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When conc. H(2)SO(4) was added to an unk...

When conc. `H_(2)SO_(4)` was added to an unknown salt present in a test which was heated, a brown gas (A) was evolved. The gas intenstified when copper turning were also added into the test tube. On cooling, the gas (A) changed into a colourless liquid (B).
(a) Identify the gases A and B
(b) Write equations for the reactions involved.

Text Solution

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When a nitrate salt is heated with conc. `H_(2)SO_(4)`, vapours of `HNO_(3)` are first produced which undergo decomposition to produce nitrogen dioxide `(NO_(2))` which is a brown gas (A) When Cu turnings are added, the gas (A) intensified due to the reduction of `HNO_(3)` by Cu.
`underset("sod. nitrate")(2NaNO_(3))+H_(2)SO_(4) overset("Heat")(rarr) underset(("Colourless"))underset("Nitric acid")(2HNO_(3))+Na_(2)SO_(4)`
`4HNO_(3) overset("Heat")(rarr) underset(("Brown gas A"))underset("Nitrogen dioxide")(4NO_(2)) + O_(2)+2H_(2)O`
`underset("Copper turnings")(CU) + 4HNO_(3) overset("Heat")(rarr) Cu(NO_(3))_(2) + underset("Nitrogen dioxide (A)")(2NO_(2))+2H_(2)O`
`NO_(2)` is an odd electron species (7 + 8 + 8 = 23 electrons). In the valence shell, it has only 7 electrons and hence is less stable. To become more stable by having 8 electrons in the valence shell, it undergoes dimerization to form a colourless gas, `N_(2)O_(4)`

Thus, (A) is nitrogen dioxide `(NO_(2))` and (B) is dinitrogen tetroxide `(N_(2)O_(4))`.
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