Question

(a) What is the significance of the critical pressure of a substance? (b) What happens to the critical temperature of a series of compounds as the force of attraction between molecules increases? (c) Which of the substances listed in Table \(11.5\) can be liquefied at the temperature of liquid nitrogen \(\left(-196^{\circ} \mathrm{C}\right)\) ?

Short answer

(a) The critical pressure is significant because it indicates the pressure required to convert a gas into a liquid without changing the temperature, helping us understand substance behavior under various conditions. (b) As intermolecular forces increase, the critical temperature also increases, as stronger forces require more energy to overcome. (c) To find substances in Table 11.5 that can be liquefied at -196°C (liquid nitrogen temperature), we must identify those with critical temperatures above -196°C.

Step-by-step solution

(a) Significance of critical pressure

Critical pressure is the pressure at which a substance in its gas phase can be converted into a liquid phase by a mere increase in pressure, with no change in temperature. It is significant because it helps us understand the behavior of substances under various temperature and pressure conditions.

(b) Critical temperature and intermolecular forces

As the force of attraction between molecules in a series of compounds increases, their critical temperature increases. This is because stronger intermolecular forces require more energy (higher temperature) to be overcome in order for the substance to transit from the liquid phase to the gas phase.

(c) Liquefiable substances at liquid nitrogen temperature

To determine which substances in Table 11.5 can be liquefied at the temperature of liquid nitrogen, we must check their critical temperatures. If a substance has a critical temperature higher than -196°C (the temperature of liquid nitrogen), it can be liquefied at that temperature. Check the table and identify substances with critical temperatures above -196°C.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine many properties of a substance. They are crucial in understanding why substances behave the way they do. There are several types of intermolecular forces to consider:

• Van der Waals Forces: Weak attractions such as London dispersion forces and dipole-dipole interactions.
• Hydrogen Bonds: A special type of dipole-dipole interaction involving hydrogen.
• Ionic Interactions: Strong forces found in ionic compounds.

Stronger intermolecular forces mean molecules are held more tightly together. Therefore, more energy (or a higher temperature) is needed to break these bonds and change the state of the substance.

Critical Temperature

The critical temperature of a substance is the highest temperature at which it can exist in liquid form, no matter how much pressure is applied. Above this temperature, it cannot be liquefied, regardless of pressure. The critical temperature gives insights into the energy needed to overcome intermolecular forces. Substances with strong intermolecular forces have higher critical temperatures. They need more energy to transition from liquid to gas because of stronger attractions between molecules. As such, understanding critical temperature helps predict a substance's phase behavior under different conditions.

Phase Transition

A phase transition refers to the change from one state of matter to another, such as solid to liquid or liquid to gas. These transitions occur because of changes in conditions like temperature and pressure. Here are common types of phase transitions:

• Melting: Solid to liquid transition.
• Boiling: Liquid to gas transition.
• Condensation: Gas to liquid transition.

During a phase transition, the energy applied is used to break intermolecular forces. This doesn't change the temperature of the substance but changes its state. Understanding phase transitions helps us predict how a substance will react under different conditions.

Liquefaction of Gases

Liquefaction is the process of turning a gas into a liquid by applying pressure and/or lowering temperature. This concept is vital for processes like natural gas storage and refrigeration. To liquefy a gas, the temperature must be lowered to below its critical temperature. Additionally, applying pressure can aid liquefaction at higher temperatures. The ability of a gas to be liquefied depends on its critical temperature; gases with higher critical temperatures can be liquefied more easily at common temperatures, like those of liquid nitrogen. This process is a practical application of understanding critical temperatures and intermolecular forces.