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Chapter 15

Chapter 15. Equilibrium. Equilibrium. N 2 + 3 H 2  2 NH 3 Both reactions occur, Closed system No change in Equilibrium reached no matter what direction. Write:. Rate law for forward Rate = Rate Law for reverse Rate =. Then…. Rate f = Rate r. So…. K f. =. k r. Or. k c.

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Chapter 15

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  1. Chapter 15 Equilibrium

  2. Equilibrium • N2 + 3 H2 2 NH3 • Both reactions occur, • Closed system • No change in • Equilibrium reached no matter what direction.

  3. Write: • Rate law for forward • Rate = • Rate Law for reverse • Rate =

  4. Then… • Ratef = Rater

  5. So… Kf = kr

  6. Or kc =

  7. Equilibrium Constants • Function of the • Always • Allows determination of

  8. Equilibrium Expression Law of mass action aA + bB  cC + d D Kc =

  9. Rules • Products in • Brackets indicate • Concentration is raised • Pure liquids and solids

  10. Practice • Write the equilibrium expression for: • H2(g) + I2(g) 2 HI(g) • C(s) + O2(g)  2 CO(g)

  11. Problem • What is the equilibrium constant for the following reaction if the concentrations are [H2]=[I2]= 0.0033 M and [HI]= 0.0099M? • H2(g) + I2(g) 2 HI(g) • Does the equilibrium lie to the left or right?

  12. Equilibrium expression N2O4(g)  2 NO2(g) Kp=

  13. Kcvs KP • These will have different values since Kp = Kc(RT)n Where R = 0.08206 (L atm)/(K mol) And T = temperature in K And n = change in moles of gas

  14. Practice 2 SO3(g)↔ 2 SO2(g) + O2(g) Kc = 4.08 E -3 What is the Kpexpression? What is the value for Kp? T = 300 K

  15. The Meaning of K… What can you say about the amount of (concentration or pressure) reactant or product if: K is much larger than 1? K is much smaller than 1? K is equal to 1?

  16. Practice • For the reaction: N2(g) + O2(g)↔ 2 NO(g) • What is the equilibrium expression? • If Kc at 25o C is 1.00 E -30, is this a good method to ‘fix’ nitrogen?

  17. Practice • For the reaction: H2(g) + I2(g)↔ 2 HI(g) Kc = 794 at 298K Kc = 54 at 700 K Which temperature is favored to produce HI? Why?

  18. Direction for equilibrium • For the reaction: H2(g) + I2(g)↔ 2 HI(g) Kc = 794 at 298K • Write the K expression. • Write the K expression for the reverse reaction. • What is the value of K for the reverse reaction?

  19. HeterogeneousEquilibria

  20. Compare N2(g) + 3 H2(g) ↔ 2 NH3(g) PbCl2(s) ↔ Pb+2(aq) + 2 Cl-(aq)

  21. Definitions… Heterogeneous equilibria Homogeneous equilibria

  22. Heterogeneous • As long as there is pure solid or liquid present, • The concentration of the solid/liquid

  23. Write Kc PbCl2(s) ↔ Pb+2(aq) + 2 Cl-(aq) Knowing what you do about solubility, would you expect the value of K to be high or low?

  24. Practice • Write the equilibrium constant expressions for Kc and Kp for: CO2(g) + H2(g)↔ CO(g) + H2O(l) SnO2(s) + 2CO(g) ↔ Sn(s) + 2CO2(g)

  25. Calculations • For the reaction: 2NO2Cl(g)↔ 2NO2(g) + Cl2(g) Write the K expression. If [NO2Cl] = 0.00106 M and [NO2] = 0.0108 M and [Cl2] = 0.00538 M, what is the value of Kcat 500. K? Kc = 0.558

  26. A mixture of 5.000 E-3 mole of hydrogen and 1.000 E-2 mole of iodine are placed in a 5.000 L container at 448 degrees C and allowed to come to equilibrium. At equilibrium, [HI] = 1.87 E-3 M. Calculate Kc for the reaction: H2(g) + I2(g) < -- > 2HI(g)

  27. ICE boxes  The reaction goes here… Use a little bit of algebra…

  28. ICE BOX STEPS • Calculate the known initial and equilibrium concentrations • Calculate the change in concentration that occurs • Use stoichiometry to calculate the changes in concentration for all other species • Calculate the desired item from the ICE box data.

  29. Sulfur trioxide decomposes at high temperature to form sulfur dioxide and oxygen. Initially, a container at 1000 K has a sulfur trioxide concentration of 6.09 E-3 M. At equilibrium the [SO3] = 2.44 E-3 M. Calculate the value of Kc at 1000 K.

  30. Q

  31. The reaction Quotient… • If you put a mixture of 1.25 mol of hydrogen, 0.880 mol of nitrogen, and 1.50 mol of ammonia in a 1.25 L container, will the reaction move to the right or left or is it at equilibrium? Kc = 0.105 N2(g) + 3 H2(g) 2 NH3(g) • Calculate ‘Q’ – the reaction quotient.

  32. Calculated the same way as Kc – but just the ‘question’, ‘Q’. • Compare to Kc. [NH3]2 (1.20)2 Q = = [H2]3 [N2] ((1.00)3(0.704)) Q =2.05

  33. Compare and decide… • Three possibilities… • If Q > K • If Q = K • If Q < K Remember… K = Products Reactants K = 0.105 Q > K This means there are too many ________ and the reaction must move to the _______. Q =2.05

  34. The Q and K relationship

  35. For the Haber process, N2(g) + 3 H2(g) 2 NH3(g)Kp = 1.45 E-5 at 500 oC. In an equilibrium mixture of the three gases at 500 oC, the partial pressure of H2 is 0.928 atm and that of N2 is 0.432 atm. What is the partial pressure of NH3 in this equilibrium mixture?

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