Problem 68 - A Complete Resource Book in Chemistry for JEE Main

Question

Nitrogen dioxide cannot be prepared by heating (a) \(\mathrm{KNO}_{3}\) (b) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) (d) \(\mathrm{AgNO}_{3}\)

Step-by-Step Solution

Answer

Nitrogen dioxide cannot be prepared by heating \( \mathrm{KNO}_{3} \).

Step 1: Analyzing Chemical Formulas

We need to determine which compound does not produce nitrogen dioxide upon heating. The options provided are: (a) \( \mathrm{KNO}_{3} \), (b) \( \mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2} \), (c) \( \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2} \), and (d) \( \mathrm{AgNO}_{3} \). All these compounds are nitrates, which are known to decompose upon heating.

Step 2: Understanding Decomposition Reactions

Typically, when nitrates of heavy metals like \( \mathrm{Pb} \), \( \mathrm{Cu} \), and \( \mathrm{Ag} \) are heated, they decompose to form nitrogen dioxide \( \mathrm{NO}_{2} \). This means (b), (c), and (d) potentially form \( \mathrm{NO}_{2} \) upon heating.

Step 3: Examining Potassium Nitrate (KNO3)

Potassium nitrate \( \mathrm{KNO}_{3} \) decomposes differently. It primarily forms potassium nitrite \( \mathrm{KNO}_{2} \) and releases oxygen \( \mathrm{O}_{2} \), without forming \( \mathrm{NO}_{2} \) when heated.

Step 4: Conclusion

The compound that does not form nitrogen dioxide upon heating is \( \mathrm{KNO}_{3} \). The reaction for \( \mathrm{KNO}_{3} \) is as follows: \( 2\mathrm{KNO}_{3} \rightarrow 2\mathrm{KNO}_{2} + \mathrm{O}_{2} \). Hence, the correct answer is option (a).

Key Concepts

Nitrate Decomposition

Nitrates are chemical compounds that consist of the nitrate ion, \( \mathrm{NO}_3^- \). These compounds are known for their decomposition upon heating, a process important in various chemical and industrial settings.
Usually, the decomposition of nitrates results in the breaking down of the compound to produce other substances. When nitrates such as those of lead, copper, or silver are subjected to heat, they typically break down into nitrogen dioxide \( \mathrm{NO}_2 \), a brown gas with a sharp odor.

• Nitrates of heavy metals like \( \mathrm{Pb(NO}_3)_2 \), \( \mathrm{Cu(NO}_3)_2 \), and \( \mathrm{AgNO}_3 \) generally decompose into their respective metal oxides, nitrogen dioxide, and oxygen.
• For example, \( \mathrm{Pb(NO}_3)_2 \) breaks down into \( \mathrm{PbO} \), \( \mathrm{NO}_2 \), and oxygen \( \mathrm{O}_2 \).

Understanding the typical decomposition patterns of nitrates helps in predicting the products formed in reactions and is significant in applications like fireworks and fertilizers.

Nitrogen Dioxide Formation

Nitrogen dioxide \( \mathrm{NO}_2 \) is a reddish-brown gas formed commonly during the decomposition of nitrates of heavy metals due to heating. It is a notable component of air pollution and can be hazardous to health.
The formation process involves the breakdown of nitrates:

• Heavy metal nitrates upon heating decompose into metal oxides, nitrogen dioxide, and oxygen.
• The brown fumes often seen during these reactions are due to \( \mathrm{NO}_2 \) gas.

This formation is crucial in understanding not only chemical reactions in laboratory and industrial settings but also its environmental impact since \( \mathrm{NO}_2 \) contributes to phenomena like smog and acid rain.

Potassium Nitrate Reaction

Potassium nitrate \( \mathrm{KNO}_3 \) stands out from its heavy metal counterparts when it comes to decomposition reactions. When \( \mathrm{KNO}_3 \) is heated, it does not produce nitrogen dioxide. Instead, it decomposes differently:

• The main reaction for \( \mathrm{KNO}_3 \) involves forming potassium nitrite \( \mathrm{KNO}_2 \) and releasing oxygen \( \mathrm{O}_2 \).
• Therefore, heating \( \mathrm{KNO}_3 \) yields an environment with less pollution compared to the formation of \( \mathrm{NO}_2 \).

This distinct reaction is useful in contexts where \( \mathrm{NO}_2 \) emissions must be minimized, making \( \mathrm{KNO}_3 \) valuable in applications like fertilizers and explosives where controlled release of oxygen is desired.