Question

A liquid is in equilibrium with its vapour at its boiling point. On the average, the molecules in the two phases have equal: (a) Kinetic energy (b) Total energy (c) Inter-molecular forces (d) Potential energy

Short answer

Molecules have equal average kinetic energy (option a).

Step-by-step solution

Understand the Problem

We need to determine which property is equal for molecules in both the liquid and vapor phases at the boiling point. The molecules are at the boiling point, meaning they coexist in both phases due to identical energy conditions.

Define Kinetic Energy Concept

Kinetic energy refers to the energy due to motion. For molecules of a substance, kinetic energy is directly related to temperature. At a specific temperature, like at boiling, molecules in both liquid and vapor phases have average kinetic energies based on the same temperature.

Evaluate the Other Options

Total energy combines both kinetic and potential energy. Inter-molecular forces and potential energy involve various factors such as state and attraction forces, differing by phase. Hence, these are not necessarily equal across liquid and vapor at the boiling point.

Conclude with Correct Answer

Boiling point tells us temperature is the same for both phases, equalizing the average kinetic energy of molecules in both the liquid and vapor phases, fitting the principle that temperature determines kinetic energy.

Key concepts

Kinetic Energy

Understanding kinetic energy is the first step to unraveling why it matters when a liquid reaches its boiling point. Kinetic energy is the energy that an object possesses due to its motion. For molecules, this energy is directly related to temperature. As temperature rises, molecules move faster, and therefore, their kinetic energy increases.

When a liquid reaches its boiling point, the average kinetic energy of the molecules in both the liquid and vapor phases is equal. This happens because the molecules are vibrating at similar energies due to the temperature being constant. Boiling point is a unique temperature where a liquid becomes a gas, and it is at this point that the kinetic energy aligns across phases.

Phase Equilibrium

Phase equilibrium is a fascinating concept that occurs when two phases, like liquid and vapor, coexist at equilibrium at a specific temperature, such as the boiling point. At this temperature, an exchange of molecules happens between phases at an equal rate, maintaining a steady phase ratio.

In the case of boiling, the molecules of the liquid are energetic enough to break free into the vapor phase, while vapor molecules, losing some energy, may condense back into the liquid. This balance results in a stable coexistence of liquid and gas phases, and is crucial for the boiling process to sustain without any net transformation from one phase to another.

Molecular Motion

Molecular motion is the fundamental movement of atoms or molecules within a substance. This motion can be translational, rotational, or vibrational, depending on the energy present.

At the boiling point, molecular motion is particularly vigorous, as molecules have enough energy to overcome inter-molecular forces keeping them in the liquid phase. In the vapor phase, molecules attain enough freedom to move randomly and independently. This increased motion at the boiling point is essential, allowing molecules to separate and form a vapor, illustrating the dynamic behavior of molecules in these conditions.

Thermodynamics

Thermodynamics plays a key role in understanding boiling. It involves the study of energy transformations, including heat, work, and the properties of systems.

At the boiling point, the thermodynamic principle that comes into play is the equality of temperature for both liquid and vapor phases. This agreement in temperature leads to an equal average kinetic energy for molecules in both phases. Observing this through the lens of the first and second laws of thermodynamics, energy within the system is conserved, and heat is absorbed to convert liquid into vapor without a temperature change. This aspect of thermodynamics helps explain why boiling occurs under these conditions and maintains phase equilibrium.