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Lecture 10: D G, Q, and K

Lecture 10: D G, Q, and K. Reading: Zumdahl 10.10, 10.11 Outline Relating D G to Q Relating D G to K The temperature dependence of K. Relating D G to Q. Recall from Lecture 6: D S = R ln ( W final / W initial ). For the expansion of a gas W final  Volume.

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Lecture 10: D G, Q, and K

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  1. Lecture 10: DG, Q, and K • Reading: Zumdahl 10.10, 10.11 • Outline • Relating DG to Q • Relating DG to K • The temperature dependence of K

  2. Relating DG to Q • Recall from Lecture 6: DS = R ln (Wfinal/Winitial) • For the expansion of a gas Wfinal Volume

  3. Relating DG to Q (cont.) • Given this relationship DS = R ln (Vfinal/Vinitial)

  4. Relating DG to Q (cont.) • This equation tells us what the change in entropy will be for a change in concentration away from standard state. Entropy change for process occurring under standard conditions Additional term for change in concentration. (1 atm, 298 K) (P ≠ 1 atm)

  5. Relating DG to Q (cont.) • How does this relate to DG?

  6. Relating DG to Q (cont.) • Generalizing to a multicomponent reaction: • Where

  7. An Example • Determine DGrxn at 298 K for: C2H4(g) + H2O(l) C2H5OH(l) where PC2H4 = 0.5 atm (others at standard state) DG°rxn = -6 kJ/mol (from Lecture 9)

  8. An Example (cont.) C2H4(g) + H2O(l) C2H5OH(l) DGrxn = -6 kJ/mol + (8.314 J/mol.K)(298K)ln(2) = -4.3 kJ/mol

  9. DG and K • The Reaction Quotient (Q) corresponds to a situation where the concentrations of reactants and products are not those at equilibrium. • At equilibrium, we have K. • What is the relationship between DG and K?

  10. DG and K (cont.) • At equilibrium, DGrxn = 0 0 K 0 = DG°rxn +RTln(K) DG°rxn = -RTln(K)

  11. DG and K (cont.) • Let’s look at the interaction between DG° and K DG°rxn = -RTln(K) If DG° < 0 then K > 1 Products are favored over reactants

  12. DG and K (cont.) • Let’s look at the interaction between DG° and K DG°rxn = -RTln(K) If DG° = 0 then K = 1 Products and reactants are equally favored

  13. DG and K (cont.) • Let’s look at the interaction between DG° and K DG°rxn = -RTln(K) If DG° > 0 then K < 1 Reactants are favored over products

  14. An Example • For the following reaction at 298 K: HBrO(aq) + H2O(l)BrO-(aq) + H3O+(aq) Ka = 2.3 x 10-9 What is DG°rxn? DG°rxn = -RTln(K) = -RTln(2.3 x 10-9) = 49.3 kJ/mol

  15. An Example (cont.) • What is DGrxn when pH = 5, [BrO-] = 0.1 M, and [HBrO] = 0.2 M ? HBrO(aq) + H2O(l) BrO-(aq) + H3O+(aq)

  16. An Example (cont.) • Then: = 49.3 kJ/mol + (8.314 J/mol.K)(298 K)ln(5 x 10-6) = 19.1 kJ/mol DGrxn < DG°rxn “shifting” reaction towards products

  17. Temperature Dependence of K • We now have two definitions for DG° DG°rxn = -RTln(K) = DH° - TDS° • Rearranging (dividing by -RT) y = m x + b • Plot of ln(K) vs 1/T is a straight line

  18. T Dependence of K (cont.) • If we measure K as a function of T, we can determine DH° by determining the slope of the line slope intercept

  19. T Dependence of K (cont.) • Once we know the T dependence of K, we can predict K at another temperature: - the van’t Hoff equation.

  20. An Example • For the following reaction: CO(g) + 2H2(g) CH3OH(l) DG° = -29 kJ/mol What is K at 340 K? • First, what is Keq when T = 298 K? DG°rxn = -RTln(K) = -29 kJ/mol ln(K298) = (-29 kJ/mol) = 11.7 -(8.314 J/mol.K)(298K) K298 = 1.2 x 105

  21. An Example (cont.) • Next, to use the van’t Hoff Eq., we need DH° CO(g) + 2H2(g) CH3OH(l) DHf°(CO(g)) = -110.5 kJ/mol DHf°(H2(g)) = 0 DHf°(CH3OH(l)) = -239 kJ/mol DH°rxn=SDH°f (products) - SDH° f (reactants) = DH°f(CH3OH(l)) - DH°f(CO(g)) = -239 kJ - (-110.5 kJ) = -128.5 kJ

  22. An Example (cont.) • With DH°, we’re ready for the van’t Hoff Eq. Why is K reduced? Reaction is Exothermic. K340 = 2.0 x 102 Increase T, Shift Eq. To React. Keq will then decrease

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