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

(a) \( \text{KNO}_3 \) cannot be used to prepare \( \text{NO}_2 \) by heating.

Step 1: Analyze the Reaction of KNO3

When potassium nitrate \( (\text{KNO}_3) \) is heated, it decomposes to form potassium nitrite \( (\text{KNO}_2) \) and oxygen gas \( (\text{O}_2) \). The reaction does not produce nitrogen dioxide \( (\text{NO}_2) \). The reaction is as follows:\[ 2\text{KNO}_3 \rightarrow 2\text{KNO}_2 + \text{O}_2 \]

Step 2: Check the Reaction of Pb(NO3)2

When lead(II) nitrate \( (\text{Pb(NO}_3)_2) \) is heated, it decomposes to produce lead(II) oxide \( (\text{PbO}) \), nitrogen dioxide \( (\text{NO}_2) \), and oxygen gas \( (\text{O}_2) \). Therefore, heating \( \text{Pb(NO}_3)_2 \) does produce \( \text{NO}_2 \). The reaction is:\[ 2\text{Pb(NO}_3)_2 \rightarrow 2\text{PbO} + 4\text{NO}_2 + \text{O}_2 \]

Step 3: Evaluate the Reaction of Cu(NO3)2

Copper(II) nitrate \( (\text{Cu(NO}_3)_2) \) decomposes upon heating to form copper(II) oxide \( (\text{CuO}) \), nitrogen dioxide \( (\text{NO}_2) \), and oxygen gas \( (\text{O}_2) \). So \( \text{NO}_2 \) is produced in this reaction:\[ 2\text{Cu(NO}_3)_2 \rightarrow 2\text{CuO} + 4\text{NO}_2 + \text{O}_2 \]

Step 4: Investigate the Reaction of AgNO3

When silver nitrate \( (\text{AgNO}_3) \) is heated, it decomposes to form silver \( (\text{Ag}) \), nitrogen dioxide \( (\text{NO}_2) \), and oxygen gas \( (\text{O}_2) \). Hence, \( \text{NO}_2 \) is produced:\[ 2\text{AgNO}_3 \rightarrow 2\text{Ag} + 2\text{NO}_2 + \text{O}_2 \]

Step 5: Conclusion: Identify the Source of No NO2

Only potassium nitrate \( (\text{KNO}_3) \) does not produce \( \text{NO}_2 \) upon decomposition. Therefore, the correct answer is \((a)\, \text{KNO}_3\).

Key Concepts

Nitrogen Dioxide Formation

Nitrogen dioxide, represented as \(\text{NO}_2\), is a reddish-brown gas with a characteristic sharp, biting odor and is one of the significant nitrogen oxides. It often forms as a byproduct in various chemical reactions, especially in combustion and decomposition reactions.

When compounds containing the nitrate ion \((\text{NO}_3^-)\) are subjected to thermal decomposition, nitrogen dioxide can be released. This process typically involves the breakdown of larger molecules containing nitrate ions upon heating, releasing various gases including nitrogen dioxide.

In the context of metal nitrates such as lead(II) nitrate \((\text{Pb(NO}_3)_2)\), copper(II) nitrate \((\text{Cu(NO}_3)_2)\), and silver nitrate \((\text{AgNO}_3)\), heating these compounds leads to the formation of \(\text{NO}_2\). These reactions, however, not only contribute to the characteristic odor and color but also significantly impact air quality and environmental health due to the harmful effects of \(\text{NO}_2\).

• Lead(II) nitrate decomposes to produce lead(II) oxide, \(\text{NO}_2\), and \(\text{O}_2\).
• Copper(II) nitrate and silver nitrate also produce \(\text{NO}_2\) alongside respective metal oxides and silver.

Understanding where and how \(\text{NO}_2\) forms is crucial for both industrial applications and environmental considerations.

Thermal Decomposition

Thermal decomposition is a process where a compound breaks down into simpler substances when heated. This type of reaction does not involve the addition of other substances or reactants but instead relies on the application of heat to instigate the breakdown.

In the context of the exercise, thermal decomposition involves heating metal nitrates which break down to yield various products including nitrogen dioxide \((\text{NO}_2)\).

Key Points:

• It is an endothermic reaction, meaning it absorbs heat from its surroundings.
• Different compounds decompose at different temperatures, releasing gas and solid byproducts.

For instance, when lead(II) nitrate \((\text{Pb(NO}_3)_2)\) is heated, it decomposes into lead(II) oxide, nitrogen dioxide, and oxygen \((\text{O}_2)\). Similar reactions occur with other nitrates like copper(II) nitrate and silver nitrate.
Understanding thermal decomposition helps in determining the conditions needed for certain reactions and how these reactions can affect industrial and environmental processes.\

Potassium Nitrate Reactions

Potassium nitrate \((\text{KNO}_3)\) serves as a classic example of a decomposition reaction without nitrogen dioxide production. During the thermal decomposition of \(\text{KNO}_3\), it breaks into potassium nitrite \((\text{KNO}_2)\) and oxygen gas \((\text{O}_2)\).

This reaction does not form nitrogen dioxide \((\text{NO}_2)\), contrasting with other metal nitrates such as lead, copper, and silver nitrates.
Key Takeaways:

• \(\text{KNO}_3\) decomposes upon heating but only releases oxygen gas and transforms into another nitrate, \(\text{KNO}_2\).
• This lack of \(\text{NO}_2\) generation makes it an exception among common nitrate compounds.

The absence of \(\text{NO}_2\) in the decomposition of potassium nitrate means this chemical behaves uniquely compared to other nitrates under thermal conditions. This specific behavior is crucial for applications where \(\text{NO}_2\) production needs to be minimized, making it valuable in controlled chemical processes and experiments.