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Entropy and Energy Dispersal

Entropy, S, is a measure of how widely energy is dispersed (spread out). Work through the three stages in order: what dispersal means, why solids, liquids and gases differ, and how the surroundings count towards the Second Law.

Left flask: volume V Right flask: empty (vacuum)
Left flask: 0 molecules
Right flask: 0 molecules

Remember

  • Entropy is a measure of how widely energy is dispersed. ΔS > 0 when energy becomes more widely dispersed.
  • S: solid < liquid < gas. Gas molecules move freely through a large volume and rotate, so their energy is dispersed over many more ways of moving.
  • Second Law: for a spontaneous process, ΔStotal = ΔSsys + ΔSsurr > 0, with ΔSsurr = −ΔH / T.
  • ΔS values are tabulated in J mol−1 K−1. S° values for liquid water and water vapour are from the CRC Handbook at 298.15 K; the value for ice is at 0 °C. ΔH and ΔS are assumed not to change with temperature.