):}` (ii) Oxidation occurs at the anode `Pb_((s)) to Pb_((aq))^(2+) + 2e^(-) "".............(1)` The `Pb^(2+)` ions COMBINE with `SO_4^(2-)` to form `PbSO_4` precipitate `Pb_((aq))^(2+) + SO_(4)^(2-) to PbSO_(4(s)) ""..........(2)` (iii) Reduction occurs at the cathode `PbO_(2(s)) + 4H_((aq))^(+) + 2e^(-) to Pb_((aq))^(2+)+ 2H_2O_((l)) ""...........(3)` The `Pb^(2+)` ions also combines with `SO_4^(2-)` ions to form sulphuric acid to form `PbSO_4` Precipitate. `Pb_((aq))^(2+) + SO_(4(aq))^(2-) to PbSO_4 "" ...............(4)` (vi) The overall reaction is, `(1) + (2) + (3) + (4)` `Pb_((s)) + PbO_(2(s)) + 4H_((aq))^(+) + 2SO_(4(aq))^(2-) to 2PbSO_(4(s)) + 2H_2O_((l))` (v) The emf of a single cell is about 2V. Usually six such cells are combined in series to produce 12 volts. (vi) The emf of the cell DEPENDS on the concentration of `H_2SO_4`.As the cell reaction uses `SO_(4)^(2-)` ions, the concentration `H_2SO_4` decreases. When the cell potential falls to about 1.8 V, the cell has to be rechanged. (vii) Recharge of the cell : During reachrge process, the role of anode and cathode is REVERSED and `H_2SO_4` is regenerated. Oxidation occurs at cathode (now anode) `overset(+2)(PbSO_(4_(s))) + 2H_2O_((l)) + overset(+4)(PbO_(2_(s))) + 4H_((aq))^(+) + overset(2+)(SO_(4(aq))) + 2e^(-)` Reduction occurs at anode (now cathode) `PbSO_(4_(s)) + 2e^(-) to Pb_((s)) + SO_(4(aq))^(2-)` Overall reaction `2PbSO_(4(s)) + 2H_2O((I)) to Pb_((s)) + PbO_(2(aq)) + 4H_((aq))^(+) + 2SO_(4(aq))^(2+)` The above reaction is exactly the reverse of redox reaction which takes place while discharging. (VIII) Uses : Lead storage battery is used in automobiles, trains, inverters. |
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