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Explain the following: (a) In the prep...

Explain the following:
(a) In the preparation of `HI` from `KI` , phosphoric acid is preferred to sulphuric acid.
(b) Boiling point of `HCl` is lower than `HF`.
(c ) Bleaching powder loses its bleaching property when kept in an open bottle for a long time.

Text Solution

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Besides acidic nature of sulphuric acid, it acts as an oxidising agent. `H_(2)SO_(4)` oxidises `HI` (resucing agent) formed from `KI` into iodine. Thus, `H_(3)PO_(4)` is preferred as it does not oxidise `HI`
`2KI+2H_(2)SO_(4) rarr 2KHSO_(4)+2HI`
`H_(2)SO_(4)+2HI rarr I_(2)+SO_(2)+2H_(2)O`
(b) In `HF`, there is hydrogen bonding. It is, therefore, an associated liquid. No hydrogen bonding is present in `HCl`. Only van der Waals' force are present. this is the reason why boiling point of `HCl` is lower then `HE`.
(c ) Bleaching action of bleaching powder is due to release of `Cl_(2)` at the time of application. Chloirne is lost by bleaching powder on long standing in open by the following two processes.
(i) `CaOCl_(2) + CO_(2)(air) rarr CaCO_(3)+Cl_(2)`
(ii) `6CaOCl_(2) rarr 5CaCl+Ca(ClO_(3))_(2)`
Thus , `CaCl_(2)` and `Ca(ClO_3)_(2)` do not release `Cl_(2)` at the time of use. Hence, bleaching property is lost by bleaching powder on exposure for long time.
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