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Explain the following (i) Excess of ca...

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.

Text Solution

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`(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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