pBlock ElementsII - Result Question 43
####43. The compound(s) which generate (s) $N _2$ gas upon thermal decomposition below $300^{\circ} C$ is (are)
(2018 Adv.)
(a) $NH _4 NO _3$
(b) $\left(NH _4\right) _2 Cr _2 O _7$
(c) $Ba\left(N _3\right) _2$
(d) $Mg _3 N _2$
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Solution:
- Among the given compounds, those which generate $N _2$ on thermal decomposition below $300^{\circ} C$ are ammonium dichromate i.e., $\left(NH _4\right) _2 Cr _2 O _7$ and barium azide or nitride i.e., $Ba\left(N _3\right) _2$. Reactions of their thermal decomposition are given below
(i) $\left(NH _4\right) _2 Cr _2 O _7 \xrightarrow[\text { Below } 300^{\circ} C]{\longrightarrow} N _2 \uparrow+Cr _2 O _3+4 H _2 O$
It is an exothemic reaction with
$$ \Delta H=-429.1 \pm 3 kcal / mol \text {. } $$
(ii) $Ba\left(N _3\right) _2 \xrightarrow[\text { Around } 160^{\circ} \text { and above }]{\Delta} Ba+3 N _2 \uparrow$
Ammonium nitrate $\left(NH _4 NO _3\right)$ on heating below $300^{\circ} C$ gives $N _2 O$ as
$$ NH _4 NO _3 \xrightarrow[\text { below } 300^{\circ} C]{\longrightarrow} N _2 O+2 H _2 O $$
However, on rapid heating or explosion
(i.e. above $300^{\circ} C$ ) it gives off nitrogen as
$$ 2 NH _4 NO _3 \xrightarrow[\text { or explosion }]{\text { Rapid heating }} 2 N _2+O _2+4 H _2 O $$
Magnesium nitride $\left(Mg _3 N _2\right)$ does not decompose at lower temperatures being comparatively more stable. Its thermal decomposition requires a minimum temperature of $700^{\circ} C$ and proceeds as
$$ Mg _3 N _2 \stackrel{700-1500^{\circ} C}{\longrightarrow} 3 Mg+N _2 \uparrow $$
