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Thermodynamics

Thermodynamics. Chapter 19. 19.1 Spontaneous Process. Occurs without assistance Could be fast or slow – rate does not determine spontaneity Exothermic reactions are always spontaneous Endothermic reactions are sometimes spontaneous

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Thermodynamics

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  1. Thermodynamics Chapter 19

  2. 19.1 Spontaneous Process • Occurs without assistance • Could be fast or slow – rate does not determine spontaneity • Exothermic reactions are always spontaneous • Endothermic reactions are sometimes spontaneous • So enthalpy is not the only thing that determines spontaneity • An irreversible process is a spontaneous process

  3. 19.2 Entropy/ 2nd Law 2nd law: Any irreversible process results in increased entropy Entropy = disorder (state function) • S = Suniv + Ssys • S = qrev/T

  4. 19.3 Entropy / 3rd Law 3rd Law = the entropy of a pure crystal at 0K = 0 Molecular entropy: S = klnW (W = microstates) Microstates • Translational (movement of atom/molecule) • Vibrational (movement of atoms in a molecule) • Rotational (spin of an atom/molecule)

  5. 19.3 continued S increases when: • Temperature ↑ • Volume ↑ • Number of independent particles ↑ • e.x. liquid evaporating, solid dissolving

  6. 19.4 chemical reactions Standard molar entropy: S° (J/mol.K) • Never zero • Gases > liquids • High mass > low mass • # atoms in molecule ↑ entropy ↑ Ssur = -qsys/T qsys ~ H

  7. 19.5 Gibbs Free Energy • A spontaneous endothermic reaction must result in an increase in entropy. • A spontaneous process that results in a decrease in entropy must be exothermic Gibbs Free Energy explains this: G = H -TS • - G = spontaneous forward reaction • G = 0 = equilibrium • +G = non-spontaneous forward reaction

  8. 19.6 Free Energy & Temperature G = H -TS -G indicates a spontaneous reaction • Spontaneous when -H and +S • Never spontaneous when +H and -S • Can be spontaneous when: • -H > T(-)S • +H < T(+)S

  9. 19.6 continuedG = H -TS

  10. 19.7 Gibbs & Equilibrium G = G° + RTlnQ • Standard conditions Q = 1 so: • G = G° • At equilibrium G = 0 so: • G° = - RTlnK • K = e-G°/RT • If -G° then K > 1 (products favored) • If +G° then K < 1 (reactants favored)

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