Nitrogen dioxide (NO2) cannot be obtained by heating certain nitrates such as potassium nitrate (KNO3) and sodium nitrate (NaNO3) because these nitrates decompose mainly to give oxygen gas and their corresponding nitrites rather than NO2. In contrast, lead nitrate (Pb(NO3)2) decomposes on heating to give lead oxide (PbO), nitrogen dioxide (NO2), and oxygen (O2), so NO2 is obtained by heating lead nitrate, but not from potassium or sodium nitrates. Lithium nitrate (LiNO3) is an exception among alkali metal nitrates, as it does decompose on heating to give nitrogen dioxide due to the small size and polarizing power of the lithium ion. Thus, nitrogen dioxide cannot be obtained by heating potassium nitrate and sodium nitrate, but it can be obtained from lead nitrate and lithium nitrate by heating. This difference is due to the stability and decomposition pathways of the nitrates involved.
Summary of nitrates and NO2 formation by heating:
- KNO3 and NaNO3: Do NOT produce NO2 on heating.
- Pb(NO3)2: Produces NO2 on heating.
- LiNO3: Produces NO2 on heating (exception among alkali nitrates).
Therefore, nitrogen dioxide cannot be obtained by heating nitrates like potassium nitrate and sodium nitrate.