(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, HCI, HBr, HI

(c) Complete the following reaction :
$X e F_{4} + S b F_{5} \to$

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Solution

(a) Salt + Conc. $H_{2} S O_{4} \to$ Brown gas (A)

Salt + conc. $H_{2} S O_{4} + c u ⟶ B r o w n g a s (A) i n t e n s i f i e d$

Brown gas (A) $C o o l i n g - --- \to$ Colourless Solid (B)

(i) A = $N O_{2}$ gas ; B = $N_{2} O_{4}$ Solid

(iii) Because $N O_{2}$ dimerises as $N_{2} O_{4}$ due to the fact that $N O_{2}$ is an odd electron species\\

(b) HI > HBr > HCI > HF

(c) $X e F_{4} + S b F_{5} ⟶ [X e F_{3}]^{+} [S b F_{6}]^{-}$

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