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Give plausiblereasonsforthefollowing: (i) Purealumina isabadconductorof electricity. To makeit conducting, twosubstances (X)and(Y) are added to moltenaluminabeforecarryingoutelectrolysis usingcarbonelectrodes. Why arecarbonelectrodes preferredover metalelectrodes ?(ii) Duringelectrolysis, thesurfaceofthemoltenelectrolyte iscovered witha substance (Z). Identify the substances (X),(Y) and(Z) and explaintheirfunctions. |
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Answer» SOLUTION :(i) Al hasa verystrongaffinity for oxygen. The ENTHALPY offormationof aluminais-1670 kJ `mol^(-1) ` , higher(more negative) thanpractically all othermetaloxides.Consequently, all the electronsare firmly heldandhenceit isbadconductor ofelectricity. (ii)The TWOSUBSTANCES usually addedto moltenaluminato makeit conductingare:cryolite(X),`Na_3 AlF_ 6`and FLUORSPAR(Y),`Ca F _ 2 `. Boththese subtancesincreasethe conductivity of thesolutionby converting` Al_ 2O _3`into` AlF_ 3`as shownbelow : ` ""Ca F_ 2hArr Ca^(2+)+2 F^(-) ` `"" Na_ 3AlF_6hArr3 Na^(+) +AlF_ 3+3 F^(-) ` At anode.`F^(-)`ionslose electronsto the electrode producingF atoms. Theatomic F thenreactswith`Al_ 2O _ 3`toform` AlF_3 andO _ 2` `""F^(-)toF+e ^(-),2Al_ 2O_ 3+12 F to4AlF_ 3+3O_ 2` The` O _ 2`thusproduced,oxidisescarbonof anode to`CO_ 2` As aresult,consumption oftheanodeoccurs. Becauseof this,theanodeshavetoreplaced fromtime to time.Sincecarbonischeaperthanany metal, THEREFORE,carbonelectrodesare preferred. (iii)Further duringelectrolysis,the surfaceof the electrolyteiscovered withcoke(Z) topreventoxidationand lossofheaddue to radiation. |
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