# CHAPTER 15 Chemical Equilibrium.

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CHAPTER 15 Chemical Equilibrium

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

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

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

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

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:

The Equilibrium Constant
When system is at equilibrium: Rf = Rr

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

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

The Equilibrium Constant
Kc is the equilibrium constant . Kc 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.

The Equilibrium Constant
Write equilibrium constant expressions for the following reactions at 500oC.

The Equilibrium Constant

The Equilibrium Constant

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

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 5000C.

Partial Pressures and the Equilibrium Constant

Relationship Between Kp and Kc
Relationship between Kp and Kc is:

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

The Equilibrium Constant
One liter of equilibrium mixture from the following system at a high temperature was found to contain mole of phosphorus trichloride, mole of chlorine, and mole of phosphorus pentachloride. Calculate Kc for the reaction. PCl5 « PCl Cl2 Equil []’s M M M

The Equilibrium Constant

The Equilibrium Constant
The decomposition of PCl5 was studied at another temperature. One mole of PCl5 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 PCl3 was present in the container. Calculate the equilibrium constant at this temperature.

The Equilibrium Constant

The Equilibrium Constant
At a given temperature 0.80 mole of N2 and 0.90 mole of H2 were placed in an evacuated 1.00-liter container. At equilibrium 0.20 mole of NH3 was present. Calculate Kc for the reaction.

The Equilibrium Constant

Variation of Kc with the Form of the Balanced Equation
Value of Kc depends upon how the balanced equation is written. PCl5 « PCl Cl2 and Kc= [PCl3][Cl2] = 0.53 [PCl5]

Variation of Kc 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

Variation of Kc with the Form of the Balanced Equation
PCl Cl2 « PCl5 Equil. []’s M M M

Variation of Kc 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.

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 CaCO3(s) « CaO(s) + CO2(g) (at 500oC)

Heterogeneous Equlibria
SO2(g) + H2O(l) « H2SO3(aq) (at 25oC) H2O(l) is the solvent

Heterogeneous Equlibria
CaF2(s) « Ca2+(aq) + 2 F-(aq) (at 25oC)

Heterogeneous Equlibria
3 Fe(s) + 4 H2O(g) « Fe3O4(s) + 4 H2(g) (at 500oC)

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

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

Solving for the Equilibrium Constant, Kp

Solving for the Equilibrium Constant, Kp

Solving for the Equilibrium Constant, Kp
The numerical value of Kc for this reaction is

Problems using the Equilibrium Constant, Kp
Kc is 49 for the following reaction at 450oC. If 1.0 mole of H2 and 1.0 mole of I2 are allowed to reach equilibrium in a 3.0-liter vessel, (a) How many moles of I2 remain unreacted at equilibrium?

Problems using the Equilibrium Constant, Kp

Problems using the Equilibrium Constant, Kp
(b) What are the equilibrium partial pressures of H2, I2 and HI?

Problems using the Equilibrium Constant, Kp

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

Problems using the Equilibrium Constant, Kp

Uses of the Equilibrium Constant, Kc
The equilibrium constant, Kc, is 3.00 for the following reaction at a given temperature. If 1.00 mole of SO2 and 1.00 mole of NO2 are put into an evacuated 2.00-liter container and allowed to reach equilibrium, what will be the concentration of each compound at equilibrium?

Uses of the Equilibrium Constant, Kc

Uses of the Equilibrium Constant, Kc
The equilibrium constant is 49 for the following reaction at 450oC. 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?

Uses of the Equilibrium Constant, Kc

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

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

The Reaction Quotient Compare Q with Kc
predict direction reaction will occur to attain equilibrium

The Reaction Quotient The equilibrium constant for the following reaction is 49 at 450oC. If 0.22 mole of I2, 0.22 mole of H2, 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?

The Reaction Quotient The equilibrium constant for the following reaction is 49 at 450oC. If 0.22 mole of I2, 0.22 mole of H2, 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?

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

Factors That Affect Equlibria
Changes in Concentration (and Pressure for reactions involving gases) Look at the following system at equilibrium at 450oC (Kc=49)

Factors That Affect Equlibria
Changes in Concentration (and Pressure for reactions involving gases) Look at the following system at equilibrium at 450oC (Kc=49)

Factors That Affect Equlibria
Changes in Concentration (and Pressure for reactions involving gases) Look at the following system at equilibrium at 450oC (Kc=49)

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

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

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

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

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

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

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.

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

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

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

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

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

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

The Haber Process Commercial production of ammonia

Application of a Stress to a System at Equilibrium
Determine the direction that the equilibrium will shift by comparing Q with Kc. 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 Kc?

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

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

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

Application of a Stress to a System at Equilibrium
Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Application of a Stress to a System at Equilibrium
Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Application of a Stress to a System at Equilibrium
Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Application of a Stress to a System at Equilibrium
Since Q<Kc the reaction will shift to the right to re-establish the equilibrium. Use algebra to represent the new concentrations.

Application of a Stress to a System at Equilibrium

Application of a Stress to a System at Equilibrium

Application of a Stress to a System at Equilibrium