Problem 14 - A Complete Resource Book in Chemistry for JEE Main

Question

The \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) angle in water molecule is about (a) \(90^{\circ}\) (b) \(105^{\circ}\) (c) \(135^{\circ}\) (d) \(180^{\circ}\)

Step-by-Step Solution

Answer

The \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) angle in water is about \(105^{\circ}\).

Step 1: Understanding molecular geometry

Water (\( \text{H}_2\text{O} \)) is a molecule with an oxygen atom bonded to two hydrogen atoms. The molecular shape is determined by the arrangement of electron pairs around the central atom.

Step 2: Applying the VSEPR theory

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts that electron pairs around a central atom will arrange themselves to minimize repulsion. In water, there are two bonding pairs and two lone pairs around the oxygen atom.

Step 3: Predicting bond angles

According to VSEPR theory, the presence of lone pairs compresses the bond angle between hydrogen atoms to less than the ideal tetrahedral angle (\(109.5^{\circ}\)). The typical \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) bond angle for water is about \(104.5^{\circ}\).

Step 4: Selecting the closest answer

Compare the possible answers to the typical \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) bond angle. The closest option to \(104.5^{\circ}\) is \(105^{\circ}\).

Key Concepts

Molecular Geometry

Understanding molecular geometry is crucial when studying chemical compounds. Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. It is determined by the number of bonding pairs (those shared between atoms) and lone pairs (those not shared with other atoms). These pairs want to be as far apart as possible because they repel each other.
The shape of the molecule is influenced by these repulsions. In a water molecule, which has the formula \( \text{H}_2\text{O} \), the central oxygen atom is bonded to two hydrogen atoms. With two bonding pairs and two lone pairs around the central oxygen atom, water adopts a bent shape rather than being linear or straight.
This molecular structure is directly related to the concept of bond angles, which we will discuss next.

Bond Angles

Bond angles are a key feature in molecular geometry. They are defined as the angles between two bonds originating from a common atom. In the context of water, the bond angle we focus on is the angle formed between the two hydrogen atoms with the central oxygen atom.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the ideal tetrahedral bond angle is \(109.5^\circ\). But in water, the presence of two lone pairs on the oxygen atom results in a bond angle that is less than this ideal angle.
Lone pairs exert greater repulsion forces than bonding pairs. This greater repulsion causes the \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) bond angle to be compressed to about \(104.5^\circ\). Hence, the water molecule's bent shape gives it a distinct bond angle, which can be approximated in practical exercises as \(105^\circ\).

Lone Pairs

Lone pairs play a vital role in shaping the molecular geometry. They consist of electrons that are not involved in bonding but are located around the central atom. These electron pairs have a significant impact on the spatial arrangement of atoms in a molecule.
In water, the oxygen atom has two lone pairs. These lone pairs occupy more space than the bonding pairs because they are not shared between atoms. Consequently, the repulsion from lone pairs is stronger, leading to a distortion of the bond angles and influencing the overall geometry of the molecule.
The presence of these lone pairs is what converts the water molecule from a theoretical linear geometry to its actual bent shape. Understanding the role of lone pairs is crucial for predicting the behavior of molecules and explaining phenomena such as the specific bond angles seen in water.