Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 12.6 ΔS_total = ΔS_system + ΔS_surroundings-Study Notes - New Syllabus

12.6 Total Entropy Change of a Reaction

The feasibility of a reaction depends on the total entropy change of both the system and its surroundings. A reaction is spontaneous only if the overall entropy increases. Therefore, both contributions must be considered together rather than focusing only on the system.

Total Entropy Change

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

This expression shows that the total entropy change is the sum of:

• \( \mathrm{\Delta S_{system}} \): entropy change within the reacting substances.
• \( \mathrm{\Delta S_{surroundings}} \): entropy change in the surroundings due to heat transfer.

Interpretation

• If \( \mathrm{\Delta S_{total} > 0} \), the reaction is spontaneous.
• If \( \mathrm{\Delta S_{total} = 0} \), the system is at equilibrium.
• If \( \mathrm{\Delta S_{total} < 0} \), the reaction is not spontaneous.

Role of the Surroundings

• The surroundings are affected by heat exchange during a reaction.
• In an exothermic reaction, heat is released to the surroundings, increasing their entropy.
• In an endothermic reaction, heat is absorbed from the surroundings, decreasing their entropy.

Link with Enthalpy

\( \mathrm{\Delta S_{surroundings} = -\frac{\Delta H}{T}} \)

• Exothermic reactions (\( \mathrm{\Delta H < 0} \)) give a positive \( \mathrm{\Delta S_{surroundings}} \).
• Endothermic reactions (\( \mathrm{\Delta H > 0} \)) give a negative \( \mathrm{\Delta S_{surroundings}} \).
• The effect depends on temperature, as it appears in the denominator.

Key Features

• Total entropy determines the direction of spontaneous change.
• Both system and surroundings must be considered.
• Enthalpy change influences the entropy of the surroundings.
• Temperature affects the magnitude of entropy change of the surroundings.