Chapter 17 Free Energy and Thermodynamics

1. Chapter 17Free Energy and Thermodynamics

2. Tro, Chemistry: A Molecular Approach 2 First Law of Thermodynamics you can�t win! First Law of Thermodynamics: Energy cannot be Created or Destroyed the total energy of the universe cannot change though you can transfer it from one place to another DEuniverse = 0 = DEsystem + DEsurroundings

3. Tro, Chemistry: A Molecular Approach 3 First Law of Thermodynamics Conservation of Energy For an exothermic reaction, �lost� heat from the system goes into the surroundings two ways energy �lost� from a system, converted to heat, q used to do work, w Energy conservation requires that the energy change in the system equal the heat released + work done DE = q + w DE = DH + PDV DE is a state function internal energy change independent of how done

4. Tro, Chemistry: A Molecular Approach 4 Energy Tax you can�t break even! to recharge a battery with 100 kJ of useful energy will require more than 100 kJ every energy transition results in a �loss� of energy conversion of energy to heat which is �lost� by heating up the surroundings

5. Tro, Chemistry: A Molecular Approach 5 Heat Tax

6. Tro, Chemistry: A Molecular Approach 6 Thermodynamics and Spontaneity thermodynamics predicts whether a process will proceed under the given conditions spontaneous process nonspontaneous processes require energy input to go spontaneity is determined by comparing the free energy of the system before the reaction with the free energy of the system after reaction. if the system after reaction has less free energy than before the reaction, the reaction is thermodynamically favorable. spontaneity ? fast or slow

7. Tro, Chemistry: A Molecular Approach 7 Comparing Potential Energy

8. Tro, Chemistry: A Molecular Approach 8 Reversibility of Process any spontaneous process is irreversible it will proceed in only one direction a reversible process will proceed back and forth between the two end conditions equilibrium results in no change in free energy if a process is spontaneous in one direction, it must be nonspontaneous in the opposite direction

9. Tro, Chemistry: A Molecular Approach 9 Thermodynamics vs. Kinetics

10. Tro, Chemistry: A Molecular Approach 10 Diamond ? Graphite

11. Tro, Chemistry: A Molecular Approach 11 Factors Affecting Whether a Reaction Is Spontaneous The two factors that determine the thermodynamic favorability are the enthalpy and the entropy. The enthalpy is a comparison of the bond energy of the reactants to the products. bond energy = amount needed to break a bond. DH The entropy factors relates to the randomness/orderliness of a system DS The enthalpy factor is generally more important than the entropy factor

12. Tro, Chemistry: A Molecular Approach 12 Enthalpy . DH related to the internal energy (kJ/mol) stronger bonds = more stable molecules if products more stable than reactants, energy released Exothermic (DH = negative) if reactants more stable than products, energy absorbed Endothermic (DH = positive) The enthalpy is favorable for exothermic reactions and unfavorable for endothermic reactions. Hess� Law: DH�rxn = S(DH�prod) - S(DH�react)

14. Tro, Chemistry: A Molecular Approach 14 Entropy entropy is a thermodynamic function that increases as the number of energetically equivalent ways of arranging the components increases, S S generally J/mol S = k ln W k = Boltzmann Constant = 1.38 x 10-23 J/K W is the number of energetically equivalent ways, unitless Random systems require less energy than ordered systems

15. Tro, Chemistry: A Molecular Approach 15 W

16. Tro, Chemistry: A Molecular Approach 16 Macrostates ? Microstates

17. Tro, Chemistry: A Molecular Approach 17 Macrostates and Probability

18. Tro, Chemistry: A Molecular Approach 18 Changes in Entropy, DS entropy change is favorable when the result is a more random system. DS is positive Some changes that increase the entropy are: reactions whose products are in a more disordered state. (solid > liquid > gas) reactions which have larger numbers of product molecules than reactant molecules. increase in temperature solids dissociating into ions upon dissolving

19. Tro, Chemistry: A Molecular Approach 19 Increases in Entropy

20. Tro, Chemistry: A Molecular Approach 20 The 2nd Law of Thermodynamics the total entropy change of the universe must be positive for a process to be spontaneous for reversible process DSuniv = 0, for irreversible (spontaneous) process DSuniv > 0 DSuniverse = DSsystem + DSsurroundings if the entropy of the system decreases, then the entropy of the surroundings must increase by a larger amount when DSsystem is negative, DSsurroundings is positive the increase in DSsurroundings often comes from the heat released in an exothermic reaction

21. Tro, Chemistry: A Molecular Approach 21 Entropy Change in State Change when materials change state, the number of macrostates it can have changes as well for entropy: solid < liquid < gas because the degrees of freedom of motion increases solid ? liquid ? gas

22. Tro, Chemistry: A Molecular Approach 22 Entropy Change and State Change

23. Tro, Chemistry: A Molecular Approach 23 Heat Flow, Entropy, and the 2nd Law

24. Tro, Chemistry: A Molecular Approach 24

25. Tro, Chemistry: A Molecular Approach 25 Temperature Dependence of DSsurroundings when a system process is exothermic, it adds heat to the surroundings, increasing the entropy of the surroundings when a system process is endothermic, it takes heat from the surroundings, decreasing the entropy of the surroundings the amount the entropy of the surroundings changes depends on the temperature it is at originally the higher the original temperature, the less effect addition or removal of heat has

26. Tro, Chemistry: A Molecular Approach 26 Gibbs Free Energy, DG maximum amount of energy from the system available to do work on the surroundings G = H � T�S DGsys = DHsys � TDSsys DGsys = � TDSuniverse DGreaction = S nDGprod � S nDGreact when DG < 0, there is a decrease in free energy of the system that is released into the surroundings; therefore a process will be spontaneous when DG is negative

27. Ex. 17.2a � The reaction C3H8(g) + 5 O2(g) ? 3 CO2(g) + 4 H2O(g) has DHrxn = -2044 kJ at 25�C. Calculate the entropy change of the surroundings.

