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Absolute Entropy and Entropy Change

Entropy (S) is a measure of disorder or randomness in a system. Absolute entropy and entropy change help us predict the spontaneity of reactions.

Absolute Entropy (S⁰)​

  • Definition: Entropy of a substance at a standard state (1 bar, usually 298 K)
  • Units: J/(mol·K)
  • Factors Affecting S⁰:
    • Phase: Gas > Liquid > Solid
    • Molecular complexity: More atoms → higher S⁰
    • Temperature: Higher temperature → higher S

Entropy Change (ΔS)​

  • Definition: Change in entropy during a process or reaction
  • Calculation:
    ΔS = ΣS⁰(products) − ΣS⁰(reactants)
  • Sign of ΔS:
    • ΔS > 0 → Increase in disorder
    • ΔS < 0 → Decrease in disorder

Entropy and Spontaneity​

  • Combined with enthalpy (ΔH) in Gibbs Free Energy:
    ΔG = ΔH − TΔS
  • ΔG < 0 → Spontaneous reaction
  • ΔS helps determine whether a reaction is favored at high or low temperatures

Key Points​

  • Entropy increases when gases are produced or solids dissolve.
  • More complex molecules generally have higher absolute entropy.
  • Entropy is temperature-dependent and critical for understanding reaction spontaneity.

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