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Chapter 17 Free Energy and Thermodynamics

Chapter 17 Free Energy and Thermodynamics

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Chapter 17 Free Energy and Thermodynamics

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  1. Chapter 17Free Energy and Thermodynamics

  2. First Law of Thermodynamics:The Conservation of Energy • You can’t win! • First Law of Thermodynamics: Energycannot be created or destroyed E can transfer from one place to another Tro: Chemistry: A Molecular Approach, 2/e

  3. 1st Law of Thermodynamics • Conservation of E • For an exothermic rxtn, “lost” heat from system goes into surroundings • 2 ways energy is “lost” from a system Tro: Chemistry: A Molecular Approach, 2/e

  4. 1st Law of Thermodynamics • E conservation requires that E D in system = heat released + work done • DE is a state function Tro: Chemistry: A Molecular Approach, 2/e

  5. The Energy Tax • You can’t break even! • To recharge a battery with 100 kJ of useful E will require Every E transition  a “loss” of E “Energy Tax” demanded by nature & conversion of E to heat Tro: Chemistry: A Molecular Approach, 2/e

  6. Heat Tax Tro: Chemistry: A Molecular Approach, 2/e

  7. Thermodynamics predicts if a process will occur under given conditions = spontaneous • Non-spontaneous processes require • Can you name some non-spontaneous rxtns? Tro: Chemistry: A Molecular Approach, 2/e

  8. Determine “Spontaneity” by comparing chem PE of system before rxtn with free E of system after rxtn • if system afterrxtn has less PE than beforerxtn, then rxtn is thermodynamically favorable. Tro: Chemistry: A Molecular Approach, 2/e

  9. Comparing Potential Energy direction of spontaneity can be determined by comparing Tro: Chemistry: A Molecular Approach, 2/e

  10. Reversibility of Process • Any spontaneous process is irreversible (net release of E in that direction) • it will proceed in • A reversible process will proceed back & forth btwn the two end conditions • any reversible process is at ____________ • results in Tro: Chemistry: A Molecular Approach, 2/e

  11. Reversibility of Process • If a process is spontaneous in one direction, Tro: Chemistry: A Molecular Approach, 2/e

  12. Thermodynamics vs. Kinetics Tro: Chemistry: A Molecular Approach, 2/e

  13. Diamond → Graphite Graphite more stable than diamond, so conversion of diamond  graphite isspontaneous Tro: Chemistry: A Molecular Approach, 2/e

  14. Spontaneous processes occur because • Most spontaneous processes go from a system of higherPE to lowerPE e.g. Tro: Chemistry: A Molecular Approach, 2/e

  15. Spontaneous Processes • Some spontaneous processes proceed from a system of lowerPE to a system at higherPE e.g. • How can something absorb PE, yet have a net release of E? Tro: Chemistry: A Molecular Approach, 2/e

  16. Tro: Chemistry: A Molecular Approach, 2/e

  17. Melting Ice When a solid melts, the particles have more Tro: Chemistry: A Molecular Approach, 2/e

  18. Melting Ice More freedom of motion __________________________ of the system. When systems become more random, E is _______________ This E= Tro: Chemistry: A Molecular Approach, 2/e

  19. Factors Affecting Whether a Reaction Is Spontaneous Two factors determine if a rxtn is spontaneous. Tro: Chemistry: A Molecular Approach, 2/e

  20. Factors Affecting Whether a Reaction Is Spontaneous • enthalpy change, DH, = difference in sum of internal E & PV work E of reactants compared to internal E & PV work E of products Tro: Chemistry: A Molecular Approach, 2/e

  21. Factors Affecting Whether a Reaction Is Spontaneous • entropy change, DS, is difference in “randomness” of reactants compared to products Tro: Chemistry: A Molecular Approach, 2/e

  22. Enthalpy Change • DH generally measured in • Stronger bonds = • A rxtn is generally exothermic if bonds in products are • exothermic = E released Tro: Chemistry: A Molecular Approach, 2/e

  23. Enthalpy Change • A rxtn is generally endothermic if bonds in products are weaker than bonds in reactants endothermic = E absorbed • DS is favorable for exothermic reactions and unfavorable for endothermic reactions Tro: Chemistry: A Molecular Approach, 2/e

  24. Entropy • Entropy= thermodynamic function • Entropy, S, increases as # of energetically equivalent ways of arranging components increases Tro: Chemistry: A Molecular Approach, 2/e

  25. S = k lnW k = Boltzmann Constant = R/Avogadro’s # k = Tro: Chemistry: A Molecular Approach, 2/e

  26. Entropy • S = k lnW k = Boltzmann Constant = 1.38 x 10−23 J/K W = (W is unitless) = • Random systems require Tro: Chemistry: A Molecular Approach, 2/e

  27. These are energetically equivalent states for expansion of a gas. W In terms of PE, doesn’t matter whether molecules are all in one flask, or evenly distributed But one of these states is more probable than other two Tro: Chemistry: A Molecular Approach, 2/e

