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1 CHAPTER 15 Chemical Equilibrium. 2 Basic Concepts Reversible reactions do not go to completion –occur in either direction –represented as:

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Presentation on theme: "1 CHAPTER 15 Chemical Equilibrium. 2 Basic Concepts Reversible reactions do not go to completion –occur in either direction –represented as:"— Presentation transcript:

1 1 CHAPTER 15 Chemical Equilibrium

2 2 Basic Concepts Reversible reactions do not go to completion –occur in either direction –represented as:

3 3 Basic Concepts Chemical equilibrium –reversible reaction that the forward reaction rate is equal to the reverse reaction rate –dynamic equilibria

4 4 Basic Concepts Graphical representation of the rates for the forward and reverse reactions for this general reaction

5 5 Basic Concepts One of the fundamental ideas of chemical equilibrium: Equilibrium can be established from either direction - forward or reverse.

6 6 The Equilibrium Constant For a simple one-step mechanism reversible reaction such as: A (g) + B (g)  C (g) + D (g) The rates of the forward and reverse reactions can be represented as:

7 7 The Equilibrium Constant When system is at equilibrium: R f = R r

8 8 The Equilibrium Constant Because the ratio of two constants is a constant we can define a new constant as follows :

9 9 The Equilibrium Constant Similarly, for the general reaction: a A (g) + b B (g)  c C (g) + d D (g) we can define a constant

10 10 The Equilibrium Constant K c is the equilibrium constant. K c is defined for a reversible reaction at a given temperature as the product of the equilibrium concentrations (in M) of the products, each raised to a power equal to its stoichiometric coefficient in the balanced equation, divided by the product of the equilibrium concentrations (in M) of the reactants, each raised to a power equal to its stoichiometric coefficient in the balanced equation.

11 11 The Equilibrium Constant Write equilibrium constant expressions for the following reactions at 500 o C.

12 12 The Equilibrium Constant

13 13 The Equilibrium Constant

14 14 Partial Pressures and the Equilibrium Constant Gas phase reactions can have equilibrium constants expressed in partial pressures rather than concentrations. –For gases the pressure is proportional to concentration –PV = nRT –P = nRT/V = []RT and [] = P/RT

15 15 Partial Pressures and the Equilibrium Constant Gas phase reactions can have equilibrium constants expressed in partial pressures rather than concentrations. –For gases the pressure is proportional to concentration –PV=nRT –P=nRT/V=[]RT and []=P/RT Consider this system in equilibrium at 500 0 C.

16 16 Partial Pressures and the Equilibrium Constant

17 17 Relationship Between K p and K c Relationship between K p and K c is:

18 18 The Equilibrium Constant Equilibrium constants are dimensionless because they actually involve a thermodynamic quantity called activity. –Activities are directly related to molarity

19 19 The Equilibrium Constant One liter of equilibrium mixture from the following system at a high temperature was found to contain 0.172 mole of phosphorus trichloride, 0.086 mole of chlorine, and 0.028 mole of phosphorus pentachloride. Calculate K c for the reaction. PCl 5  PCl 3 + Cl 2 Equil []’s 0.028 M 0.172 M 0.086 M

20 20 The Equilibrium Constant

21 21 The Equilibrium Constant The decomposition of PCl 5 was studied at another temperature. One mole of PCl 5 was introduced into an evacuated 1.00 liter container. The system was allowed to reach equilibrium at the new temperature. At equilibrium 0.60 mole of PCl 3 was present in the container. Calculate the equilibrium constant at this temperature.

22 22 The Equilibrium Constant

23 23 The Equilibrium Constant At a given temperature 0.80 mole of N 2 and 0.90 mole of H 2 were placed in an evacuated 1.00-liter container. At equilibrium 0.20 mole of NH 3 was present. Calculate K c for the reaction.

24 24 The Equilibrium Constant

25 25 Variation of K c with the Form of the Balanced Equation Value of K c depends upon how the balanced equation is written. PCl 5  PCl 3 + Cl 2 and K c = [PCl 3 ][Cl 2 ] = 0.53 [PCl 5 ]

26 26 Variation of K c with the Form of the Balanced Equation Calculate the equilibrium constant for the reverse reaction by two methods, i.e, the equilibrium constant for the reaction

27 27 Variation of K c with the Form of the Balanced Equation PCl 3 + Cl 2  PCl 5 Equil. []’s 0.172 M 0.086 M 0.028 M

28 28 Variation of K c with the Form of the Balanced Equation Large equilibrium constants indicate that most of the reactants are converted to products. Small equilibrium constants indicate that only small amounts of products are formed.

