AP Chemistry Unit 9.2 Absolute Entropy and Entropy Changes

Absolute Entropy and Entropy Changes

  • The Third Law of Thermodynamics: the entropy of a perfect (ionic) crystal at 0K is zero bcus there is NO disorder (everythings is positioned perfectly)
    • S values at other temps must be higher

Standard Entropy

  • Standard Entropies S° (at 298K and 1 atm) (must be given)
  • Pure elements have lower entropy values than larger/complex molecules
    • More complex molecules have higher S° because more ways the molecules can be arranged
  • S° is a state function
    • Unit: j/K
  • = J/K x M
    • Total entropy of products – total entropy of reactants
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