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How fast will it occur – Kinetics How much heat will it give off or absorb – D H (enthalpy)

Questions We Can Ask. How fast will it occur – Kinetics How much heat will it give off or absorb – D H (enthalpy) Will it create more or less disorder – D S (entropy) Will it occur at all? - D G (Gibb’s Free Energy). Spontaneous Reactions.

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How fast will it occur – Kinetics How much heat will it give off or absorb – D H (enthalpy)

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  1. Questions We Can Ask • How fast will it occur – Kinetics • How much heat will it give off or absorb – DH (enthalpy) • Will it create more or less disorder – DS (entropy) • Will it occur at all? - DG (Gibb’s Free Energy)

  2. Spontaneous Reactions • Spontaneous Rxns – occur without an outside source of energy • Direction – Rxns are spontaneous in one direction only (eggs breaking picture) • Temperature has an effect Ice Water (spont. Above 0oC, reverse not spontaneous)

  3. Spontaneous Reactions • Many spontaneous rxns: 1. Exothermic – Give off heat 2. Increase disorder

  4. Entropy • Entropy – measure of disorder • More disorder, larger the entropy • DS+ More disorder (shuffling a deck of cards) • DS- Less disorder (ordering a deck of cards)

  5. Boltzmann

  6. Entropy • States of matter Solid  Liquid  Gas  Plasma • Dissolving - DS+ NaOH(s)  Na+(aq) + OH-(aq)

  7. Entropy • Decomposing CaCO3(s)  CaO(s) + CO2(g) 4. Increase in # gas molecules PCl5(g)  PCl3(g) + Cl2(g)

  8. Entropy Is DS is positive or negative for: • H2O(l)  H2O(g) • Ag+(aq) + Cl-(aq)  AgCl(s) • 2Fe2O3(s) 4Fe(s) + 3O2(g) • CaO(s) + CO2(g)  CaCO3(s) • N2(g) + 3H2(g)  2NH3(g) • N2(g) + O2(g)  2NO(g)

  9. The First Law • Energy is conserved • Energy cannot be created or destroyed, it only changes form • DE = q + w • Battery in a motorized toy car Chemical  Electrical Mechanical Work Done Some waste heat produced

  10. The Second Law • Entropy is not conserved • The universe is getting more disordered. • Can only create “local order.” 4. Ex: Cleaning locker – You sweat and threw a lot of things away. Universe got more disordered.

  11. The Third Law • The entropy of a pure crystal at absolute zero is zero • Theoretical state of perfect order • Above 0 KAt 0 K atoms wiggle no molec. motion

  12. Laws of Thermodynamics

  13. Which of the following pairs has the higher entropy? • H2O(s) or H2O(l) • NaCl(s) or HCl(g) • HCl(g) or Ar(g) • N2(g) at 78 K or N2(g) at 298 K • H2(g) or SO2(g) • 1 mol of HCl(g) or 2 mol HCl(g)

  14. Calculating Entropy • Standard Molar Entropies – at 1 atm and 25oC (298 K) • Unit – Joule/ mol K • Standard molar entropies of element is not zero DHofSo Fe(s) 0 27.2 4. DSor = SnSoprod – S mSoreactants

  15. Calculating Entropy • Calculate DSo for: N2(g) + 3H2(g)  2NH3(g) (Ans: -198.3 J/ mol K, more order) 2. Calculate DSo for: Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g) (Ans: 180.4 J/ mol K, more disorder)

  16. Gibbs Free Energy • Josiah Gibbs – 1st Ph.D. in science from a U.S. University (Yale, 1863) • “Free Energy” – Maximum amount of work you can get from a chemical reaction • DG < 0 Rxn will occur DG = 0 Rxn at equilibrium DG > 0 Rxn will NOT occur 4. DG NEVER tells you how fast a rxn will occur

  17. Gibbs Free Energy • Combustion of Methane CH4 + O2 DG (-) CO2 + H2O Reaction position

  18. DGor = SnDGfoprod – S mDGforxts • Will the following rxn occur? N2(g) + 3H2(g)  2NH3(g) (Ans: DGor = -33.32 kJ) 2. Will the following rxn occur? CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) (Ans: DGor = -800.7 kJ)

  19. Gibbs Free Energy DG = DH – TDS T must be in Kelvin • Use DH and DS to determine if the following reaction occurs spontaneously at 500oC. N2(g) + 3H2(g)  2NH3(g) (Ans: DG = 61 kJ) 2. At what temperature will it become spontaneous?

