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Chapter 17 : Equilibrium

Chapter 17 : Equilibrium. Mr Belland Presents - How my Son and I play. Chemical equilibrium is when two opposing reactions occur simultaneously and at the same rate. For example: A + B → C + D and C + D → A + B This is usually written as a reversible equation using double arrows:

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Chapter 17 : Equilibrium

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  1. Chapter 17 : Equilibrium

  2. Mr Belland Presents -How my Son and I play

  3. Chemical equilibrium is when two opposing reactions occur simultaneously and at the same rate. For example: A + B → C + D and C + D → A + B This is usually written as a reversible equation using double arrows: A + B ⇌ C + D

  4. What equilibrium is not: • equal amounts of products and reactants • permanent • instantly establish • a stop in the chemical reaction • for reactions that go to completion (like dissociation of a strong acid) • the same for every condition of T

  5. Example case 1: in a 1 L vessel R 2SO2 + O2⇌ 2SO3 I 0.400 mol 0.200 mol 0 C -0.056 mol -0.028 mol + 0.056 mol E 0.344 mol 0.172 mol 0.056 mol in 1 L vessel the above mols are also M Describe what occurred here.

  6. Example case 2: in a 1 L vessel R 2SO2 + O2⇌ 2SO3 I 0 0 0.500 mol C +0.424 mol +0.212 mol -0.424 mol E 0.424 mol 0.212 mol 0.076 mol in 1 L vessel the above mols are also M Describe what occurred here.

  7. Suppose we want a rate law: for case 1 (forward reaction) ratef = kf [SO2]2[O2] for case 2 (reverse reaction) rater = kr [SO3]2 (for only 1 step it is the slow step, so the coefficients = order) At equilibrium ratef = rater (by definition)

  8. If ratef = rater then kf [SO2]2[O2] = kr [SO3]2 rearranging to group terms… What is on top, and what is on bottom, in relationship to the forward reaction? Kc is called the equilibrium (concentration) constant (capitol K) and the c is sometimes left off

  9. What is the value of the equilibrium constant, Kc, for case 1? Kc is unitless, so be sure the units on all your numbers match!

  10. What is the value of the equilibrium constant, Kc, for case 2? What do you notice?

  11. Did you notice for both cases we used the forward reaction? If we used the reverse reaction, we would have gotten a different K, but it would still be related to the forward K, it is just the inverse. So Kforward = 1/Kreverse or Kreverse = 1/Kforward Let’s try it…

  12. Example case 2: in a 1 L vessel R 2SO3 ⇌ 2SO2 + O2 I 0.500 mol 0 0 C -0.424 mol +0.424 mol +0.212 mol E 0.076 mol 0.424 mol 0.212 mol What is Kc and what is 1/Kc for this reaction?

  13. One other thing, if perhaps many equilibrium reactions are involved in a mechanism, and you are given the K for each step, the total can be found as… Ktotal = K1 K2 K3…

  14. In general for aA + bB⇌cC + dD How would this change if the number of products or reactants was different? Remember, the assumption is this is the only step in the mechanism or is the slow step, otherwise the coefficients don’t necessarily match the exponents.

  15. So Kc is a measure of how much the reaction has occurred (the activity of the reaction). The bigger the Kc the more products and less reactants. Explain. What would be Kc for no reaction? What would be Kc for a strong acid/base?

  16. Practice: in a 5 L vessel R N2 + 3H2⇌ 2NH3 I C E 3.01 mol 2.10 mol 0.565 mol Find Kc

  17. Practice: in a 2 L vessel R 2N2O ⇌ 2N2 + O2 I 10.0 mol 0 0 C E 2.20 mol Find Kc

  18. In the previous example the reactant and products were gases, but we used Molarity anyway. Sometimes when all the substances are gases it is easier to use their partial pressures instead of concentrations, so… Why call it Kp? What would the units be?

  19. Kp is not the same as Kc, because [] is only dependent on the moles and volume where P is also dependent on temperature and the gas constant, but if these are accounted for, then… Kp = Kc(RT)Δn where Δn = ngas products - ngas reactants When would Kp = Kc?

  20. What if we don’t know if a reaction is at equilibrium yet? We then determine the reaction quotient, Q if Q < Kc , then it is not yet at equilibrium forward reaction will still be faster if Q > Kc, then it is past equilibrium reverse reaction will now be faster if Q = Kc, then it is at equilibrium

  21. Practice: For 2HI ⇌ H2 + I2 the Kc = 65.0 if E 0.500M 0.280 M 3.40 M is this at equilibrium?

  22. Typically Kc is used to find the concentration of reactants and products at equilibrium.For example, what is the equilibrium concentrations if Kc = 49.0 R A + B ⇌ C + D I 0.200 M 0.200 M 0 0

  23. What is the equilibrium concentrations if Kc = 49.0 R A + B ⇌ C + D I 0.300 M 0.100 M 0 0

  24. Changes to equilibrium: • if [] or pressure/volume changes or a catalyst is added, then Q changes, but Kcwill not change, so the reaction rates will change to get Q = Kc again (LeChatelier’s can tell you how it will change, shifting right or left means speeding up forward or reverse reaction) • if T changes then Kcwill change, so Q = Kc will happen, but it will be a new Kc

  25. Let’s practice using LeChatelier’s Principle If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) Then for each change, state how it will shift (forward or reverse) and why it will shift (Q less than K or Q greater than K)

  26. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) Pressure is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  27. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) Pressure is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  28. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) Volume is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  29. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) Volume is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  30. Explain why pressure and volume can only affect Kp and not Kc. What change would affect Kc?

  31. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) [A] is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  32. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) [A] is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  33. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) [D] is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  34. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) [D] is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  35. Why would catalysts not change the equilibrium? Why would the addition of inert gases not change the equilibrium? What affect does changing temperature have on equilibrium? What can we do with LeChatelier's principle?

  36. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) + Heat Temperature is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  37. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) + Heat Temperature is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  38. If 2A(g) + 3B(g) + Heat ⇌ C(g) + 2D(g) Temperature is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  39. If 2A(g) + 3B(g) + Heat ⇌ C(g) + 2D(g) Temperature is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  40. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) ∆H = + 76 kJ/mol Temperature is decreased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  41. If 2A(g) + 3B(g) ⇌ C(g) + 2D(g) ∆H = - 76 kJ/mol Temperature is increased: What affect does that have? How will the equilibrium shift? Why will the equilibrium shift?

  42. Practice: in a 1 L vessel, H2 + I2⇌ 2HI at equilibrium [HI] = 0.490 M, [H2] = 0.080 M and [I2] = 0.060 M then 0.300 more moles of HI is added, what will be the new equilibrium concentrations?

  43. First, find the Kc

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