29 29 Heterogeneous Equlibria Heterogeneous equilibria have two or more phases –pure solids and liquids have activities of unity –solvents in very dilute solutions have activities that are essentially unity CaCO 3(s)  CaO (s) + CO 2(g) (at 500 o C)

30 30 Heterogeneous Equlibria SO 2(g) + H 2 O (l)  H 2 SO 3(aq) (at 25 o C) H 2 O (l) is the solvent

31 31 Heterogeneous Equlibria CaF 2(s)  Ca 2+ (aq) + 2 F - (aq) (at 25 o C)

32 32 Heterogeneous Equlibria 3 Fe (s) + 4 H 2 O (g)  Fe 3 O 4(s) + 4 H 2(g) (at 500 o C)

33 33 Uses of the Equilibrium Constant, K p Nitrosyl bromide, NOBr, is 34% dissociated by the following reaction at 25 o C, in a vessel in which the total pressure is 0.25 atmosphere. What is the value of K p ?

34 34 Solving for the Equilibrium Constant, K p Nitrosyl bromide, NOBr, is 34% dissociated by the following reaction at 25 o C, in a vessel in which the total pressure is 0.25 atmosphere. What is the value of K p ?

35 35 Solving for the Equilibrium Constant, K p

36 36 Solving for the Equilibrium Constant, K p

37 37 Solving for the Equilibrium Constant, K p The numerical value of K c for this reaction is

38 38 Problems using the Equilibrium Constant, K p K c is 49 for the following reaction at 450 o C. If 1.0 mole of H 2 and 1.0 mole of I 2 are allowed to reach equilibrium in a 3.0-liter vessel, (a) How many moles of I 2 remain unreacted at equilibrium?

39 39 Problems using the Equilibrium Constant, K p

40 40 Problems using the Equilibrium Constant, K p (b) What are the equilibrium partial pressures of H 2, I 2 and HI?

41 41 Problems using the Equilibrium Constant, K p

42 42 Problems using the Equilibrium Constant, K p (c) What is the total pressure in the reaction vessel?

43 43 Problems using the Equilibrium Constant, K p

44 44 Uses of the Equilibrium Constant, K c The equilibrium constant, K c, is 3.00 for the following reaction at a given temperature. If 1.00 mole of SO 2 and 1.00 mole of NO 2 are put into an evacuated 2.00-liter container and allowed to reach equilibrium, what will be the concentration of each compound at equilibrium?

45 45 Uses of the Equilibrium Constant, K c

46 46 Uses of the Equilibrium Constant, K c The equilibrium constant is 49 for the following reaction at 450 o C. If 1.00 mole of HI is put into an evacuated 1.00-liter container and allowed to reach equilibrium, what will be the equilibrium concentration of each substance?

47 47 Uses of the Equilibrium Constant, K c

48 48 The Reaction Quotient Q - Mass action expression or reaction quotient –same form as K c –concentrations are not necessarily equilibrium values

49 49 The Reaction Quotient Q - Mass action expression or reaction quotient –same form as K c –concentrations are not necessarily equilibrium values

50 50 The Reaction Quotient Compare Q with K c –predict direction reaction will occur to attain equilibrium

51 51 The Reaction Quotient The equilibrium constant for the following reaction is 49 at 450 o C. If 0.22 mole of I 2, 0.22 mole of H 2, and 0.66 mole of HI were put into an evacuated 1.00-liter container, would the system be at equilibrium? If not, what must occur to establish equilibrium?

52 52 The Reaction Quotient The equilibrium constant for the following reaction is 49 at 450 o C. If 0.22 mole of I 2, 0.22 mole of H 2, and 0.66 mole of HI were put into an evacuated 1.00-liter container, would the system be at equilibrium? If not, what must occur to establish equilibrium?

