Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.2 Entropy as disorder-Study Notes - New Syllabus

12.2 Entropy as a Measure of Disorder

Entropy (\( \mathrm{S} \)) is a thermodynamic quantity that describes the degree of disorder in a system. It reflects how energy and particles are distributed within a system. A system with greater randomness and more possible arrangements of particles and energy has higher entropy.

Entropy

Entropy is a measure of the disorder of a system in terms of the random dispersal of molecules and of energy quanta between molecules.

Disorder in Terms of Molecules

• Entropy increases when particles are more randomly distributed.
• Gases have higher entropy than liquids, and liquids have higher entropy than solids.
• This is because gas particles are free to move and occupy a larger volume.
• An increase in the number of particles also increases disorder.

Disorder in Terms of Energy

• Entropy also depends on how energy is distributed among particles.
• When energy is spread out over many particles, entropy is higher.
• If energy is concentrated in fewer particles, entropy is lower.
• Greater dispersal of energy quanta leads to greater disorder.

Key Features

• High entropy corresponds to high randomness and disorder.
• Low entropy corresponds to more ordered systems.
• Entropy increases when particles become more dispersed in space.
• Entropy increases when energy is more evenly distributed among particles.

Illustrative Changes in Entropy

• \( \mathrm{solid \rightarrow liquid \rightarrow gas} \): entropy increases.
• Dissolving a solid in water often increases entropy due to dispersion of ions.
• Reactions producing more gas molecules generally increase entropy.