  28. These microstates all have the same macrostate So there are six different particle arrangements that result in the same macrostate Macrostates → Microstates Tro: Chemistry: A Molecular Approach, 2/e

  29. This macrostate can be achieved through several different arrangements of the particles Macrostates → Microstates Tro: Chemistry: A Molecular Approach, 2/e

  30. Macrostates and Probability There is only one possible arrangement that gives State A and one that gives State B There are six possible arrangements that give State C The macrostate with the highest entropy also has the greatest dispersal of energy Tro: Chemistry: A Molecular Approach, 2/e

  31. Macrostates and Probability Therefore State C has higher entropy than either State A or State B There is 6x probability of having the State C macrostate than either State A or State B Tro: Chemistry: A Molecular Approach, 2/e

  32. Changes in Entropy, DS DS = Sfinal − Sinitial • Entropy change is favorable when result is a more Tro: Chemistry: A Molecular Approach, 2/e

  33. Some D’s that increase entropy are: • rxtns whose products are more random state e.g. • rxtns that have larger #’s of product molecules than reactant molecules Tro: Chemistry: A Molecular Approach, 2/e

  34. Increases in Entropy Tro: Chemistry: A Molecular Approach, 2/e

  35. DS For a process where final condition is more random than initial condition, DSsystemis ________________ and entropy D is ___________________ for the process to be spontaneous Tro: Chemistry: A Molecular Approach, 2/e

  36. DS For a process where final condition is more orderly than initial condition, DSsystemis ____________and entropy change is ___________________ for the process to be spontaneous DSsystem = DSreaction = Sn(S°products) −Sn(S°reactants) Tro: Chemistry: A Molecular Approach, 2/e

  37. Entropy Change in State Change • When materials D state, # of macrostates it can have D’s as well • more degrees of freedom molecules have, more macrostates are possible Tro: Chemistry: A Molecular Approach, 2/e

  38. Entropy Change in State Change • solids have fewer macrostates than liquids, which have fewer macrostates than gases Tro: Chemistry: A Molecular Approach, 2/e

  39. Entropy Change and State Change Tro: Chemistry: A Molecular Approach, 2/e

  40. Try this: Predict whether DSsystem is (+) or (−) for each of the following • A hot beaker burning your fingers • Water vapor condensing • Separation of oil and vinegar salad dressing • Dissolving sugar in tea • 2 PbO2(s)  2 PbO(s) + O2(g) • 2 NH3(g)  N2(g) + 3 H2(g) • Ag+(aq) + Cl−(aq)  AgCl(s) Tro: Chemistry: A Molecular Approach, 2/e

  41. Chapter 17Free Energy and ThermodynamicsPART 2

  42. 2nd Law of Thermodynamics • says that total entropy D of universe must be positive for a process to be spontaneous • for irreversible (spontaneous) process • for reversible process • DSuniverse = DSsystem + DSsurroundings Tro: Chemistry: A Molecular Approach, 2/e

  43. The 2nd Law of Thermodynamics • DSuniverse = DSsystem + DSsurroundings • If entropy of system ____________________, then entropy of surroundings must increase by ___________________________ when DSsystem is __________________, DSsurroundingsmust be ____________________ for a spontaneous process Tro: Chemistry: A Molecular Approach, 2/e

  44. Heat Flow, Entropy, and the 2nd Law When ice is placed in water, heat flows from 2nd Law: heat must flow from water to ice because it results in Entropy of universe Tro: Chemistry: A Molecular Approach, 2/e

  45. Heat Transfer &DS of Surroundings • 2nd Law demands that Suniverseincrease for a spontaneous process • Yet processes like water vapor condensing are spontaneous, even though water vapor is more random than liquid water. • What’s driving that process then? Tro: Chemistry: A Molecular Approach, 2/e

  46. Heat Transfer &DS of Surroundings • If a process is spontaneous, yet DS of process is unfavorable, there must have been • Entropy increase must heat released by system: Tro: Chemistry: A Molecular Approach, 2/e

  47. Heat Transfer &DS of Surroundings When the DSsystem is unfavorable (neg) DSsurroundings must be favorable (pos), and large to allow process to be spontaneous Tro: Chemistry: A Molecular Approach, 2/e

  48. Heat Exchange and DSsurroundings • When a process is exothermic, heat is added to surroundings, increasing Ssurroundings • When a process is endothermic, it takes heat from surroundings, decreasing Ssurroundings Tro: Chemistry: A Molecular Approach, 2/e

  49. Heat Exchange and DSsurroundings • Amount of Dssurroundingsdepends on original temp • higher original temp, Tro: Chemistry: A Molecular Approach, 2/e

  50. Temp Dependence of DSsurroundings • When heat is added to surroundings that are cool greater effect on DS than it would have if the surroundings were already hot Tro: Chemistry: A Molecular Approach, 2/e