Question

Nitrogen dioxide cannot be obtained by heating:
This question has multiple correct options
A. ${{KN}}{{{O}}_{{3}}}$
B. ${{LiN}}{{{O}}_{{3}}}$
C. ${{Pb(N}}{{{O}}_{{3}}}{{{)}}_{{2}}}$
D. ${{NaN}}{{{O}}_{{3}}}$

Answer 1

Hint:We will write down the reaction for each given option, then we would know which nitrate does not decompose into nitrogen dioxide on heating. Alkali metal nitrates on heating decompose into a metal nitrite, and oxygen gas. In this case, the exception is lithium metal.

Complete step by step answer:
Now, we will write the reactions one by one. First, we have potassium nitrate.
-As we know, potassium is an alkali metal. So, as mentioned potassium nitrate will decompose into metal nitrite and oxygen gas.
-Thus, the chemical reaction can be written as:
${{KN}}{{{O}}_{{3}}}\xrightarrow{{{\Delta }}}{{KN}}{{{O}}_{{2}}}{{ + }}\dfrac{{{1}}}{{{2}}}{{{O}}_{{2}}}$
-We can say that potassium nitrate does not form nitrogen dioxide on heating.
-Second, we have lithium nitrate. We already mentioned that lithium is the only alkali metal which decomposes into nitrogen dioxide.
-So, we can write the chemical reaction as:
${{Li(N}}{{{O}}_{{3}}}{{)}}\xrightarrow{{{\Delta }}}{{L}}{{{i}}_{{2}}}{{O + N}}{{{O}}_{{2}}} \uparrow {{ + }}\dfrac{{{1}}}{{{2}}}{{{O}}_{{2}}}$
Here, we can say that lithium nitrate evolves nitrogen dioxide on heating.
-Third, we have lead nitrate. Lead nitrate will decompose into the lead, nitrogen dioxide, and oxygen on heating.
-Thus, the chemical reaction can be written as:
${{Pb(N}}{{{O}}_{{3}}}{{{)}}_{{2}}}\xrightarrow{{{\Delta }}}{{Pb}} \downarrow {{ + 2N}}{{{O}}_{{2}}} \uparrow {{ + }}{{{O}}_{{2}}} \uparrow $
-We can see that nitrogen dioxide is evolved by heating lead nitrate.
-Fourth, we have sodium nitrate. We know that sodium is an alkali metal. So, as mentioned it will form sodium nitrite on heating.
-So, the chemical equation can be written as:
${{NaN}}{{{O}}_{{3}}}\xrightarrow{\Delta }{{NaN}}{{{O}}_{{2}}}{{ + }}\dfrac{{{1}}}{{{2}}}{{{O}}_{{2}}}$
-We can say that nitrogen dioxide is not evolved on heating.
In the last, we can conclude that potassium nitrate and sodium nitrate does not form nitrogen dioxide on heating.
-Hence, the correct options are A and D.

Note:
-In the case of alkali metals, the nitrate of lithium (lithium nitrate) is the only one that decomposes into nitrogen dioxide on heating. There are specific reasons why lithium nitrate forms nitrogen dioxide on heating. The reasons are as follows:
-The nitrates of other alkali metals like sodium, potassium are thermodynamically stable but lithium nitrate is not stable.
-Lithium has a relatively small size as compared to other metals.
-Lithium cation is polarizing which leads to the formation of oxide rather than nitrite.