Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

Two oxides of nitrogen, \(\mathrm{NO}\) and \(\mathrm{NO}_{2}\) react together at \(253 \mathrm{~K}\) and form a compound of nitrogen \(\mathrm{X} . \mathrm{X}\) reacts with water to yield another compound of nitrogen \(Y\). The shape of the anion of \(Y\) molecule is ..... (a) triangular planar (b) pyramidal (c) tetrahedral (d) square planar

Step-by-Step Solution

Answer

The shape of the anion \( \text{NO}_2^- \) in \( \text{Y} \) is bent.

Step 1: Understanding the Reaction

We start with the reaction of nitrogen oxides \( \text{NO} \) and \( \text{NO}_2 \) to form a compound \( \text{X} \). The probable reaction is \( \text{NO} + \text{NO}_2 \rightarrow \text{N}_2\text{O}_3 \). This is because \( \text{N}_2\text{O}_3 \) is a known compound that forms under these conditions.

Step 2: Reacting with Water

Compound \( \text{X} \), which is \( \text{N}_2\text{O}_3 \), reacts with water. \( \text{N}_2\text{O}_3 \) is known to form nitrous acid \( \text{HNO}_2 \) when it reacts with water, as per the reaction: \( \text{N}_2\text{O}_3 + \text{H}_2\text{O} \rightarrow 2\text{HNO}_2 \). Thus, compound \( \text{Y} \) is \( \text{HNO}_2 \).

Step 3: Analyzing the Anion in Y

\( \text{HNO}_2 \) contains the nitrite ion \( \text{NO}_2^- \) as its anion. To determine the shape of the \( \text{NO}_2^- \) ion, we first consider its electron geometry. \( \text{NO}_2^- \) consists of nitrogen with 3 electron domains (two bonding pairs and one lone pair).

Step 4: Applying VSEPR Theory

Using Valence Shell Electron Pair Repulsion (VSEPR) theory, a molecule with 3 electron domains (triatomic with one lone pair) adopts a trigonal planar electron domain geometry but a bent molecular geometry. The lone pair causes the \( \text{O-N-O} \) bond angle to decrease from \(120^{\circ}\) to about \(115^{\circ}\). This tilted formation gives the nitrite ion its shape.

Key Concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. Understanding it helps us predict the behavior and reactivity of molecules. VSEPR theory, or Valence Shell Electron Pair Repulsion theory, plays a key role in determining molecular geometry. It proposes that electron pairs around a central atom will arrange themselves to minimize repulsion. This results in various geometric shapes, such as linear, trigonal planar, tetrahedral, and more.

For molecules like the nitrite ion (_2^-), the geometry is influenced by both bonding pairs and lone pairs of electrons. Even though the electron domains are arranged in a trigonal planar fashion due to three domains (including a lone pair), the actual shape observed is bent due to the repulsion caused by the lone pair. This is a crucial point in predicting the molecular shape and revealing the chemical properties of the molecule.

Electron Domains

Electron domains consist of bonds (single, double, or triple) and lone pairs around a central atom. They determine the molecule's electron geometry and influence molecular shape. A molecule can have different types of electron domains, which dictate how atoms are spatially arranged.

In the nitrite ion (_2^-), there are three electron domains: two bonding pairs with oxygen atoms and one lone pair of electrons on the nitrogen atom. This trident-like configuration forms the basis for predicting the spatial arrangement of the atoms using the VSEPR theory.

• Bonding Pair: Shared between two atoms, forming the framework of a molecule.
• Lone Pair: Unshared electrons that occupy more space, causing deviations in bond angles.

These elements together create a specific geometric configuration, leading to the bent shape of the nitrite ion.

Nitrite Ion

The nitrite ion, denoted as _2^-, is a negatively charged ion often involved in various chemical reactions. It forms the anion component of compounds like nitrous acid, playing a crucial role in nitrogen cycles in nature and industrial processes.

The nitrite ion comprises a central nitrogen atom connected to two oxygen atoms. Because of the presence of a lone pair on nitrogen, the ion does not follow a linear geometry; instead, it assumes a bent or angular shape. This affects its reactivity and interaction with other compounds. The nitrite ion serves as a key compound in the synthesis of many other chemicals and is essential in the study of both inorganic and organic chemistry.

Nitrogen Oxides

Nitrogen oxides encompass a variety of compounds made of nitrogen and oxygen. Common examples include  and _2. These compounds play significant roles in atmospheric chemistry and industrial applications.

In the reaction context mentioned above,  and _2 interact to form , a dinitrogen trioxide. This intermediate can further react with water to produce nitrous acid _2. Nitrogen oxides are versatile substances in environmental chemistry, contributing to phenomena like smog and acid rain and are explored for their roles in air pollution.

• : Nitrogen monoxide, a contributor to ground-level ozone.
• _2: Nitrogen dioxide, known for its reddish-brown color and role in pollution.

Understanding their interconversions helps manage and mitigate environmental impacts.

Nitrous Acid

Nitrous acid (_2) is a weak and somewhat unstable acid known for its ability to participate in various chemical processes. Upon dissociation, it yields the nitrite ion (_2^-), making it crucial in multiple chemical and environmental pathways.

This acid forms when dinitrogen trioxide interacts with water, a reaction that plays a significant role in nitrogen chemistry and industrial processes. Although nitrous acid is not typically isolated due to its instability, it's integral in processes like the formation of diazonium salts, used in dye manufacture.

• Dye Chemistry: Provides intermediates in dye production.
• Nitrogen Cycle: Acts as a transitional species in soil and water environments.

Understanding nitrous acid and its derivatives, like the nitrite ion, underlines the importance of nitrogen-based compounds in both nature and industry.