Problem 38 - A Complete Resource Book in Chemistry for JEE Main

Question

The critical temperature of water is higher than that of \(\mathrm{O}_{2}\), because the \(\mathrm{H}_{2} \mathrm{O}\) molecule has (a) fewer electrons than \(\mathrm{O}_{2}\) (b) two covalent bonds (c) V-shape (d) dipole moment

Step-by-Step Solution

Answer

(d) dipole moment

Step 1: Understanding Critical Temperature

The critical temperature is the temperature above which a substance cannot exist in the liquid state regardless of pressure. It is related to intermolecular forces, as substances with stronger intermolecular forces generally have higher critical temperatures.

Step 2: Analyzing Water (H2O) vs. Oxygen (O2)

Water (H_{2}O) is a polar molecule with a dipole moment due to its bent V-shape and polar bonds (oxygen being more electronegative than hydrogen). In contrast, O_{2} is a nonpolar molecule as it consists of two identical atoms sharing electrons equally.

Step 3: Influence of Molecular Properties

The polarity of a molecule affects its intermolecular forces. Water has hydrogen bonding due to its dipole moment, resulting in stronger intermolecular forces compared to the weaker London dispersion forces between nonpolar O_{2} molecules.

Step 4: Identifying the Correct Reason

The key reason for water's higher critical temperature is its dipole moment, which leads to hydrogen bonding. This is the strongest type of intermolecular force among the given molecules, contributing significantly to water's high critical temperature.

Key Concepts

Understanding Dipole Moment

The concept of a dipole moment is central to understanding why certain molecules, like water (H_2O), exhibit higher critical temperatures. A dipole moment occurs in molecules where there is a significant difference in electronegativities between bonded atoms. This difference causes one part of the molecule to be slightly negative and the other to be slightly positive, resulting in a dipole. Water is a perfect example as it has a bent V-shape geometry, which means the dipole moments from its polar O-H bonds do not cancel out. Instead, they add up to create a significant overall dipole moment. This feature is crucial as it contributes to the molecule's ability to engage in strong intermolecular forces.

Exploring Intermolecular Forces

Intermolecular forces are the forces that hold molecules together in a liquid or solid. These forces include

• London dispersion forces
• Dipole-dipole interactions
• Hydrogen bonding

In the case of water, the presence of a strong dipole moment leads to hydrogen bonds, a type of strong intermolecular force. In comparison, nonpolar molecules, like O_2, only exhibit London dispersion forces, which are much weaker. Stronger intermolecular forces, like hydrogen bonding, result in a higher critical temperature for substances, as more energy (in the form of heat) is needed to break these interactions and transition the substance into a gas.

Hydrogen Bonding: A Special Kind of Attraction

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs in molecules where hydrogen is directly bonded to highly electronegative atoms, such as nitrogen, oxygen, or fluorine. In water, each hydrogen atom shares bonds with an oxygen atom, creating partial positive charges on the hydrogens and a partial negative charge on the oxygen. This enables water molecules to form strong hydrogen bonds with each other.

Impact on Physical Properties: Hydrogen bonds significantly influence the physical properties of a substance, including its critical temperature and boiling point.

Examples: Water's high boiling point and critical temperature are key examples of the impact of hydrogen bonding. These bonds must be broken for water to transition from liquid to gas, requiring significant energy.

What Makes a Polar Molecule?

A polar molecule is characterized by an uneven distribution of electron density, resulting in a permanent dipole moment. Water is a classic example of a polar molecule due to its asymmetrical shape and the unequal sharing of electrons between hydrogen and oxygen. The oxygen atom, being more electronegative, pulls the shared electrons closer, resulting in a slight negative charge at one end and a slight positive charge at the other. This polarity is what makes molecules like water highly interactive, influencing their intermolecular forces and, consequently, their physical properties.
Key factors that contribute to molecular polarity include:

• The difference in electronegativity between atoms
• The shape of the molecule
• The resulting dipole moment

Thus, the strong dipole moment in polar molecules like water leads to higher critical temperatures.