Entropy (AQA A-Level Chemistry): Revision Notes

5.1.6 Entropy

What is Entropy ($S$)?

Entropy, symbolised as $S$, is a measure of the disorder or randomness within a system. The higher the disorder, the greater the entropy. Standard entropy values ($S^\circ$) are measured in joules per kelvin per mole ($J K⁻¹ mol⁻¹$) and provide a reference for the degree of disorder in various substances.

Importance of Entropy in Feasibility of Reactions

While enthalpy ($\Delta H$) helps us understand the energy changes in a reaction, it alone is not sufficient to determine whether a reaction will occur spontaneously. Entropy changes ($\Delta S$) play a crucial role in predicting reaction feasibility, especially when changes in disorder significantly affect the reaction.

Factors Affecting Entropy

Physical State:

• Gases have the highest entropy due to the high degree of movement and disorder.
• Liquids and Solutions are more disordered than solids but less than gases.
• Solids have the lowest entropy as particles are in fixed positions.

Dissolution:

• Dissolving a solid increases entropy because particles can move more freely in solution, increasing disorder.

Number of Particles:

• An increase in the number of particles, especially in reactions that produce more moles of gas, leads to higher entropy due to more possible arrangements of particles and their energy.

Standard (Absolute) Entropy Values

Standard entropy values are based on a scale where the entropy of a substance is defined as zero at 0 K, assuming a perfect crystal structure at this temperature. As temperature increases, the entropy also increases. Thus, gases, with their high degree of disorder, have the highest standard entropy values.

Calculating Entropy Change ($ΔS$) in Reactions

The entropy change of a reaction can be calculated using absolute entropy values with the formula:

$$\Delta S = \sum S^\circ_{\text{products}} - \sum S^\circ_{\text{reactants}}$$

This calculation provides insight into whether a reaction increases or decreases in disorder.