Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.3 Entropy and temperature/states-Study Notes - New Syllabus

12.3 Factors Affecting Entropy

The entropy of a substance depends on temperature and physical state. Entropy increases as the disorder of a system increases, which can occur through greater molecular motion or increased freedom of particles. Certain fundamental principles describe how entropy varies under different conditions.

Effect of Temperature on Entropy

Entropy increases with temperature because particles gain kinetic energy and move more randomly.

• At higher temperatures, particles have greater kinetic energy.
• This leads to increased molecular motion and randomness.
• Energy is more widely dispersed among particles.
• Therefore, entropy increases as temperature increases.

Effect of Physical State

Entropy increases in the order: \( \mathrm{solid < liquid < gas} \).

• In solids, particles are arranged in a fixed, ordered lattice with limited movement.
• In liquids, particles can move past each other, increasing disorder.
• In gases, particles move freely and are widely spaced, giving maximum disorder.
• Therefore, entropy increases from solid to liquid to gas.

Entropy at Absolute Zero

A perfect crystal at \( \mathrm{0\ K} \) has zero entropy.

• At absolute zero, particles have minimum possible energy.
• There is no thermal motion of particles.
• The arrangement of particles is perfectly ordered.
• Therefore, there is only one possible arrangement, giving zero entropy.

Key Features

• Entropy increases with increasing temperature due to greater energy dispersal.
• Entropy increases as particles become less ordered (solid → liquid → gas).
• A perfect crystal at \( \mathrm{0\ K} \) has zero entropy.
• Entropy reflects both particle arrangement and energy distribution.