53 53 Factors That Affect Equlibria LeChatelier’s Principle - If a change of conditions (stress) is applied to a system in equilibrium, the system responds in the way that best tends to reduce the stress in reaching a new state of equilibrium. Some stresses are changes in: –concentration –pressure –temperature

54 54 Factors That Affect Equlibria 1Changes in Concentration (and Pressure for reactions involving gases) Look at the following system at equilibrium at 450 o C (K c =49)

55 55 Factors That Affect Equlibria 1Changes in Concentration (and Pressure for reactions involving gases) Look at the following system at equilibrium at 450 o C (K c =49)

56 56 Factors That Affect Equlibria 1Changes in Concentration (and Pressure for reactions involving gases) Look at the following system at equilibrium at 450 o C (K c =49)

57 57 Factors That Affect Equlibria 2Changes in Volume (and Pressure for reactions involving gases) Change the volume by changing the pressure at constant temperature on the following system at equilibrium:

58 58 Factors That Affect Equlibria 2Changes in Volume (and Pressure for reactions involving gases) Change the volume by changing the pressure at constant temperature on the following system at equilibrium:

59 59 Factors That Affect Equlibria 2Changes in Volume (and Pressure for reactions involving gases) Change the volume by changing the pressure at constant temperature on the following system at equilibrium:

60 60 Factors That Affect Equlibria 3Changing the Temperature Consider the following reaction at equilibrium

61 61 Factors That Affect Equlibria 3Changing the Temperature Consider the following reaction at equilibrium

62 62 Factors That Affect Equlibria 3Changing the Temperature Consider the following reaction at equilibrium

63 63 Factors That Affect Equlibria Introduction of a Catalyst Catalysts decrease the activation energy of both the forward and reverse reaction equally. Does not affect the position of equilibrium.

64 64 Factors That Affect Equlibria Given the reaction below at equilibrium in a closed container at 500 o C. How would the equilibrium be influenced by the following?

65 65 Factors That Affect Equlibria Given the reaction below at equilibrium in a closed container at 500 o C. How would the equilibrium be influenced by the following?

66 66 Factors That Affect Equlibria How will an increase in pressure (caused by decreasing the volume) affect the equilibrium in each of the following reactions?

67 67 Factors That Affect Equlibria How will an increase in pressure (caused by decreasing the volume) affect the equilibrium in each of the following reactions?

68 68 Factors That Affect Equlibria How will an increase in temperature affect each of the following reactions?

69 69 Factors That Affect Equlibria How will an increase in temperature affect each of the following reactions?

70 70 The Haber Process Commercial production of ammonia

71 71

72 72 Application of a Stress to a System at Equilibrium Determine the direction that the equilibrium will shift by comparing Q with K c. An equilibrium mixture from the following reaction was found to contain 0.20 mol/L of A, 0.30 mol/L of B, and 0.30 mol/L of C. What is the value for K c ?

73 73 Application of a Stress to a System at Equilibrium If the volume of the reaction vessel were suddenly doubled while the temperature remained constant, what would be the new equilibrium concentrations? Calculate Q, after the volume has been doubled

74 74 Application of a Stress to a System at Equilibrium If the volume of the reaction vessel were suddenly doubled while the temperature remained constant, what would be the new equilibrium concentrations? Calculate Q, after the volume has been doubled

75 75 Application of a Stress to a System at Equilibrium If the volume of the reaction vessel were suddenly doubled while the temperature remained constant, what would be the new equilibrium concentrations? Calculate Q, after the volume has been doubled

76 76 Application of a Stress to a System at Equilibrium Since Q { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/10/2758839/slides/slide_76.jpg", "name": "76 Application of a Stress to a System at Equilibrium Since Q

77 77 Application of a Stress to a System at Equilibrium Since Q { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/10/2758839/slides/slide_77.jpg", "name": "77 Application of a Stress to a System at Equilibrium Since Q

78 78 Application of a Stress to a System at Equilibrium Since Q { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/10/2758839/slides/slide_78.jpg", "name": "78 Application of a Stress to a System at Equilibrium Since Q

79 79 Application of a Stress to a System at Equilibrium Since Q { "@context": "http://schema.org", "@type": "ImageObject", "contentUrl": "http://images.slideplayer.com/10/2758839/slides/slide_79.jpg", "name": "79 Application of a Stress to a System at Equilibrium Since Q

80 80 Application of a Stress to a System at Equilibrium

81 81 Application of a Stress to a System at Equilibrium

82 82 Application of a Stress to a System at Equilibrium


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