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Pure HI kept in a bottle axquires a brow...

Pure `HI` kept in a bottle axquires a brown colour after sometime. Why?
(b) Ferric ioddic is very unstable but ferric chloride is stable. Why?
(c ) `KHF_(2)` is well known but `KHCl_(2)` and `KHBr_(2)` do not exist. Why?

Text Solution

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(a) `HI` is a strong reducing agent . It is oxidised by oxygen of the air, ie., aerial oxidised of `HI` occurs.
`{:(O_(2)+4H^(o+)+4e^(Ө)to2H_(2)O),([2I^(Ө)toI_(2)+2e^(Ө)]xx2),(ulbar(O_(2)+4H^(o+)+4I to2H_(2)O+2I_(2))):}`
The iodine liberated gives a brown colour to the solution.
(b) Iodide ion is a strong reducing agent and reduces ferric ions into ferrous ions, hence ferric iodide is unstable. On the other hand, chloride ion is a weak reducing agent and does not reduce ferric to ferrous ion and thus ferric chloride is stable.
(c ) Hydrofluoric acid exists as dimeric as dimeric molecule `(H_(2)F_(2))` due to hydrogen bonding . `HF` thus exhibits dibasic nature and forms two series of salts, `KHF_(2)` (as `K^(o+)` and `F^(Ө)....H-F)` and `KF(K^(o+) and F^(Ө))` .`HCl and HBr` exist as monomeric molecules as hydrogen bonding is not present.
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