Question

Which one of the following nitrates will decompose to give NO₂ on heating?

• A. NaNO₃
• B. KNO₃
• C. RbNO₃
• D. LiNO₃

Hint:

Lithium nitrate decomposes uniquely, forming lithium oxide, NO₂, and O₂, unlike other alkali nitrates that form nitrite and O₂.

Answer 1

The Correct Option is D

To determine which nitrate will decompose to give nitrogen dioxide (\(NO_2\)) upon heating, let's first understand the thermal decomposition behavior of alkali metal nitrates.

Theoretical Explanation: 

Nitrates of alkali metals (Group 1) typically decompose on heating to form different compounds. Here's what generally happens:

• For most alkali nitrates like sodium nitrate (\(NaNO_3\)), potassium nitrate (\(KNO_3\)), and rubidium nitrate (\(RbNO_3\)), decomposition results in the formation of the metal nitrite and oxygen gas. The equation for sodium nitrate is as follows:
  • \(2NaNO_3 \rightarrow 2NaNO_2 + O_2\)
• However, lithium nitrate (\(LiNO_3\)) behaves differently due to the small size and high charge density of the lithium ion. On heating, it decomposes to produce lithium oxide (\(Li_2O\)), nitrogen dioxide (\(NO_2\)), and oxygen gas. The decomposition of lithium nitrate is given by:
  • \(4LiNO_3 \rightarrow 2Li_2O + 4NO_2 + O_2\)
• This is because lithium exhibits properties somewhat similar to the alkaline earth metals, influencing its decomposition behavior.

Conclusion:

Given the options:

• \(NaNO_3\): Does not produce \(NO_2\) upon decomposition.
• \(KNO_3\): Does not produce \(NO_2\) upon decomposition.
• \(RbNO_3\): Does not produce \(NO_2\) upon decomposition.
• \(LiNO_3\): Decomposes to produce \(NO_2\).

The correct answer is LiNO₃ because it is the only nitrate among those listed that decomposes with the liberation of nitrogen dioxide.