Entropy (S) is a measure of the disorder or number of possible arrangements (microstates) of a system. The greater the disorder, the higher the entropy.
Key Definition
Entropy (S) — a thermodynamic quantity representing the number of ways energy can be distributed among particles. Units: J K⁻¹ mol⁻¹
\( \Delta S = \sum S(\text{products}) - \sum S(\text{reactants}) \)
Predicting Entropy Changes
Entropy Increases With...
Rules for Predicting ΔS
- ΔS > 0 (positive): solid → liquid → gas, fewer → more moles of gas, dissolving
- ΔS < 0 (negative): gas → liquid → solid, more → fewer moles of gas
- Entropy of a perfect crystal at 0 K is zero (Third Law of Thermodynamics)
Think About It
CaCO₃(s) → CaO(s) + CO₂(g). Predict the sign of ΔS.
ΔS > 0 (positive) — a gas is produced from a solid. The number of moles of gas increases from 0 to 1, greatly increasing disorder.