1. Chemical Equilibrium Be sure you download this powerpoint from the website.

2. The Concept of Chemical Equilibrium

3. Chemical Equilibrium

4. Physical equilibrium

5. The Equilibrium Constant (K) AP exam questions will either give you K to calculate a concentration of a substance, or give you concentrations to calculate K. – K is the mathematical expression of the law of mass action – Law of mass action For a reversible reaction at equilibrium and a constant temperature, a certain ratio of reactant and product concentrations has a constant value, K.

6. The Equilibrium Constant (K)

16. Homework… Read Read Read Read Read Read Read Read Read Read Read Read Read Read Read Read Read chapter on Gases and on Chemical Equilibrium!!!!!! You will have a quiz tomorrow on gas stoichiometry. I will assign you homework on equilibrium tomorrow. You will have a quiz on equilibrium Thursday.

17. Chemical Equilibrium Part II

18. Heterogeneous Equilibrium

23. Just in case you were wondering why…

24. Multiple Equilibria The reactions we have considered are relatively simple, a more complicated situation is one in which the product molecules in one equilibrium system are in coved in a second process. aA + bB↔cC + dD cC + dD ↔ eE + fF

25. Multiple Equilibria

26. When: aA + bB ↔ cC + dD cC + dD ↔ eE + fF (don’t write this down because it is confusing) The overall reaction is given by the sum of the two reactions: aA + bB ↔ cC + dD K’c cC + dD ↔ eE + fF K’’c aA + bB ↔ eE + fF Kc

27. Multiple Equilibria

28. Therefore: if a reaction can be expressed as the sum of two or more reactions, the equilibrium constant for the overall reaction is given by the product of the equilibrium constants of the individual reactions

29. Multiple Equilibria

30. The relationship between equilibrium and rate of reaction (kinetics) For equilibrium reactions, we have both forward and reverse reactions, therefore; aA + bB ↔ cC + dD Where K f is our forward equilibrium constant, and K r is our reverse constant. Rate f = Rate r KfKf KrKr

31. The relationship between equilibrium and rate of reaction (kinetics)

32. Next Week We will cover – The reaction quotient (Q) – Le Chateliers Principle. – Please read sections 13.5, 13.6, and 13.7 in your book.

33. Homework due Thursday. Pgs 650-651, 17-35 odd #’s Pgs 651-653, #’s 37-45 odd, 51, 55, 57.

34. Chemical Equilibrium Part III

35. The reaction quotient (Q) For reactions that have not reached equilibrium, we obtain the reaction quotient (Q) instead of the equilibrium constant (K). – Simply put, the reaction quotient uses the initial concentrations of reactants and products in the same equation as K. – You then compare Q to K to determine whether or not the reaction is at equilibrium. Simple enough.

36. The reaction quotient (Q)

37. Example Consider the following reaction: N 2(g) + 3H 2(g) ↔2NH 3(g) at the start of the reaction there are.249 mol of N 2, 3.21 x 10 -2 mol H2, and 6.42 x 10 -4 mol NH 3 in a 3.5L reaction vessel at 375°C, If the equilibrium constant (K) is 1.2, decide whether the system is in equilibrium. If it is not, predict which way the net reaction will occur.

38. Example

39. Q=.611 Q < K Therefore, there are more reactants than products, and the reaction must proceed from left to right to reach equilibrium.

40. LeChatlier’s Principle if an external stress is applied to a system at equilibrium, the system will adjust in such a way that the stress will is partially offset as the system reaches a new equilibrium position. Stresses include: change in Temp, volume, pressure, or a change in moles.

41. LeChatlier’s Principle A classic example from Biology: Your body is in a state of dynamic equilibrium with your environment. When you dramatically change environments, you body feels the effects and works to return to equilibrium. This is a process called “acclimatization”.

42. A classic example from Biology The combination of oxygen with a hemoglobin molecule produces the polymer Oxyhemoglobin which transports oxygen to body tissues. Hb (aq) + O 2 ↔ HbO 2(aq)

43. At higher altitudes the concentration of oxygen decreases, thus leading to a state of disequilibrium. This is known as altitude sickness and can be fatal. Yuppies call this “the height hangover”.

44. So according to LeChatlier: Hb (aq) + O 2 ↔ HbO 2(aq) A decrease in oxygen would shift the equilibrium from left to right. In other words, the left side of the arrow above would be bigger that the left; and Q >K. This would mean that there is more HbO 2 than what the environment realistically allows as a result of a smaller O 2 concentration. Your body (as the system) reacts by creating more Hb to capture more oxygen. This process takes weeks, even years.

45. Tomorrow We will wrap up this chapter – Calculating equilibrium concentrations – Addressing system stresses Use the rest of the period to work on your homework.

46. Chemical Equilibrium Part IV

47. Calculating Equilibrium Concentrations If we know the equilibrium constant K for a particular reaction, we can calculate the equilibrium concentrations from the initial concentrations Using the ICE method – Initial, Change, Equilibrium We do this by solving algebraically.

48. Calculating Equilibrium Concentrations Consider the following reaction of cis-stilbene and tran-stilbene, two isomers that exist in equilibrium in a closed system.

49. K c for this system at 200° C is 24.0. if the initial concentration of cis-stilbene is.850 mol/L, and no trans-stilbene, what is the concentrations for both substances at equilibrium? – Solve algebraically using the ICE method

51. Example #2 A mixture of.500 mol of H 2 and.500 mol of I 2 was placed in a 1L flask at 430°C. The equilibrium constant K c for the reaction is 54.3 at this temperature. Calculate the concentrations of H 2 and I 2 at equilibrium. H 2(g) + I 2(g) ↔ 2HI (g)

52. Example #2

53. X=.393 M [H2]=.107M [I2]=.107M [HI]=.786M

54. Example #3 same method, different execution For the same reaction as Example #2, the initial concentrations of.00623 mol of H 2,.00414 mol of I 2, and.0224 mol of HI. Calculate the concentrations at equilibrium. Your first instinct will be to calculate K, but you cant since you have the initial concentrations. In this case you have to calculate Q first and compare to K to determine the direction of the reaction.

55. Example #3

59. Addressing system stress… Here's what you need to know. The only thing that can act on a system and change its K, is temperature.