  20. Gibbs Free Energy 3. Use DH and DS to determine if the following reaction occurs spontaneously at 25 oC? 2SO2(g) + O2(g)  2SO3(g) (Ans: DG = -140.1 kJ) 4. At what temperature will it become non-spontaneous?

  21. Gibbs Free Energy DG = DH – TDS

  22. Gibbs Free Energy • A certain rxn is exothermic and becomes more ordered. Will the rxn occur at any temperature? • A certain rxn is endothermic and becomes less ordered. Will it occur?

  23. Gibbs Free Energy 3. A certain rxn is exothermic and occurs at any temperature. What does this reveal?

  24. A solution of sodium chloride is added to a solution of silver(I)nitrate. • Write the net ionic reaction • Predict and explain the sign of DG (a reaction does occur) • Predict and explain the sign of DS • Predict and explain the sign of DH • High or low temperature spontaneity?

  25. Overview DH Is heat produced or must heat a rxn DHor = SnDHoprod – S mDHorxts DS more or less order DSor = SnDSoprod – S mDSorxts

  26. Overview DG • Tells you whether a rxn will occur spontaneously • Considers DH, DS and temperature DG = DH – TDS • Does NOT tell you speed

  27. Free Energy and K DG = 0 at equilibrium DG = DGo + RT lnQ • = DGo + RT lnK DGo = -RT lnK or K = e-DG/RT DGo negative K>1 DGo zero K=1 DGo positive K<1

  28. Example 3 Calculate the value of K for the following equation if DGo = -33.32 kJ: N2(g) + 3H2(g)  2NH3(g) DGo = -RT lnK lnK = DGo = -33,320 J -RT (-8.314 J/mol-K)(298K) lnK = 13.4 K = e13.4 = 6.6 X 105

  29. Example 4 Calculate DGoand K for the following reaction at 298 K. H2(g) + Br2(g)  2HBr(g) ANS: -109.6 kJ/mol, 1.6 X 1019

  30. Free Energy and K DGo is at 298 K (25oC) Can calculate DG at other temperatures using: DG = DGo + RT lnQ DGo = Free energy change at 25oC R = 8.314 J/mol-K T = Temperature in Kelvin (Absolute temperature) Q = Reaction Coefficient

  31. Example 1 Calculate the DG at 298 K for a reaction mixture of 1.0 atm of N2, 3.0 atm of H2, and 0.50 atm of NH3. N2(g) + 3H2(g)  2NH3(g)

  32. Q = [NH3]2 = (0.50)2 = 0.0093 [N2][H2]3 (1.0)(3.0)3 DGo = -33.32 kJ DG = DGo + RT lnQ DG = -33,320 J+ (8.3.14 J/mol-K)(298 K)(ln0.0093) DG = - 44.9 kJ/mol (larger driving force for making NH3)

  33. Example 2 Calculate the DG at 298 K for a reaction mixture of 0.50 atm of N2, 0.75 atm of H2, and 2.0 atm of NH3. (ANS: -26.0 kJ/mol)

  34. a) As4 b) 1 mol H2O(g) c) 0.5 mol CH4 d) 100 g Na2SO4(aq) • a) DS – (fewer moles gas) b) DS+ (more moles gas) c) DS- (fewer moles gas) d) DS – (formation of solid) • a) 15.9 b) -147.2 c) -207.2 d) -221.5 54. a) DH- b) DS+ c) DG =-32.2 kJ (spon) • a) -2116.4 kJ b) 218.1 kJ c) -895.0 kJ d) -240.6 kJ • a) -190.5 kJ b) 69.1 kJ c) 192.7 kJ d) 40.8 kJ

  35. 62. a) -795 kJ b) +519 kJ, 3200K • a) DG becomes more negative, more spon b) DG becomes more positive, less spon c) DG becomes more negative, more spon 76. a) DGo = 29.2 kJ b) DG = -9.3 kJ • a) K = 2 X 10-14 b) K = 1.6 X 1014 c) K = 3.5 X 1024

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