Question

Oxide of nitrogen $N{O_2}$ is obtained by heating:
A) $CsN{O_3}$
B) $KN{O_3}$
C) $LiN{O_3}$
D) $NaN{O_3}$

Answer 1

Here, all compounds are alkali metal nitrates which on thermal decomposition give metal nitrite and oxygen gas. But there are some exceptions which release nitrogen dioxide gas. The metal which is highly polarisable releases nitrogen dioxide gas.

Complete solution:
Alkali metal nitrates on heating produce respective nitrite and oxygen gas. The general representation of the reaction is as follows:

MN{O_3}\,\xrightarrow{\Delta }\,MN{O_2}\,\,\, + \,\,\,\,\,\frac{1}{2}{O_2} \\\ nitrate \,\,\,\,\,\,\,\,\,\,\,\,\,nitrite\,\,\,\,\,oxygen gas \\\ \end{gathered} $$ Here, M represents the alkali metals. The decomposition reaction of the caesium is as follows: $$CsN{O_3}\,\xrightarrow{\Delta }\,CsN{O_2}\, + \,\frac{1}{2}{O_2}$$ Here we can see caesium nitrate on heating gives caesium nitrite and oxygen gas indicates,$$N{O_2}$$ is not produced from the caesium nitrate. Hence, option (a)$$CsN{O_3}$$ is incorrect. The decomposition reaction of the potassium is as follows: $$KN{O_3}\,\xrightarrow{\Delta }\,KN{O_2}\,\,\, + \,\,\,\frac{1}{2}{O_2}$$ Here we can see potassium nitrate on heating gives potassium nitrite and oxygen gas indicates,$$N{O_2}$$ is not produced from the potassium nitrate. Hence, option (b)$$KN{O_3}$$ is incorrect. The decomposition reaction of the lithium is as follows: But there is an exception to this decomposition reaction. Lithium metal on heating decomposes into lithium oxide, nitrogen dioxide and oxygen gas $$2LiN{O_3}\,\xrightarrow{\Delta }\,L{i_2}O\, + \,2N{O_2}\, + \frac{1}{2}{O_2}$$ Lithium cation is more polarisable among all alkali metal cations because of the smaller size of the lithium. It leads to produce lithium oxide instead of the lithium nitrite and shows different behaviour than other alkali nitrates. Here, the decomposition of the lithium nitrate produces$$N{O_2}$$.The decomposition reaction of the sodium is as follows: $$NaN{O_3}\,\xrightarrow{\Delta }\,NaN{O_2}\,\,\, + \,\,\,\frac{1}{2}{O_2}$$ Here, sodium nitrate on heating gives sodium nitrite and oxygen gas indicates$$N{O_2}$$ is not produced from the sodium nitrate. Hence, option (d)$$NaN{O_3}$$ is incorrect. **Hence, the option (C) $$LiN{O_3}$$ is the correct answer to the question.** **Note:** Along with the group from top to bottom of the periodic table atomic as well as ionic size increases. Smaller is the size of the metal cation, more will be the polarisability. The metal cation with higher polarisability releases nitrogen dioxide gas on heating.