Chapter 15: Chemical Equilibrium

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Chapter 15 Chemical Equilibrium
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Chapter 15 Outline

• Concept of Equilibrium
• The Equilibrium Constant (Kc Kp)
• Heterogeneous Equilibrium
• Calculating and Applying Equilibrium Constants
• Le Châteliers Principle

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The Concept of Chemical Equilibrium
Animation
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What is Equilibrium A case study

• Using the collision model
• as the amount of NO2 builds up, there is a chance
  that two NO2 molecules will collide to form N2O4.
  
• At the beginning of the reaction, there is no NO2
  so the reverse reaction (2NO2(g) ? N2O4(g)) does
  not occur.

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Equilibrium Case Study

• The point at which the rate of decomposition
• N2O4(g) ? 2NO2(g)
• equals the rate of dimerization
• 2NO2(g) ? N2O4(g).
• The system is dynamic equilibrium.
• The equilibrium is dynamic because the reaction
  has not stopped the opposing rates are equal.
• Consider frozen N2O4 only white solid is
  present. On the microscopic level, only N2O4
  molecules are present.

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The Concept of Equilibrium
Equilibrium is a state in which there are no
observable changes as time goes by.

• Chemical equilibrium is achieved when
• the rates of the forward and reverse reactions
  are equal and
• the concentrations of the reactants and products
  remain constant
• Chemistry is still happening!!

Physical equilibrium
A
Chemical equilibrium
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The Concept of Equilibrium
Start with NO2
Start with N2O4
Start with NO2 N2O4
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The Concept of Equilibrium
kf N2O4 kr NO22

Kc
constant
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Equilibrium Constant
N2O4 (g) 2NO2 (g)
4.63 x 10-3
Law of Mass Action
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Lets Practice!

• Write the equilibrium constant for following
  reactions
• a. H2 I2 2HI
• b. Cd2 4Br- CdBr42-

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Equilibrium Constant
Equilibrium will lie to the right Favor the
Products
Equilibrium will lie to the left Favor the
Reactants
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Conceptual Question

• The equilibrium constant for the reaction H2
  I2 2HI varies with temperature as
  follows
• K 794 at 298 K K 54 at 700K
• Is the formation of HI favored at higher or
  lower temperatures?

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Conceptual Question

• 2SO2 (g) O2 (g) 2SO3 (g)
• The equilibrium cannot be established
  when_________is/are placed in a 1.0 L container.
• A. 0.25 mol SO2 (g) and 0.25 mol O2 (g)
• B. 0.75 mol SO2 (g)
• C. 0.25 mol SO2 (g) and 0.25 mol SO3 (g)
• D. 0.50 mol O2 (g) and 0.50 mol SO3 (g)
• E. 1.0 mol SO3 (g)

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Student Practice Calculation
The equilibrium concentrations for the reaction
between carbon monoxide and molecular chlorine to
form COCl2 (g) at 740C are CO 0.012 M, Cl2
0.054 M, and COCl2 0.14 M. Calculate the
equilibrium constant.
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Equilibrium Case Study 2
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Equilibrium Expressions Involving Pressures
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Equilibrium Expressions Involving Pressures
or C P/RT
In terms of concentration
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Relationship between Kc and Kp
K NH32 C NH3 2 Kc
N2H23 (CN2) (CH23) C P/RT so
substitute into above expression (P NH3/RT) 2
(PN2/RT) (PH2/RT)3
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Student Practice Calculation
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Student Practice Calculation
Kp K(RT) Dn, K Kp / (RT) Dn, Dn 2 - 3
- 1 K 0.25 x (0.08206 L/atm mol K)(1100K) 23
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Equilibrium Constant
Kc
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.
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Equilibrium Constant
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When the equation for a reversible reaction is
written in the opposite direction, the
equilibrium constant becomes the reciprocal of
the original equilibrium constant.
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Writing Equilibrium Constant Expressions

• The concentrations of the reacting species in the
  liquid phase are expressed in M. In the gaseous
  phase, the concentrations can be expressed in M
  or in atm.
• The concentrations of pure solids, pure liquids
  and solvents do not appear in the equilibrium
  constant expressions.
• The equilibrium constant is a dimensionless
  quantity.
• When discussing equilibrium you must indicate the
  balanced equation and the temperature.
• If a reaction can be expressed as a 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.

