An oxide of nitrogen is reddish brown and paramagnetic at room temperature but it decolourises and also…

An oxide of nitrogen is reddish brown and paramagnetic at room temperature but it decolourises and also loses its paramagnetism on freezing it. The oxide at room temperature is
  1. Pure $\mathrm{NO}_{2}$
  2. Pure $\mathrm{N}_{2} \mathrm{O}_{4}$
  3. equilibrium mixture of $\mathrm{N}_{2} \mathrm{O}_{4}$ and $\mathrm{NO}_{2}$
  4. $\mathrm{N}_{2} \mathrm{O}_{5}$

Solution

$2 \mathrm{Pb}\left(\mathrm{NO}_{3}ight)_{2} ightarrow 2 \mathrm{PbO}+4 \mathrm{NO}_{2}+\mathrm{O}_{2}$
i.e. can be prepared by heating heavy metal nitrate of $\mathrm{Pb}\left(\mathrm{NO}_{3}ight)_{2}$. It is a red brown gas but on cooling below $273 \mathrm{~K}$ converts to colourless liquid and retains as dimer. In $\mathrm{NO}_{2}$ molecules $17$ valence $\mathrm{e}^{-}$are there ($12$ that of $\mathrm{O}$ & $5$ that of $\mathrm{N}$ ) i.e. one $\mathrm{e}^{-}$unpaired results to paramagnetic nature. So this is an odd $\mathrm{e}^{-}$molecule on being dimer with even $\mathrm{e}^{-}$becomes stable & loses paramagnetic characteristics.

Asked in: JEE-TOPICTESTS-CHEMISTRY

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