Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.4 Direction of change (ΔS)-Study Notes - New Syllabus

12.4 Direction of Spontaneous Change and Total Entropy

The natural direction of any process is determined by the change in total entropy of the system and its surroundings. A process is spontaneous if it leads to an overall increase in entropy. This reflects the tendency of particles and energy to become more randomly dispersed over time.

Spontaneous Change

A process is spontaneous if it occurs in the direction of increasing total entropy (\( \mathrm{\Delta S_{total} > 0} \)).

Total entropy includes both the system and the surroundings:

• \( \mathrm{\Delta S_{total} = \Delta S_{system} + \Delta S_{surroundings}} \)

Even if the entropy of the system decreases, the process can still be spontaneous if the increase in entropy of the surroundings is greater, resulting in an overall positive entropy change.

Interpretation of Direction of Change

• Processes tend to move towards greater disorder and randomness.
• Energy becomes more widely dispersed among particles.
• The most probable state is the one with the highest entropy.

Example: Gases Spreading

When a gas is released into a room:

• Gas particles spread out to occupy the entire available volume.
• The particles become more randomly distributed.
• The number of possible arrangements of particles increases significantly.
• Therefore, entropy increases and the process occurs spontaneously.

The reverse process (gas spontaneously gathering into one corner) would decrease entropy and is therefore extremely unlikely.

Key Features

• Spontaneous processes increase total entropy.
• Entropy increase reflects greater dispersal of particles and energy.
• Both system and surroundings must be considered.
• Natural processes move towards the most probable (most disordered) state.