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Explain the following `(i)` Excess of carbon is added to zinc metallurgy. `(ii)` In the elctrolysis of alumina, cryolite is added to alumina. `(iii)` Coke does not cause reduction of `Al_(2)O_(3)`. `(iv)` Gold is soluble in aquaregia, silver is not. |
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Answer» `(i)` As both `Zn` and `C` have nearly equal affinities for `O` atoms i.e. `(DeltaG^(@)=-360kJ "for" Zn)(DeltaG^(@)=-460kJ "for" C)` So excess of carbon is added to reduce `ZnO` to `Zn`. `2Zn+O_(2)to2ZnO` DeltaG^(@)=-360kJ`…….`(i)` `2C+O_(2)to2CO` `DeltaG^(@)=-460kJ`.........`(ii)` From equation `(i)` and `(ii)` we get, `ZnO+CtoZn+CO`, `DeltaG=-50kJ//mol` `(ii)` Pure alumina has very high fusion temperature and is bad conductor of electricity . To reduce its fusion temperature and to make it good conductor of electricity, cryolite is added. `(iii)` `Al_(2)O_(3)+Cto2Al+3CO` `DeltaG^(@)=3DelltaG_(f)^(@)(CO)-DeltaG_(f)^(@)(Al_(2)O_(3))` `=-3(137.2)+1582` `=+ve` Hence, reaction is not spontaneous `(iv)` Gold forms soluble `AuCl_(3)` while silver forms white precipitate. |
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