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Thermodynamics

Thermodynamics. Chapter 20. Thermodynamics. Prediction of whether change will occur No indication of timeframe Spontaneous: occurs without external intervention Nonspontaneous: requires outside “push”. Entropy and Spontaneity. Driving force for a spontaneous change is an

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Thermodynamics

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  1. Thermodynamics Chapter 20

  2. Thermodynamics • Prediction of whether change will occur • No indication of timeframe • Spontaneous: • occurs without external intervention • Nonspontaneous: • requires outside “push”

  3. Entropy and Spontaneity • Driving force for a spontaneous change is an • increase in entropy of the universe • Entropy, S: measure of disorder • Spontaneous change implies: • more order  less order • fewer ways of arranging particles  more

  4. Second Law of Thermodynamics • In any spontaneous change, there is always an increase in entropy of the universe. • Units: J • K

  5. Entropy • 1877 Ludwig Boltzmann: • k = Boltzmann constant, R/NA • W = no. of possible arrangements • Third Law of Thermodynamics: • The entropy of a perfect crystal at 0 K is zero.

  6. Positional Entropy • Why does a gas expand into a vacuum? • Expanded state has highest positional probability of states available.

  7. Other factors in entropy • Size: • increase in S with increasing size (mass) • Molecular complexity: • increase in S with increasing complexity • Generally effect of physical state >> complexity

  8. Reactions • For a spontaneous reaction: • NaOH(s) + CO2(g) Na2CO3(s) + H2O(l) • S0 64.45 213.7 139 69.94 J/K • Is the reaction spontaneous as written?

  9. Spontaneity and S • Spontaneous: Suniv > 0 • Nonspontaneous: Suniv < 0 • At equilibrium: Suniv = 0 • DSsys can be positive if DSsurr increases enough

  10. Surroundings and Suniv • Surroundings add or remove heat • Exothermic: • Dqsys < 0 • Dqsurr > 0 so DSsurr > 0 • Endothermic: • Dqsys > 0 • Dqsurr < 0 so DSsurr < 0

  11. DSsurr and DSsys • DSsurr: DSsurr - Dqsys • DSsurr 1/T • At constant pressure:

  12. The Math • Given: • @constant P: • Multiply by  T: • Result:

  13. Reactions and DG • DG0: Standard Free Energy • Reactants in standard states are • converted to products in standard states

  14. Gibb’s Free Energy • Overall criterion for spontaneity • from the standpoint of the system • A process at constant temp. and pressure is spontaneous in the direction DG decreases

  15. G = H - TS

  16. Summary • DG < 0 Spontaneous as written • DG > 0 Not spontaneous as written • Reverse process spontaneous • DG = 0 At equilibrium

  17. A Closer Look… • TDS: • energy not avail. for doing work • DG: • E avail. as heat – E not avail. for work •  max. work available (constant T and P) • Amount of work actually obtained depends on path

  18. DG and Work • DG • Spontaneous max. work obtainable • Nonspontaneous min. work required • Work and path-dependence • wmax (wmin) process performed reversibly • theoretical • wactual < wmax performed irreversibly • real world

  19. Reversible vs. 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.

  20. Free Energy and Pressure • Q: reaction quotient from mass action law

  21. Free Energy and Equilibrium • K: equilibrium constant • At • equilibrium: DG = 0 • K = Q

  22. A B

  23. DG and Extent of Reaction A B DG0B < DG0A Spontaneous C D DG0D> DG0C Nonspontaneous

  24. Temperature Dependence of K • Plot lnK vs. 1/T • slope = -DH0/R intercept = DS0/R • *assumes DH0, DS0 relatively T independent

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