Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.5 Entropy changes -Study Notes - New Syllabus

12.5 Entropy Changes

Entropy changes occur during changes of state because the arrangement and freedom of movement of particles change. These changes affect the degree of disorder and the dispersal of energy within the system.

(i) Entropy Change in Changes of State

Entropy increases when a substance changes to a more disordered state due to greater freedom of particle movement and increased dispersal of energy.

Solid → Liquid (Melting)

• In a solid, particles are arranged in a fixed, ordered lattice.
• During melting, particles gain energy and begin to move past each other.
• The structure becomes less ordered.
• Therefore, entropy increases.

Liquid → Gas (Boiling/Evaporation)

• In a liquid, particles are close together but can move.
• In a gas, particles are far apart and move freely in all directions.
• There is a large increase in volume and number of possible arrangements.
• Therefore, entropy increases significantly.

Gas → Liquid (Condensation)

• Gas particles become closer together and lose freedom of movement.
• The system becomes more ordered.
• Therefore, entropy decreases.

Liquid → Solid (Freezing)

• Particles become fixed in a regular arrangement.
• Movement is restricted to vibrations about fixed positions.
• The system becomes highly ordered.
• Therefore, entropy decreases.

Explanation in Terms of Energy Dispersal

• During melting and boiling, energy is absorbed and spread among particles.
• This increases the number of accessible energy states.
• Greater dispersal of energy contributes to higher entropy.

Key Features

• Entropy increases when moving to a less ordered state.
• Entropy decreases when moving to a more ordered state.
• Changes of state involve both particle rearrangement and energy redistribution.
• The largest entropy increase occurs during liquid → gas transition.

(ii) Entropy Changes during Dissolving of a Solid Ionic Lattice

When a solid ionic lattice dissolves in water, entropy changes occur due to changes in both the arrangement of ions and the distribution of energy. The overall entropy change depends on the balance between increased disorder from ion separation and any ordering effects caused by interactions with water molecules.

Entropy Change on Dissolving

Dissolving an ionic solid usually increases entropy because ions become more randomly dispersed in solution.

Breakdown of the Ionic Lattice

• In a solid ionic lattice, ions are arranged in a highly ordered, regular structure.
• When the solid dissolves, the lattice breaks apart into individual ions.
• These ions become free to move throughout the solution.
• This increases the number of possible arrangements of particles.

Hydration of Ions

• Water molecules surround the ions (hydration).
• The partially charged ends of water molecules align around ions.
• This can create some local ordering of water molecules.

Overall Entropy Change

• The increase in disorder due to ion separation generally outweighs any ordering caused by hydration.
• Therefore, entropy usually increases when an ionic solid dissolves.
• However, in some cases, strong hydration can reduce entropy increase or even lead to a decrease.

Explanation in Terms of Energy Dispersal

• Energy is used to break the ionic lattice and is then distributed among ions and surrounding water molecules.
• The dispersal of energy among more particles contributes to an increase in entropy.

Key Features

• Entropy increases due to dispersion of ions in solution.
• Hydration may introduce some ordering of water molecules.
• The overall entropy change depends on the balance between these effects.
• Most ionic solids show an overall increase in entropy when dissolving.

(iii) Entropy Changes in Reactions Involving Change in Number of Moles

Entropy changes during chemical reactions are influenced by changes in the number of particles, especially in gaseous systems. A change in the number of moles of reactants to products affects the degree of disorder and the dispersal of molecules in the system.

Entropy Change and Number of Moles

An increase in the number of moles of particles leads to an increase in entropy, while a decrease leads to a decrease in entropy.

Increase in Number of Moles

• When more moles of particles are formed, there are more particles present in the system.
• This increases the number of possible arrangements of particles.
• The particles become more widely dispersed.
• Therefore, entropy increases (\( \mathrm{\Delta S > 0} \)).

Decrease in Number of Moles

• When fewer moles of particles are formed, the number of particles decreases.
• The number of possible arrangements of particles is reduced.
• The system becomes more ordered.
• Therefore, entropy decreases (\( \mathrm{\Delta S < 0} \)).

Importance of Gaseous Systems

• The effect is most significant for gases because gas particles occupy large volumes and have high freedom of movement.
• Changes in moles of solids or liquids have a smaller effect on entropy.

Illustrative Examples

• \( \mathrm{N_2O_4(g) \rightarrow 2NO_2(g)} \): number of moles increases, so entropy increases.
• \( \mathrm{2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)} \): number of moles decreases, so entropy decreases.

Explanation in Terms of Energy Dispersal

• More particles allow energy to be distributed among a greater number of molecules.
• This increases the number of accessible energy states.
• Greater energy dispersal contributes to higher entropy.

Key Features

• Increase in moles → increase in entropy.
• Decrease in moles → decrease in entropy.
• Effect is most significant for gaseous reactions.
• Entropy change depends on particle dispersal and energy distribution.