Question

Which occurs when a substance \(X\) is converted from liquid to vapour phase at the standard boiling point? I. Potential energy of the system decreases II. The distance between molecules increases III. The average kinetic energy of the molecules in both phases are equal (a) I only (b) II only (c) III only (d) II and III only

Short answer

The correct options are II and III only since the distance between molecules increases and the average kinetic energy of both phases remains equal at the standard boiling point.

Step-by-step solution

Understanding Phase Change from Liquid to Vapour

When a substance transitions from the liquid to the vapour phase, there is an increase in the energy of the system due to the absorption of heat. This heat is known as the latent heat of vaporization. The phase change occurs at the standard boiling point, implying the temperature remains constant during this process.

Analyzing Statement I: Potential Energy of the System

The potential energy of the system does not decrease; it increases because the molecules must overcome the intermolecular forces that are keeping them together in the liquid state. As the molecules move apart to form vapour, potential energy increases.

Analyzing Statement II: Distance Between Molecules

In the vapour phase, the distance between molecules is greater than in the liquid phase because the molecules have sufficient energy to overcome intermolecular forces and move apart. Thus, this statement is true.

Analyzing Statement III: Average Kinetic Energy of Molecules

The average kinetic energy of the molecules in a substance is directly related to its temperature. As both the liquid and vapour are in equilibrium at the boiling point, they have the same temperature. Therefore, the average kinetic energy of the molecules in both phases is equal.

Combining the Valid Statements

From the analysis above, we conclude that Statements II and III are correct. Statement I is incorrect as the potential energy increases rather than decreases during vaporization.

Key concepts

Latent Heat of Vaporization

The latent heat of vaporization is a critical concept when understanding phase changes in physical chemistry. It refers to the amount of energy in the form of heat that is required to convert a substance from the liquid phase to the vaporous or gaseous phase, at a constant temperature - the boiling point. This energy is essential because it is used to overcome the intermolecular forces that hold the molecules together in the liquid state.

During vaporization, even though the temperature of the substance does not change, its potential energy increases as the molecules separate and move farther apart, leading to an expansive state with greater volume. For students trying to visualize this process, it may be helpful to imagine energy being injected into the system, not to increase the speed of the particles (kinetic energy) but to break the 'bonds' between them, allowing them to disperse and move freely in the vapor phase.

Significance of Latent Heat in Everyday Processes

Latent heat of vaporization not only explains the behavior of boiling water but also plays a role in weather phenomena such as evaporation and condensation that are crucial to the water cycle.

Intermolecular Forces

Intermolecular forces are the forces of attraction and repulsion between molecules that affect all physical states: solid, liquid, and gas. These forces dictate how molecules interact with one another, and their strength can determine the state of a substance at a given temperature. In liquids, these forces are strong enough to keep molecules close together but not at fixed positions as in solids.

When a liquid turns into vapor, the intermolecular forces are overcome by the input of energy, i.e., the latent heat of vaporization, allowing the molecules to move apart. The type and strength of these forces - such as hydrogen bonding, van der Waals forces, or dipole-dipole interactions - will determine the amount of energy needed for this transition to occur.

Types of Intermolecular Forces

• Van der Waals Forces: Includes London dispersion forces, existing in all molecular interactions.
• Dipole-Dipole Interactions: Occur between polar molecules with permanent dipole moments.
• Hydrogen Bonds: Strong forms of dipole-dipole interactions involving hydrogen atoms.

Equilibrium at Boiling Point

When a substance is at its boiling point, it has reached a stage where the liquid and vapor phases coexist in equilibrium. This means that the rate of evaporation (liquid converting to vapor) is equal to the rate of condensation (vapor converting back to liquid). At this precise point, the average kinetic energy of the molecules in the liquid is the same as that in the vapor phase.

Being at equilibrium does not mean that all molecular activity ceases. Rather, it signals a dynamic balance where individual molecules are constantly changing phases, but the overall amount of liquid and vapor remains constant. A key takeaway for students is that at this equilibrium, the temperature remains constant despite continuous energy exchanges; thus the average kinetic energy of the molecules does not change with time.

Dynamic Nature of Boiling

The term 'boiling' might suggest vigorous activity, yet at the microscopic level, it represents a finely balanced process where two opposite phase changes occur at equal rates.