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Lecture 22

Lecture 22. Chemical Equilibria 9.1-9.5 27-October Assigned HW 9.1, 9.2, 9.6, 9.8, 9.12, 9.14, 9.16a,b, 9.18, 9.24, 9.28 Due : Monday 1-Nov. Review. Vapor Pressure is a result of a dynamic equilibrium between molecules at the surface of a liquid and the gas phase

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Lecture 22

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  1. Lecture 22 Chemical Equilibria 9.1-9.5 27-October Assigned HW 9.1, 9.2, 9.6, 9.8, 9.12, 9.14, 9.16a,b, 9.18, 9.24, 9.28 Due: Monday 1-Nov

  2. Review • Vapor Pressure is a result of a dynamic equilibrium between molecules at the surface of a liquid and the gas phase • At lower temperatures, not as many liquid molecules have enough energy to ‘escape’ • At the boiling point, the vapor pressure = atmospheric pressure • Phase diagrams are a map of the most stable phase at various temperature and pressures • Triple Point  the point where all three phases are in dynamic equilibrium • Critical Point  where the liquid-gas phase line ends • Beyond this point is a supercritical fluid • Line slope  predicts density of phases • Solubility depends on Intermolecular forces • Hydrophobic  afraid of water – does not dissolve in water • Hydrophilic  water loving – readily dissolves in water

  3. Dynamic Equilibria for a Physical Process gas liquid Rate of vaporization liquid gas Rate of condensation liquid gas

  4. Dynamic Equilibria for a Chemical Process Products reactants Rate Reactants Products Rate Products Reactants

  5. Dynamic Equilibria for a Chemical Process Rate of N2O4 production 2 NO2 N2O4(g) (Brown) (Colorless) Rate of N2O4 decomposition If we start with 0.08 M NO2(g), how much N2O4(g) do you expect?

  6. Dynamic Equilibria for a Chemical Process Rate of N2O4 production 2 NO2 N2O4(g) (Brown) (Colorless) Rate of N2O4 decomposition If we start with 0.08 M NO2(g), how much N2O4(g) do you expect? Is [NO2] changing at the same rate as [N2O4]? 0.0337 M N2O4 0.0125 M NO2

  7. Dynamic Equilibria for a Chemical Process Rate N2O4(g) 2 NO2 Rate (Brown) (Colorless) If we start with 0.04 M N2O4(g), how much NO2(g) do you expect? 0.0337 M N2O4 0.0125 M NO2

  8. Dynamic Equilibria for a Chemical Process Rate of forward reaction N2O4(g) 2 NO2 Rate of reverse reaction

  9. Dynamic Equilibria for a Chemical Process No product yet N2O4(g) N2O4(g) N2O4(g) 2 NO2 2 NO2 N2O4(g) 2 NO2 Note that the rates seem to be related to the concentrations N2O4(g) Reaction time 2 NO2 N2O4(g) 2 NO2 Chemical Equilibrium

  10. Dynamic Equilibria and Concentrations Rate of forward reaction N2O4(g) 2 NO2 Rate of reverse reaction Goal: Find a mathematical relationship between the equilibrium concentrations

  11. Dynamic Equilibria and Concentrations Goal: Find a mathematical relationship between the equilibrium concentrations At equilibrium: Equilibrium Constant

  12. Dynamic Equilibria and Concentrations Goal: Find a mathematical relationship between the equilibrium concentrations N2O4(g) 2 NO2 0.0046 0.0046 0.0046 0.0046 0.0046 0.371 0.371 0.299 0.435 0.327 NOT CONSTANT

  13. Dynamic Equilibria and Concentrations D + A D + A 2 A A A A A B B + C B B + C 2 B B + B 2 B

  14. Dynamic Equilibria and Concentrations 2A 4B A B B A 2B 4A

  15. Dynamic Equilibria and Concentrations If Figure 1 represents the above chemical reaction at equilibrium, what other figure represents this reaction at equilibrium? A B How many of each? A 2 B 6

  16. Dynamic Equilibria and Concentrations A + B2 AB + B If the first figure represents the above chemical reaction at equilibrium, what other figure(s) represents this reaction at equilibrium? 2 4 4 A 1 1 2 1 B2 2 2 1 2 AB 1 1 1 2 B 2

  17. Equilibrium Constants and Partial Pressures Note: we will cover the equilibrium relationship between pressure and concentration in more detail on Friday. So adding SO2(g) and O2(g) at different ratios results in an equilibrium represented by K

  18. Equilibrium Constants and Partial Pressures Starting with the equilibrium constant below, determine the balanced chemical equation and express the equilibrium as KC.

  19. Equilibrium Constants and Reaction Yield Do you expect Big or Small Equilibrium Constants to produce more products?

  20. Equilibrium Constants Thermodynamics At equilibrium, ΔG = 0, so we might expect there to be a relationship between ΔG° and K cC + dD aA + bB Reaction Quotient This allows you to predict the direction of any reaction at given concentrations of the reactants and products

  21. Equilibrium Constant vs. Reaction Quotient cC + dD aA + bB If Q > K, are the products or reactants favored? ΔGr > 0 If Q > K, are the products or reactants favored? ΔGr < 0 If Q = K, are the products or reactants favored? ΔGr = 0

  22. Equilibrium Constant vs. Reaction Quotient If Q > K, are the products or reactants favored? ΔGr > 0 If Q > K, are the products or reactants favored? ΔGr < 0 If Q = K, are the products or reactants favored? ΔGr = 0

  23. Equilibrium Constant vs. Reaction Quotient cC + dD aA + bB If Q > K, are the products or reactants favored? ΔGr > 0 If Q > K, are the products or reactants favored? ΔGr < 0 If Q = K, are the products or reactants favored? ΔGr = 0 At equilibrium, ΔGr = 0 and Q = K

  24. Equilibrium Constants Thermodynamics Consider the following reaction at 500. K ΔGr° = -21.1 kJ mol-1 a. Calculate K H2(g) + I2(g) 2 HI(g) b. Calculate ΔGr when:

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