← Thermochemical principles and the properties of particles and substances

6 Entropy (ΔS)

Entropy (S) is a measure of the disorder or number of ways energy can be arranged in a system. Units: J K⁻¹ mol⁻¹ (note joules, not kilojoules).

Predicting the sign of ΔS

  • Entropy increases (ΔS > 0) going solid → liquid → gas.
  • ΔS > 0 when the number of moles of gas increases. Count gas moles on each side.
  • ΔS > 0 when a solid dissolves, or a large molecule breaks into smaller ones.
  • ΔS < 0 when gas moles decrease, or particles become more ordered (e.g. freezing, precipitation).
Entropy change of reaction ΔS° = Σ S°(products) − Σ S°(reactants) Elements have non-zero standard entropies (unlike ΔHf). ✓

Quick check

For N₂(g) + 3H₂(g) → 2NH₃(g): 4 mol gas → 2 mol gas, so ΔS is negative (fewer gas particles = more order).

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