AP Chemistry Unit 9.1 Introduction to Entropy

Introduction to Entropy

  • Thermodynamics let us predict whether a process will occur but does not tell us about the amount of time required
  • A spontaneous or “thermodynamically favored” process is one that occurs without intervention; may be fast or slow
    • Ex: a ball rolls down a hill but never spontaneously rolls back up


  • Entropy (S): measure of the randomness/disorder of a system
    • More disorder = greater entropy
  • Trends in entropy:
    • Ssolid < Sliquid << Sgas
    • More complex molecules have higher entropy
    • Weaker forces of attraction = higher S
    • Greater mass = higher S
Sign of ΔSΔS = +ΔS = –
MeaningThings are becoming more disordered = thermodynamically favoredThings are becoming more organized

Decomposition reaction (one reactant becoming two products) Dissolving

Endothermic reactions

Products have more moles of gas

Synthesis reactions (two reactants become one product)

Exothermic reactions

Molecules dispersed equally → move to single chamber

Products have less moles of gas

Second Law of Thermodynamics

  • In any thermodynamically favored process (energy transfer or transformation), there is usually an increase in the entropy of the universe
  • Since energy never flows spontaneously in the other direction, the entropy of the universe is always increasing
  • > 0 → usually a favored process
  • < 0 → usually NOT a favored process (favored in the opposite direction)
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