Chemical Equilibria and Reaction Quotients — Transcript

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1. 0:00hey it's professor Dave, let's look at chemical equilibria.
2. 0:09sometimes chemical reactions are not unidirectional, they are reversible. the reactants create the
3. 0:16products which then go right back and create reactants. this means there is
4. 0:20both a forward and reverse reaction. when the rates of the forward and reverse
5. 0:25reactions are the same this system is at dynamic equilibrium. chemistry is
6. 0:31happening but because the reactions are happening at the same rate there doesn't
7. 0:36appear to be any activity. there is a balance. we learned how to use
8. 0:40stoichiometry to discuss limiting reagents and how much of the products to
9. 0:45expect but these were for unidirectional reactions where we assume that all the
10. 0:50reactants make products and then the reaction is over. with equilibria it's a
11. 0:56little more complicated to calculate what the concentrations of each
12. 1:00substance will be at equilibrium so we're going to have to do a little bit
13. 1:06of math to be able to describe the system. for example let's look at this
14. 1:11equilibrium let's say we start with one mole of PCl5, allow the system to reach
15. 1:18equilibrium, and then once at equilibrium we measure that there are 0.135 moles of
16. 1:26PCl3. so how much of the other two things are there? to answer this we can
17. 1:32make something called an icebox. these letters stand for initial, change, and
18. 1:39equilibrium. we set them up like this. for initial we put the initial amount of
19. 1:45each thing we started with, just one mole of reactant so we put one there and
20. 1:50zeros for the products since there wasn't any of those at first. then for
21. 1:55the change we don't know exactly how much the change was to get to
22. 1:58equilibrium so we call it x. for the reactant we put negative x because
23. 2:04that is being depleted and in this case all of the stoichiometric coefficients
24. 2:08are one so as x moles of reactant are being depleted
25. 2:13x moles of each product are being formed. we would make these two or three x as
26. 2:19necessary if the coefficients were different. and for the products x is
27. 2:24positive because these are being formed. then lastly we add up the initial and
28. 2:30the change to give the amounts present at equilibrium. for the reactant this is
29. 2:361-x and for the products it is simply x. we measured the final concentration
30. 2:42of PCl3 as 0.135 which here will correspond to x and we therefore know
31. 2:50all the other concentrations by plugging in x
32. 2:54simple, no? well it can get trickier as we will see but first let's learn about
33. 3:00equilibrium expressions. every equilibrium has an equilibrium constant
34. 3:05Kc. it will be given by the concentrations of the products each
35. 3:09raised to the power of their stoichiometric coefficients over the
36. 3:13concentration of the reactants each raised to the power of their
37. 3:17stoichiometric coefficients. this is called the equilibrium constant
38. 3:21expression. one thing this constant tells us is whether the products or reactants
39. 3:27are favored in the equilibrium. if Kc is much greater than one that means the
40. 3:33numerator is bigger so we are creating more products. if Kc is much less than
41. 3:39one that means the denominator is bigger and we are creating more reactants. when
42. 3:44we write this expression we only take into account gases and aqueous species
43. 3:49solids and pure liquids will not be included in the expression. for example
44. 3:55here
45. 3:56notice how carbon is not included in the expression since it doesn't make sense
46. 4:01to discuss a solid in terms of a concentration. sometimes we may want to
47. 4:06predict which way a mixture will go given some non equilibrium quantities. to
48. 4:12do this we plug in non equilibrium values into the Kc expression to
49. 4:17calculate the reaction quotient, Q. if Kc is bigger than Q, Q is more on
50. 4:24the reactant side so it'll make more products to equilibrate. if Kc is
51. 4:29smaller than Q, Q is more on the product side so it'll make more
52. 4:34reactants to equilibrate. when Kc equals Q the system is at equilibrium.
53. 4:39let's try a more difficult icebox. given the following information
54. 4:44calculate the equilibrium concentrations of each substance in terms of molarity
55. 4:48remember that molarity is moles per liter. so we set up our icebox and
56. 4:54calculate the concentration of our reactant. that'll be the initial value and
57. 4:59zero for the products. now here the change will be different for each substance
58. 5:04because of the stoichiometry
59. 5:07for every two moles of reactant we get one mole of each product so we have to
60. 5:12subtract 2x here and add x here. then equilibrium is just the sum of
61. 5:20initial and change
62. 5:22plug them into the equilibrium expression and solve for x. luckily on
63. 5:28the right
64. 5:29both the numerator and denominator are squared so we can take the square root
65. 5:33of both sides. if it wasn't this convenient we may have had to use the
66. 5:38quadratic equation. but as is we can solve for x and use that to determine
67. 5:45all the equilibrium concentrations. let's check comprehension
68. 6:19thanks for watching guys, subscribe to my channel for more tutorials and as always feel free to email me