(a) When concentrated sulphuric acid was added to an unknown salt present in a test tube a brown gas (A) was evolved . This gas intensified when copper turnings were added to this test tube . On cooling, the gas (A) changed into a colourless solid (B) .
(i) Identify (A) and (B).
(ii) Write the structures of (A) and (B).
(iii) Why does gas (A) change to solid on cooling ?
(b) Arrange the following in the decreasing order of their reducing character :
HF , HCl , HBr , HI
(c) Complete the following reaction :
`XeF_(4) + SbF_(5) to `

Explanation

(i) `underset("Sodium nitrate")(2NaNO_(3)) + H_(2)SO_(4) overset(Delta)(to) underset("nitric acid")(2HNO_(3)) + Na_(2)SO_(4)`
`4HNO_(3) overset(Delta)(to) underset("Brown gas(A)")underset("Nitrogen dioxide")(4NO_(2)) + O_(2) + 2H_(2)O`
`Cu + 4HNO_(3) overset(Delta)(to) Cu(NO_(3))_(2) + underset((A))(2N)O_(2) + 2H_(2)O` and (B) is Dinitrogen tetraoxide `(N_(2)O_(4))`.
(ii) Structure : `NO_(2)(A) :`

(iii) `NO_(2)` (A) is an odd electron species which is less stable . To become more stable , it undergoes dimerization to form a colourless solid (B) `N_(2)O_(4)`.
(b) `HF gt HCl gt HBr gt HI` : decreasing order of their reducing character .
(c) `XeF_(4) + SbF_(5) to [XeF_(3)]^(+) [SbF_(6)]^(-)`