Examkin

Question

Difficulty: Very hardPostulates of Kinetic Theory and States of Matter

A pure liquid sample held at its boiling point absorbs heat until it completely transforms into a gas at constant temperature and pressure. According to the Kinetic Molecular Theory of matter, what occurs to the average kinetic energy and the potential energy of the molecules during this phase change?

  1. The average kinetic energy remains unchanged, while the potential energy increases as work is done against intermolecular forces of attraction.Answer
  2. B
    Both the average kinetic energy and the potential energy increase simultaneously because thermal energy is continuously added to the system.
  3. C
    The average kinetic energy increases, but the potential energy remains constant because phase changes depend only on molecular speed.
  4. D
    The average kinetic energy decreases to compensate for the sharp increase in potential energy required to overcome intermolecular forces.

Answer

The average kinetic energy remains unchanged, while the potential energy increases as work is done against intermolecular forces of attraction.
According to the Kinetic Molecular Theory, absolute temperature is a direct measure of average kinetic energy. During boiling, temperature remains constant, so the average kinetic energy of the particles does not change. Instead, the heat energy added goes entirely into doing work to break intermolecular forces of attraction, increasing the potential energy as molecules transition from the liquid state to the gaseous state.

Step-by-Step Solution

1
Relate absolute temperature to kinetic energy using KMT postulates.
Since the phase transition occurs at a constant boiling temperature (TT), the average kinetic energy (Ek=32kTE_k = \frac{3}{2}kT) of the molecules remains strictly constant.
Postulates of the Kinetic Molecular Theory explicitly state that average kinetic energy is directly proportional to absolute temperature.
2
Analyze the role of absorbed heat during a phase change from liquid to gas.
The absorbed energy (latent heat of vaporization) is utilized exclusively to perform work against cohesive intermolecular forces.
Separating liquid molecules to the large intermolecular distances characteristic of gases increases the system's microscopic potential energy without changing temperature.

Key Concept

Phase Transitions, Intermolecular Forces, and Molecular Energy in KMT
Estimated Time:1m 30s
Rate this question

Topics

Postulates of Kinetic Theory and States of Matter