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Equilibrium Constant
Heterogeneous equilibrium applies to reactions in
which reactants and products are in different
phases.
CaCO3 constant CaO constant
Kc CO2
The concentration of solids and pure liquids are
not included in the expression for the
equilibrium constant.
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Consider the following equilibrium at 295
K The partial pressure of each gas is 0.265
atm. Calculate K for this reaction?
0.265 x 0.265 0.0702
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More Calculations

• How do we calculate K if we dont know the
  concentrations of all of the chemical species in
  an equilibrium?
• Use an equilibrium Table
• Tabulate the known initial concentrations
• Calculate the change for known species
• Use stoichiometry to calculate change in other
  unknown species
• Calculate K
• Lets do an example

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Enough ammonia is dissolved in 5.00 L of water at
25C to produce a solution that is 0.0124 M in
ammonia. The solution is then allowed to come to
equilibrium. Analysis of the equilibrium mixture
shows that the concentration of OH- is 4.64 x
10-4 M. Calculate Keq at 25C for the reaction.

• NH3 (aq) H2O (l) NH4
  (aq) OH- (aq)

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You Try!

• A flask is charged with 1.500 atm N2O4(g) and
  1.00 atm NO2 (g). After equilibrium is reached,
  the partial pressure of NO2 is 0.512 atm.
• A. what is the equilibrium partial pressure of
  N2O4?
• B. Calculate the value of Keq for the reaction

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Conceptual Question

• A mixture of 0.682 mol H2 and 0.440 mol of Br2 is
  combined in a reaction vessel with volume of 2.0
  L. At equilibrium at 700 K, there are 0.566 mol
  of H2 present. At equilibrium, there are
  __________ mol of Br2 present in the reaction
  vessel.
• H2 (g) Br2 (g) 2HBr (g)
• A. 0.000
• B. 0.440
• C. 0.566
• D. 0.232
• E. 0.324

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Application of Equilibrium Constants

• Predicting the direction of reactions

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Application of Equilibrium Constants
The reaction quotient (Qc) is calculated by
substituting the initial concentrations of the
reactants and products into the equilibrium
constant (Kc) expression.

• IF
• Qc gt Kc system proceeds from right to left to
  reach equilibrium
• Qc Kc the system is at equilibrium
• Qc lt Kc system proceeds from left to right to
  reach equilibrium

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Example

• Keq 4.51 x 10-5 at 450C for
• N2(g) 3H2(g) 2NH3 (g)
• Indicate which direction the mixture must shift
  to reach equilibrium when 105 atm NH3, 35 atm N2
  and 495 atm H2 are present.

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Application of Equilibrium Constants
Calculating Equilibrium Concentrations Chemists
often need to know the amounts of reactants and
products at equilibrium. How do we do this?

• Express the equilibrium concentrations of all
  species in terms of the initial concentrations
  and a single unknown x, which represents the
  change in concentration.
• Write the equilibrium constant expression in
  terms of the equilibrium concentrations. Knowing
  the value of the equilibrium constant, solve for
  x.
• Having solved for x, calculate the equilibrium
  concentrations of all species.

Example problems on board N2O4 Ex Quadratic
Formula Ex
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Le Châteliers Principle

• What happens to a system at equilibrium when a
  stress is applied?

What Happens?
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Le Châteliers Principle

• If an external stress is applied to a system at
  equilibrium, the system adjusts in such a way
  that the stress is partially offset as the system
  reaches a new equilibrium position.

Change in Concentration
Change
Shifts the Equilibrium
Increase concentration of product(s)
left
Decrease concentration of product(s)
right
Increase concentration of reactant(s)
right
Decrease concentration of reactant(s)
left
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LeChateliers Principle

• Addition of a reaction component

Increase Concentrations of products
Add acetic acid
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LeChateliers Principle

• Removal of a reaction component

Remove ethyl acetate
Equilibrium shifts to the right
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Conceptual Question

• Problem when acid rain attacks marble (calcium
  carbonate), the following equilibrium can be
  written
• how does the fact that CO2 is a gas influence
  the equilibrium?

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What about Pressure and Temp?
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Le Châteliers Principle
Changes in Volume and Pressure
Change
Shifts the Equilibrium
Increase pressure
Side with fewest moles of gas
Decrease pressure
Side with most moles of gas
Increase volume
Side with most moles of gas
Decrease volume
Side with fewest moles of gas
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Le Châteliers Principle
Changes in Temperature
Change
Exothermic Rx
Endothermic Rx
Increase temperature
K decreases
K increases
Decrease temperature
K increases
K decreases

• Adding a Catalyst
• does not change K
• does not shift the position of an equilibrium
  system
• Allows system to reach equilibrium sooner though!

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At 12800C the equilibrium constant (Kc) for the
reaction Is 1.1 x 10-3. If the initial
concentrations are Br2 0.063 M and Br
0.012 M, calculate the concentrations of these
species at equilibrium.
Let x be the change in concentration of Br2
Initial (M)
0.063
0.012
ICE
Change (M)
-x
2x
Equilibrium (M)
0.063 - x
0.012 2x