pBlock ElementsII 1 Question 43
43. The compound(s) which generate (s) $\mathrm{N} _{2}$ gas upon thermal decomposition below $300^{\circ} \mathrm{C}$ is (are)
(2018 Adv.)
(a) $\mathrm{NH} _{4} \mathrm{NO} _{3}$
(b) $\left(\mathrm{NH} _{4}\right) _{2} \mathrm{Cr} _{2} \mathrm{O} _{7}$
(c) $\mathrm{Ba}\left(\mathrm{N} _{3}\right) _{2}$
(d) $\mathrm{Mg} _{3} \mathrm{N} _{2}$
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Answer:
Correct Answer: 43. (b,c)
Solution:
- Among the given compounds, those which generate $\mathrm{N} _{2}$ on thermal decomposition below $300^{\circ} \mathrm{C}$ are ammonium dichromate i.e., $\left(\mathrm{NH} _{4}\right) _{2} \mathrm{Cr} _{2} \mathrm{O} _{7}$ and barium azide or nitride i.e., $\mathrm{Ba}\left(\mathrm{N} _{3}\right) _{2}$. Reactions of their thermal decomposition are given below
(i) $\left(\mathrm{NH} _{4}\right) _{2} \mathrm{Cr} _{2} \mathrm{O} _{7} \xrightarrow[\text { Below } 300^{\circ} \mathrm{C}]{\longrightarrow} \mathrm{N} _{2} \uparrow+\mathrm{Cr} _{2} \mathrm{O} _{3}+4 \mathrm{H} _{2} \mathrm{O}$
It is an exothemic reaction with
$$ \Delta H=-429.1 \pm 3 \mathrm{kcal} / \mathrm{mol} \text {. } $$
(ii) $\mathrm{Ba}\left(\mathrm{N} _{3}\right) _{2} \xrightarrow[\text { Around } 160^{\circ} \text { and above }]{\Delta} \mathrm{Ba}+3 \mathrm{N} _{2} \uparrow$
Ammonium nitrate $\left(\mathrm{NH} _{4} \mathrm{NO} _{3}\right)$ on heating below $300^{\circ} \mathrm{C}$ gives $\mathrm{N} _{2} \mathrm{O}$ as
$$ \mathrm{NH} _{4} \mathrm{NO} _{3} \xrightarrow[\text { below } 300^{\circ} \mathrm{C}]{\longrightarrow} \mathrm{N} _{2} \mathrm{O}+2 \mathrm{H} _{2} \mathrm{O} $$
However, on rapid heating or explosion
(i.e. above $300^{\circ} \mathrm{C}$ ) it gives off nitrogen as
$$ 2 \mathrm{NH} _{4} \mathrm{NO} _{3} \xrightarrow[\text { or explosion }]{\text { Rapid heating }} 2 \mathrm{N} _{2}+\mathrm{O} _{2}+4 \mathrm{H} _{2} \mathrm{O} $$
Magnesium nitride $\left(\mathrm{Mg} _{3} \mathrm{N} _{2}\right)$ does not decompose at lower temperatures being comparatively more stable. Its thermal decomposition requires a minimum temperature of $700^{\circ} \mathrm{C}$ and proceeds as
$$ \mathrm{Mg} _{3} \mathrm{N} _{2} \stackrel{700-1500^{\circ} \mathrm{C}}{\longrightarrow} 3 \mathrm{Mg}+\mathrm{N} _{2} \uparrow $$
