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AP Chem

AP Chem. Get Thermo Practice WS stamped off Today: Entropy, Gibbs Free Energy Spontaneous reactions Unit 4 Quest Th 11/8. First Law of Thermodynamics. Energy cannot be created nor destroyed. Therefore, the total energy of the universe is a constant.

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AP Chem

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  1. AP Chem • Get Thermo Practice WS stamped off • Today: • Entropy, Gibbs Free Energy • Spontaneous reactions • Unit 4 Quest Th 11/8

  2. First Law of Thermodynamics • Energy cannot be created nor destroyed. • Therefore, the total energy of the universe is a constant. • Energy can, however, be converted from one form to another or transferred from a system to the surroundings or vice versa

  3. Spontaneous Process • Spontaneous processes can occur without any outside intervention ** the spontaneous process may be slow or fast

  4. 1. Directionality • If one direction of a process is spontaneous the other direction is not • A nail will spontaneously rust but not spontaneously unrust

  5. 2. Temperature/Pressure • Pressure and temperature affect spontaneity • Above 0 C it is spontaneous for ice to melt. • Below 0 C the reverse process (freezing) is spontaneous. • At 0oC both processes are in equilibrium and neither reaction is favored

  6. Example • Determine whether the following processes are spontaneous: a) when a piece of metal heated to 150oC is added to water at 40oC, the water gets hotter b) water at room temperature decomposes into H2(g) and O2(g) c) Benzene vapor, C6H6(g), at a pressure of 1atm, condenses to a liquid at the normal boiling point of benzene, 80.1oC

  7. What is the driving force for reactions? • Thought used to be enthalpy – most spontaneous reactions are exothermic • negative ∆H is “good” – more favorable • leads to lower potential energy, which is more stable

  8. What is the driving force for reactions? • Most spontaneous reactions are exothermic. BUT……. We do have reactions that are endothermic that occur spontaneously • So another driving force could be ENTROPY (S) Units = J/mol x K ** NOTE THE UNITS! The standard for this is in J, while ∆H is in kJ/mol

  9. Entropy • Measure of degree of disorder, randomness, or chaos in a system • Described as the # of possible arrangements that are available to the system • Reactions will “tend towards disorder”

  10. Spontaneous Reaction Scorecard • Enthalpy, H, negative is “good” lowering of energy • Entropy, S, positive is “good” increase in possibilities/randomness

  11. Increasing Entropy • Entropy tends to increase with increases in • Temperature • Volume • The number of independently moving molecules • State change, solubility

  12. # Moving particles 1. Change in State • Entropy increases with the freedom of motion of molecules. • Therefore, S(g) > S(l) > S(s) • Solubility • Generally, when a solid is dissolved in a solvent, entropy increases

  13. Example 1. Choose the sample of matter that has a greater entropy in each pair: • 1mol NaCl(s) or 1mol HCl(g) • 2mol HCl(g) or 1mol HCl(g) • 1mol of HCl(g) or 1mol Ar(g)

  14. Example 2. Predict whether S is positive or negative for each of the following processes: a) H2O(l)  H2O(g) b) Ag+(aq) + Cl-(aq)  AgCl(s) c) 4Fe(s) + 3O2(g)  2Fe2O3(s) d) N2(g) + O2(g)  2NO(g)

  15. Second Law of Thermodynamics • The entropy of the universe increases for spontaneous processes, and the entropy of the universe does not change for reversible processes.

  16. Main Entropy Calculation Entropy changes for a reaction can be estimated in a manner analogous to that by which H is estimated: S = S(products) —S(reactants)

  17. Example Calculate the entropy of the following reaction: 2NiS(s) + 3O2(g) --> 2SO2(g) + 2NiO(s) SO2 = 248 J/mol K NiO = 38 J/mol K O2 = 205 J/mol K NiS = 53 J/mol K

  18. Example Calculate the entropy of the following reaction: 2NiS(s) + 3O2(g) --> 2SO2(g) + 2NiO(s) S =(2(248) + 2(38)) - (2(53) + 3(205))

  19. Example Calculate the entropy of the following reaction: 2NiS(s) + 3O2(g) --> 2SO2(g) + 2NiO(s) S = (2(248) + 2(38)) - (2(53) + 3(205) (496+76) - (106+615) 572-721 S = -149 J / K

  20. Example Calculate the entropy of the following reaction: 2NiS(s) + 3O2(g) --> 2SO2(g) + 2NiO(s) S is decreasing “good or bad?” Why? Gas molecules are decreasing

  21. Example • Calculate the standard entropy change for the reaction below at 298K: Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g)

  22. 3rd Law of Thermodynamics • The entropy of a pure solid at Absolute Zero is equal to 0 • ONLY AT ABSOLUTE ZERO! • Hf of an element/diatomic = 0, but the S of one is usually NOT – be careful

  23. Gibbs Free Energy (G) • Can be thought of as the amount of available energy to do useful work. • Mathematically combines enthalpy and entropy to predict whether a reaction at constant temperature and pressure will be spontaneous ΔG = ΔH - T ΔS

  24. Gibbs Free Energy • If DG < 0 • the forward reaction is spontaneous. • If DG = 0 • the system is at equilibrium. • If G > 0 • the reaction is spontaneous in the reverse direction.

  25. Calculations • G = H - T S Problem with units: H = kJ/mole S = J / mole x K T = K *****Convert J to kJ******

  26. Example 1. Calculate the standard free energy change for the formation of NO(g) from N2(g) and O2(g) at 298K given that ΔHo=180.7kJ and ΔSo=24.7J/K. Is the reaction spontaneous under these conditions? ΔG = ΔH - T ΔS ΔG = (180.7 kJ) – (298 K)(kJ/K) ΔG = + 173.3 kJ, Not spontaneous

  27. Example 2. A particular reaction has a ΔHo = 24.6kJ and ΔSo =132J/K at 298K. Calculate ΔGo. Is the reaction spontaneous under these conditions?

  28. Standard Free Energy Changes Analogous to standard enthalpies of formation are standard free energies of formation, G. DG = SDG products)  SG (reactants)

  29. Example 1. Use data from appendix C to calculate the free-energy change for the following reaction at 298K: P4(g) + 6Cl2(g)  4PCl3(g) . What is ΔGo for the reverse reaction?

  30. Example 2. The ΔHo for the combustion of propane is -2220kJ : C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l) a) Predict whether ΔGo for this reaction is more or less negative than ΔHo b) Calculate ΔGo. Was your prediction correct?

  31. Free Energy and Temperature ΔG = ΔH - T ΔS

  32. Example 1. The Haber process is used for the production of ammonia. Assume ΔHo and ΔSo for this reaction do not change with temperature. a) Predict the direction in which ΔGo for this reaction changes with increasing temperature b) Calculate the ΔGo values for the reaction at 25oC and 500oC.

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