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Thermodynamics

Thermodynamics. Chapter 19. Spontaneous Processes and Entropy. Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required for the process. A spontaneous process is one that occurs without outside intervention. Entropy.

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Thermodynamics

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  1. Thermodynamics Chapter 19

  2. Spontaneous Processes and Entropy • Thermodynamics lets us predict whether a process will occur but gives no information about the amount of time required for the process. • A spontaneous process is one that occurs without outside intervention.

  3. Entropy • The driving force for a spontaneous process is an increase in the entropy of the universe. • Entropy, S, can be viewed as a measure of randomness, or disorder.

  4. Positional Entropy • A gas expands into a vacuum because the expanded state has the highest positional probability of states available to the system. • Therefore, • Ssolid < Sliquid << Sgas

  5. The Second Law of Thermodynamics • . . . in any spontaneous process there is always an increase in the entropy of the universe. • Suniv > 0 for a spontaneous process.

  6. Free Energy • G = HTS(from the standpoint of the system) • A process (at constant T, P) is spontaneous in the direction in which free energy decreases: • Gmeans+Suniv

  7. Effect of H and S on Spontaneity

  8. The Third Law of Thermodynamics • . . . the entropy of a perfect crystal at 0 K is zero. • Because S is explicitly known (= 0) at 0 K, S values at other temps can be calculated.

  9. Free Energy Change and Chemical Reactions • G = standard free energy change that occurs if reactants in their standard state are converted to products in their standard state. • G = npGf(products)nrGf(reactants)

  10. Entropy and Enthalpy

  11. Free Energy and Pressure • G = G + RT ln(Q) • Q = reaction quotient from the law of mass action.

  12. Free Energy and Equilibrium • G = RT ln(K) • K = equilibrium constant • This is so because G = 0 and Q = K at equilibrium.

  13. Temperature Dependence of K • y = mx + b • (H and S independent of temperature over a small temperature range) ln(K) = -Ho/R*(1/T) + So/R

  14. Reversible v. Irreversible Processes • Reversible: The universe is exactly the same as it was before the cyclic process. • Irreversible: The universe is different after the cyclic process. • All real processes are irreversible -- (some work is changed to heat).

  15. END

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