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Describe about the working principle of Leclanche cell. |
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Answer» Solution :(i) Leclanche cell : `{:("Anode",:,"Zinc container"),("Cathode",:,"GRAPHITE rod in contact with" MnO_2),("ELECTROLYTE",:,"Ammonium chloride and Zinc chloride in water"):}` emf of the cell = 1.5 V. (ii) Cell reaction: Oxidation at anode `Zn_((s)) to Zn_((AQ))^(2+) + 2e^(-) ""............(1)` Reduction at cathode `2NH_(4(aq))^(+) + 2e^(-) to 2NH_(3(aq)) + H_(2(g)) "" ...........(2)` (iii) The hydrogen gas is oxidised to water by `MnO_2` `H_(2_(g)) + 2MnO_(2_(s)) to Mn_2O_(3_(s)) + H_2O_((l))"" .........(3) ` Adding equations 1,2,3 the overall redox reaction `Zn_((s)) + 2NH_(4(aq))^(+) + 2MnO_(2(s)) to Zn_((aq))^(2+) + Mn_2O_(3(s)) + H_2O_((l)) + 2NH_3 "" ............(4)` (iv) The AMMONIA produced at the cathode combines with `Zn^(2+)` to form a complex ion `[Zn(NH_3)_(4)]^(2+) (aq)`. As the reaction proceeds, the concentration of `NH_4^+` will decrease and the aqueous `NH_3` will INCREASE which lead to the decrease in the emf of the cell. |
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