28. Tro, Chemistry: A Molecular Approach 28 Free Energy Change and Spontaneity

29. Tro, Chemistry: A Molecular Approach 29 Gibbs Free Energy, DG process will be spontaneous when DG is negative DG will be negative when DH is negative and DS is positive exothermic and more random DH is negative and large and DS is negative but small DH is positive but small and DS is positive and large or high temperature DG will be positive when DH is + and DS is - never spontaneous at any temperature when DG = 0 the reaction is at equilibrium

30. Tro, Chemistry: A Molecular Approach 30 DG, DH, and DS

31. Ex. 17.3a � The reaction CCl4(g) ? C(s, graphite) + 2 Cl2(g) has DH = +95.7 kJ and DS = +142.2 J/K at 25�C. Calculate DG and determine if it is spontaneous.

32. Ex. 17.3a � The reaction CCl4(g) ? C(s, graphite) + 2 Cl2(g) has DH = +95.7 kJ and DS = +142.2 J/K. Calculate the minimum temperature it will be spontaneous.

33. Tro, Chemistry: A Molecular Approach 33 The 3rd Law of ThermodynamicsAbsolute Entropy the absolute entropy of a substance is the amount of energy it has due to dispersion of energy through its particles the 3rd Law states that for a perfect crystal at absolute zero, the absolute entropy = 0 J/mol�K therefore, every substance that is not a perfect crystal at absolute zero has some energy from entropy therefore, the absolute entropy of substances is always +

34. Tro, Chemistry: A Molecular Approach 34 Standard Entropies S� Extensive (depends on amount) entropies for 1 mole at 298 K for a particular state, a particular allotrope, particular molecular complexity, a particular molar mass, and a particular degree of dissolution

36. Tro, Chemistry: A Molecular Approach 36 Relative Standard EntropiesStates the gas state has a larger entropy than the liquid state at a particular temperature the liquid state has a larger entropy than the solid state at a particular temperature

37. Tro, Chemistry: A Molecular Approach 37 Relative Standard EntropiesMolar Mass the larger the molar mass, the larger the entropy available energy states more closely spaced, allowing more dispersal of energy through the states

38. Tro, Chemistry: A Molecular Approach 38 Relative Standard EntropiesAllotropes the less constrained the structure of an allotrope is, the larger its entropy

39. Tro, Chemistry: A Molecular Approach 39 Relative Standard EntropiesMolecular Complexity larger, more complex molecules generally have larger entropy more available energy states, allowing more dispersal of energy through the states

40. Tro, Chemistry: A Molecular Approach 40 Relative Standard EntropiesDissolution dissolved solids generally have larger entropy distributing particles throughout the mixture

41. Ex. 17.4 �Calculate DS? for the reaction4 NH3(g) + 5 O2(g) ? 4 NO(g) + 6 H2O(l)

42. Tro, Chemistry: A Molecular Approach 42 Calculating DG? at 25?C: DGoreaction = SnGof(products) - SnGof(reactants) at temperatures other than 25?C: assuming the change in DHoreaction and DSoreaction is negligible DG?reaction = DH?reaction � TDS?reaction

43. 43

44. Ex. 17.7 �Calculate DG? at 25?C for the reactionCH4(g) + 8 O2(g) ? CO2(g) + 2 H2O(g) + 4 O3(g)

45. Ex. 17.6 � The reaction SO2(g) + � O2(g) ? SO3(g) has DH? = -98.9 kJ and DS? = -94.0 J/K at 25�C. Calculate DG? at 125?C and determine if it is spontaneous.

46. Tro, Chemistry: A Molecular Approach 46 DG Relationships if a reaction can be expressed as a series of reactions, the sum of the DG values of the individual reaction is the DG of the total reaction DG is a state function if a reaction is reversed, the sign of its DG value reverses if the amounts of materials is multiplied by a factor, the value of the DG is multiplied by the same factor the value of DG of a reaction is extensive

47. Tro, Chemistry: A Molecular Approach 47 Free Energy and Reversible Reactions the change in free energy is a theoretical limit as to the amount of work that can be done if the reaction achieves its theoretical limit, it is a reversible reaction

48. Tro, Chemistry: A Molecular Approach 48 Real Reactions in a real reaction, some of the free energy is �lost� as heat if not most therefore, real reactions are irreversible

49. Tro, Chemistry: A Molecular Approach 49 DG under Nonstandard Conditions DG = DG? only when the reactants and products are in their standard states there normal state at that temperature partial pressure of gas = 1 atm concentration = 1 M under nonstandard conditions, DG = DG? + RTlnQ Q is the reaction quotient at equilibrium DG = 0 DG? = -RTlnK

50. Tro, Chemistry: A Molecular Approach 50

51. 51 Example - DG Calculate DG at 427�C for the reaction below if the PN2 = 33.0 atm, PH2= 99.0 atm, and PNH3= 2.0 atm N2(g) + 3 H2(g) � 2 NH3(g)

52. Tro, Chemistry: A Molecular Approach 52

53. 53 Example - K Estimate the equilibrium constant and position of equilibrium for the following reaction at 427�C N2(g) + 3 H2(g) � 2 NH3(g)

54. Tro, Chemistry: A Molecular Approach 54 Temperature Dependence of K for an exothermic reaction, increasing the temperature decreases the value of the equilibrium constant for an endothermic reaction, increasing the temperature increases the value of the equilibrium constant