Question

A liquid is in equilibrium with its vapour at its boiling point. On the average, the molecules in the two phase have equal (a) intermolecular forces (b) potential energy (c) temperature (d) kinetic energy

Short answer

The molecules have equal kinetic energy at the boiling point.

Step-by-step solution

Understanding the Question

The problem asks us to identify which property is equal for molecules in both the liquid and vapor phases when a liquid is at its boiling point and in equilibrium with its vapor.

Analyzing the Properties

1. **Intermolecular forces**: These are typically stronger in the liquid phase than in the vapor phase, as molecules are more tightly packed in a liquid. 2. **Potential energy**: Potential energy is related to the position and interactions of the molecules. In the liquid phase, potential energy is generally lower due to stronger intermolecular attractions. 3. **Temperature**: At the boiling point, the liquid and its vapor are in thermal equilibrium, meaning they have the same temperature. 4. **Kinetic energy**: Temperature is a measure of the average kinetic energy of molecules. Therefore, molecules in both phases at the boiling point will have the same average kinetic energy as they are at the same temperature.

Identifying the Correct Property

Given that temperature is the measure of average kinetic energy, the molecules in both phases must have equal kinetic energy when at the boiling point and in equilibrium.

Key concepts

Boiling Point

The boiling point is a significant concept when studying phase changes in chemistry. It refers to the temperature at which a liquid's vapor pressure equals the external pressure, causing the liquid to turn into vapor. At this specific temperature, a unique balance—known as equilibrium—exists between the liquid and its vapor. During this state, the liquid turns into vapor not only at the surface but throughout the liquid. This means that at the boiling point, both the liquid and vapor phases coexist comfortably in equilibrium. Understanding the boiling point is crucial because it signifies a transition in the state of matter and is an essential component of physical chemistry.

Intermolecular Forces

Intermolecular forces determine how molecules interact with each other. In the liquid phase, these forces are quite strong due to the proximity of molecules, keeping them tightly packed. However, once the liquid reaches its boiling point and becomes vapor, these forces weaken significantly as molecules spread out. This reduction in intermolecular attraction allows molecules more freedom of movement in the vapor phase. Common types of intermolecular forces include hydrogen bonds, dipole-dipole interactions, and London dispersion forces. These forces greatly influence the physical properties of substances, such as boiling point, viscosity, and surface tension. A stronger intermolecular force in a substance usually means a higher boiling point because more energy is needed to break these interactions to change phases.

Kinetic Energy

Kinetic energy, in the context of molecules, is the energy that those molecules possess due to their motion. It is directly related to temperature, as an increase in kinetic energy corresponds to an increase in temperature. At the boiling point, the average kinetic energy of molecules in a liquid is equal to that of molecules in vapor due to thermal equilibrium, meaning there is no net flow of thermal energy between the two phases. This balance ensures that the transformation between liquid and vapor occurs continuously and at a consistent rate. It's important to understand that while kinetic energy may remain the same, individual molecules can have varying energies, but the average remains constant for the temperature. This principle plays a crucial role in understanding thermal dynamics in chemistry.

Phase Changes

Phase changes occur when a substance transitions from one state of matter to another, such as from liquid to gas at the boiling point. These changes are physical processes and involve the alteration of the energy states within a substance. During a phase change, the temperature remains constant as the energy supplied or removed is used in changing the state—instead of affecting the kinetic energy. For example, during boiling, energy is used to overcome intermolecular forces rather than raising the temperature. This demonstrates how energy conversion in phase changes involves potential and kinetic energy. When studying phase changes, it is vital to understand both the energy involved in breaking intermolecular forces and the role of environmental pressure which can alter the temperature at which these changes occur.

Potential Energy

Potential energy in molecules is related to their positions relative to each other. In a liquid, molecules are closely packed, so they have relatively low potential energy because they are stabilized by strong intermolecular forces. As a liquid reaches its boiling point and transforms into vapor, its potential energy increases because molecules move to a higher energy state as they overcome these intermolecular forces. This increase in potential energy during vaporization signifies the energy needed to separate molecules from each other. Potential energy, therefore, plays a crucial role in understanding why energy is needed for phase changes. While kinetic energy remains constant during an equilibrium phase transition, potential energy changes significantly because it measures the energy required for these transformations.