Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

On heating, lead (II) nitrate gives a brown gas (A). The gas (A) on cooling changes to a colourless solid/liquid (B). (B) on heating withNO changes to a blue solid (C). The oxidation number of nitrogen in solid (C) is : [Main Sep. 04, 2020 (I)] (a) \(+5\) (b) \(+2\) (c) \(+3\) \((\mathrm{d})+4\)

Step-by-Step Solution

Answer

The oxidation number of nitrogen in solid (C) is +3.

Step 1: Identify the Brown Gas (A)

When lead(II) nitrate \(Pb(NO_3)_2\) is heated, it decomposes to produce a brown gas which is nitrogen dioxide \(NO_2\). The equation for this decomposition is: \[ 2Pb(NO_3)_2(s) \rightarrow 2PbO(s) + 4NO_2(g) + O_2(g) \]Thus, the brown gas (A) is \(NO_2\).

Step 2: Cooling of the Brown Gas (A)

When the nitrogen dioxide gas \(NO_2\) is cooled, it dimerizes to form dinitrogen tetroxide \(N_2O_4\), which is a colorless solid or liquid. Hence, the compound (B) is \(N_2O_4\).

Step 3: Reaction of (B) with NO

The reaction of dinitrogen tetroxide \(N_2O_4\) with nitric oxide \(NO\) yields the blue solid (C), which is a nitrosyl complex such as nitrosyl nitrate \(N_2O_3\). The reaction can be simplified as follows:\[ N_2O_4 + NO \rightarrow N_2O_3 \] This compound has a blue color, so (C) is \(N_2O_3\).

Step 4: Determine the Oxidation Number of Nitrogen in (C)

In nitrosyl nitrate \(N_2O_3\), we know that the overall charge of the compound is zero. The known oxidation number of oxygen is \(-2\). For \(N_2O_3\), the formula can be written as \(2x + 3(-2) = 0\) where \(x\) is the oxidation number of nitrogen. Solving this gives:- \(2x - 6 = 0\)- \(2x = 6\)- \(x = +3\) The oxidation number of nitrogen in the compound is \(+3\).

Key Concepts

Lead Nitrate Decomposition

Lead (II) nitrate, represented by the chemical formula \( \text{Pb(NO}_3\text{)}_2 \), undergoes decomposition upon heating. This process results in the formation of several products, including a brown gas known as nitrogen dioxide \( \text{NO}_2 \). The thermal decomposition can be represented by the balanced chemical equation:\[2 \text{Pb(NO}_3\text{)}_2(s) \rightarrow 2 \text{PbO}(s) + 4 \text{NO}_2(g) + \text{O}_2(g)\]Key points to consider in this reaction:

• Lead(II) oxide \( \text{PbO} \) is a solid product.
• Oxygen \( \text{O}_2 \) is released as a gaseous product.
• The brown color observed is primarily due to the nitrogen dioxide \( \text{NO}_2 \) gas.

Understanding this reaction is crucial, as it demonstrates the breakdown of a compound into simpler products under the influence of heat. This aspect is essential in various chemical processes and environmental studies.

Nitrogen Dioxide

Nitrogen dioxide \( \text{NO}_2 \) is a reddish-brown gas commonly produced during the decomposition of nitrates, such as lead nitrate. It plays a pivotal role in both chemistry and environmental science. A unique property of nitrogen dioxide is its ability to exist in equilibrium with dinitrogen tetroxide \( \text{N}_2\text{O}_4 \), a colorless compound, especially at lower temperatures.Key Characteristics of Nitrogen Dioxide:

• It is a paramagnetic gas, meaning it is attracted by magnetic fields.
• It has significant implications in atmospheric reactions, contributing to phenomena like photochemical smog.

When nitrogen dioxide is cooled, it dimerizes to form dinitrogen tetroxide through the reaction:\[2 \text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g)\]This transformation showcases how temperature influences the physical manifestation of chemical compounds. Moreover, these transitions are crucial when examining the behavior of gases in different states and conditions.

Nitrosyl Complex

Nitrosyl complexes are fascinating compounds that contain the nitrosyl group \( \text{NO} \), which is bonded to a metal center. In this context, we are particularly interested in the formation of a blue-colored nitrosyl complex, such as \( \text{N}_2\text{O}_3 \), when \( \text{N}_2\text{O}_4 \) reacts with nitric oxide \( \text{NO} \).In the reaction:\[\text{N}_2\text{O}_4 + \text{NO} \rightarrow \text{N}_2\text{O}_3\]One gets a distinctive blue solid, denoting the formation of a specific nitrosyl complex. The formation and presence of these complexes are imperative to various fields of chemistry, especially coordination chemistry and industrial catalysis.Understanding the Oxidation Number:To determine the oxidation number of nitrogen in \( \text{N}_2\text{O}_3 \), we use the oxidation number rules:

• Oxygen typically has an oxidation number of \(-2\).
• The sum of oxidation numbers in a neutral compound is zero.

Setting up the equation:\[ 2x + 3(-2) = 0 \]Solving gives:

• \(2x - 6 = 0\)
• \(2x = 6\)
• \(x = +3\)

Therefore, the oxidation number of nitrogen in \( \text{N}_2\text{O}_3 \) is \(+3\). This detailed understanding of oxidation numbers aids in comprehending chemical reactivity and bonding.