Question

Gases do not liquefy above the critical temperature because above critical temperature (a) the gases become ideal. (b) the intermolecular attraction vanishes. (c) the kinetic energy of molecules become so large that the attractive forces become unable to hold the molecules together. (d) the repulsive forces dominates in the molecules.

Short answer

(c) Gases do not liquefy above the critical temperature because the kinetic energy of molecules becomes so large that attractive forces cannot hold the molecules together.

Step-by-step solution

- Understanding Critical Temperature

The critical temperature of a substance is the highest temperature at which it can exist as a liquid. Above this temperature, the gas phase cannot be liquefied by pressure alone.

- Analyzing the Options

Analyze each option given in the exercise to determine which statement correctly explains why gases do not liquefy above the critical temperature.

- Identifying the Correct Option

Option (c) is the correct answer since at temperatures above the critical temperature, the kinetic energy of the gas molecules is sufficiently high that it overcomes the intermolecular attractive forces, making liquefaction impossible.

Key concepts

Liquefaction of Gases

Liquefaction of gases is a process that transforms a gas into a liquid state. This transformation is essential in various industrial applications, such as the transport and storage of natural gas. To achieve liquefaction, a gas must be cooled below its boiling point and, in many cases, compressed to increase the pressure.

However, there are limitations to this process. For instance, no matter how much pressure we apply, a gas cannot be liquefied above its critical temperature, which brings us to the underlying principles of why this is the case: kinetic energy and intermolecular forces, which we will explore further in the following sections.

Kinetic Energy in Gases

Kinetic energy, the energy possessed by an object due to its motion, is particularly significant in the context of gases. In gases, molecules are constantly in rapid, random motion, and the kinetic energy of these molecules determines the temperature of the gas.

At higher temperatures, when the gas molecules have more kinetic energy, they move faster and are more likely to overcome the attractive forces between them. This is crucial in understanding gas behavior, especially when it comes to liquefaction, as sufficient kinetic energy can prevent the gas molecules from coming close enough together to condense into a liquid, particularly above the critical temperature.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). These forces are weaker than the intramolecular forces that hold substances together, yet they are significant in determining the physical properties of substances, such as boiling points, melting points, and the ability to liquefy a gas.

In gases, when the temperature drops (and, correspondingly, the kinetic energy decreases), intermolecular forces, like Van der Waals forces or dipole-dipole interactions, become more influential, allowing the molecules to attract each other and condense into a liquid. However, as indicated in the solution to the exercise, above the critical temperature, these forces are not strong enough to overcome the high kinetic energy of the gas molecules, making liquefaction unachievable.