Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

An ideal gas, obeying kinetic theory of gases cannot be liquefied, because (a) its critical temperature is above \(0^{\circ} \mathrm{C}\) (b) its molecules are relatively small in size (c) it solidifies before becoming a liquid (d) forces acting between its molecules are negli-gible.

Step-by-Step Solution

Answer

(d) An ideal gas cannot be liquefied because intermolecular forces are negligible.

Step 1: Understand the Question

The question asks why an ideal gas, as described by the kinetic theory of gases, cannot be liquefied. Liquefaction refers to the conversion of a gas into a liquid, which involves intermolecular forces at work.

Step 2: Review Kinetic Theory Assumptions

According to the kinetic theory of gases, an ideal gas assumes there are no intermolecular forces between the gas molecules, meaning the molecules do not attract or repel each other.

Step 3: Analyze Each Option

- (a) An ideal gas's critical temperature, if above 0°C, does not prevent liquefaction, as it's unrelated to this intrinsic property. - (b) The size of the molecules does not directly impact their ability to liquefy. - (c) Solidification before liquefaction is not a feature of an ideal gas; it's more about temperature conditions. - (d) Lack of intermolecular forces implies that they cannot form a liquid, as condensation requires molecular attraction.

Step 4: Determine the Correct Answer

The correct answer is (d) forces acting between its molecules are negligible. Without intermolecular forces, the gas molecules cannot form a liquid state.

Key Concepts

Ideal Gas

An ideal gas is a simplified model that helps us understand the behavior of gases under different conditions. According to the kinetic theory of gases, ideal gases are composed of particles that move randomly and constantly in straight lines.
These gas particles are assumed to be perfectly elastic, meaning they do not lose energy when they collide. A crucial assumption of the ideal gas model is that there are no intermolecular forces acting between the molecules.
This means the gas molecules neither attract nor repel each other, making interactions purely due to collisions.
The ideal gas law, expressed as \(PV = nRT\), allows us to predict the behavior of gases when one of these conditions changes. However, it is important to note that real gases do not always behave like ideal gases, especially under high pressure or low temperature conditions.

Intermolecular Forces

Intermolecular forces are the forces that act between molecules within a substance. These forces are crucial when it comes to the physical changes like phase transitions. There are several types of intermolecular forces:

• London dispersion forces: Weak forces found in all molecules, arising due to temporary dipoles caused by electron movement.
• Dipole-dipole interactions: Occur between polar molecules where positive and negative ends attract.
• Hydrogen bonds: Strong types of dipole-dipole interactions that occur specifically when hydrogen is bonded to a highly electronegative element like nitrogen, oxygen, or fluorine.

These forces determine the physical properties of substances, such as boiling point and melting point. For gases to liquefy, attractive intermolecular forces must be present to pull molecules closer together. Since ideal gases ignore these forces in their model, they inherently lack the capability to condense into a liquid form with the same principles.

Liquefaction of Gases

Liquefaction is the process through which a gas transitions into a liquid. This process involves cooling or compressing a gas so that its molecules are close enough to allow intermolecular forces to effectively act.
To liquefy a gas, it must reach its critical temperature, above which it remains gaseous regardless of pressure.
The absence of intermolecular forces in an ideal gas means it cannot be liquefied. In real-world conditions, increasing pressure and lowering the temperature can promote liquefaction by enhancing molecular attractions.
However, ideal gases, by their theoretical model, lack these attractions entirely. Thus, an ideal gas, as per the kinetic theory, remains unbound by such intermolecular interactions and therefore cannot transition to a liquid state.