Chemical Equilibrium

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Chemical Equilibrium
Chapter 13 AP CHEMISTRY
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Chemical Equilibrium

• The state where the concentrations of all
  reactants and products remain constant with time.
• On the molecular level, there is frantic
  activity. Equilibrium is not static, but is a
  highly dynamic situation.

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The Concept of Equilibrium

• Chemical equilibrium occurs when a reaction and
  its reverse reaction proceed at the same rate.

4
The Concept of Equilibrium

• As the substance warms it begins to decompose
• N2O4(g) ? 2NO2(g)
• A mixture of N2O4 (initially present) and NO2
  (initially formed) appears brown.
• When enough NO2 is formed, it can react to form
  N2O4
• 2NO2(g) ? N2O4(g).
• At equilibrium, as much N2O4 reacts to form NO2
  as NO2 reacts to re-form N2O4

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The Concept of Equilibrium

• As a system approaches equilibrium, both the
  forward and reverse reactions are occurring.
• At equilibrium, the forward and reverse reactions
  are proceeding at the same rate.

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A System at Equilibrium

• Once equilibrium is achieved, the amount of each
  reactant and product remains constant.

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Depicting Equilibrium

• Since, in a system at equilibrium, both the
  forward and reverse reactions are being carried
  out, we write its equation with a double arrow.

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Notes on Equilibrium Expressions (EE)

• For a given equation jA kB ? lC mD
• K does not include any pure solids or liquids
• The expression shows products divided by
  reactants
• Like the rate constant, k, the units of K depend
  on the experiment being performed
• For the reverse reaction K 1/K (reactants
  and products switch)
• Sometimes you will see K written as Kc
• Law of mass action

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Equilibrium Expression

• Write the equilibrium expression for
• 4NH3(g) 7O2(g) 4NO2(g) 6H2O(g)
• Complete sample problems 1-4. See sample
  problems 13.1 and 13.2 in your textbook for more
  worked examples.

Complete sample problems 1-4.
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Significance of Equilibrium Expression

• The inherent tendency for a reaction is occur is
  indicated by the magnitude of K.
• A K value much larger than 1 means that at
  equilibrium the reaction system will consist of
  mainly product equilibrium lies to the right
• A very small K means that the system at
  equilibrium will consist mainly of reactants
  equilibrium position is far to the left
• The size of K and the time required to reach
  equilibrium are NOT directly related.

Complete sample problem 5.
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Notes on Equilibrium Expressions (EE)

• For a reaction multiplied by an integer, n,
  Knew (Korig)n
• See sample exercise 13.2(c) on page 614.
• For a given reaction, K is dependent only on
  temperature

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Heterogeneous Equilibria

• . . . are equilibria that involve more than one
  phase.
• CaCO3(s) CaO(s) CO2(g)
• K CO2
• The position of a heterogeneous equilibrium does
  not depend on the amounts of pure solids or
  liquids present.

13
Practice Problem

• Consider the reaction represented by the
  equation
• Fe3(aq) SCN-(aq) ? FeSCN2(aq)
• In trial 1, you start with 6.00 M Fe3(aq) and
  10.0 M SCN-(aq), and at equilibrium the
  concentration of FeSCN2(aq) is 4.00 M.
• What is the value of the equilibrium constant for
  this reaction?

Fe3(aq) SCN-(aq) ?
FeSCN2(aq) Initial
Change
Equilibrium
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Practice Problem

• Fe3(aq) SCN-(aq) ?
  FeSCN2(aq)
• Equilibrium 2.00 6.00 4.00

Complete sample problems 6 7 for more practice
using ICE charts.
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Equilibrium Constant in Terms of Pressure

• Equilibria involving gases can be described in
  terms of either pressure or concentrations.
• The relationship can be seen using the Ideal Gas
  Law, PVnRT.
• Rearranging this equation gives
• n/V represents concentration in M.

Complete sample problem 8 now.
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K vs. Kp

• For any reaction
• Kp K(RT)Dn
• Dn sum of coefficients of gaseous products
  minus sum of coefficients of gaseous reactants.

Complete Sample Problem 9.
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Reaction Quotient

• After the equilibrium constant (K) is known, we
  can use it to determine if a reaction is at
  equilibrium.
• The reaction quotient, Q, has the same form as
  the equilibrium constant expression EXCEPT
  initial concentrations are used instead of
  equilibrium concentrations.

• H2(g) F2(g) 2HF(g)

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Predicting the Direction of a Reaction Using
Reaction Quotient

• If Q gt K then the reverse reaction must occur to
  reach equilibrium (i.e., products are consumed,
  reactants are formed, the numerator in the
  equilibrium constant expression decreases and Q
  decreases until it equals K).
• If Q lt K then the forward reaction must occur to
  reach equilibrium.
• If Q K then the reaction is at equilibrium.

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Practice Problem Part 2

• Using the previous reaction
• Fe3(aq) SCN-(aq) ? FeSCN2(aq) and the K
  value we determined K 0.33 determine if the
  following concentrations are at equilibrium
• Initial10.0 M Fe3(aq), 8.00 M SCN-(aq), and
  2.00 M FeSCN2-

Complete sample problems 10 11.
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Solving Equilibrium Problems

• 1. Balance the equation.
• 2. Write the equilibrium expression.
• 3. List the initial concentrations.
• 4. Calculate Q and determine the shift to
  equilibrium.

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Solving Equilibrium Problems(continued)

• 5. Define the change needed to reach equilibrium.
• 6. Substitute equilibrium concentrations into
  equilibrium expression and solve.
• 7. Check calculated concentrations by calculating
  K.

Complete more complex sample problems 12-15.
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Le Châteliers Principle

• . . . if a change is imposed on a system at
  equilibrium, the position of the equilibrium will
  shift in a direction that tends to reduce that
  change.

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Le Châteliers Principle

• Consider the production of ammonia
• As the pressure increases, the amount of ammonia
  present at equilibrium increases.
• As the temperature decreases, the amount of
  ammonia at equilibrium increases.
• Can this be predicted?
• Le Châteliers Principle if a system at
  equilibrium is disturbed, the system will move in
  such a way as to counteract the disturbance.

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Le Châteliers Principle
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Effects of Changes on the System
1. Concentration The system will shift away
from the added component. 2. Temperature K will
change depending upon the temperature (treat the
energy change as a reactant).
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Increase of Pressure to an Equilibrium.
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Effects of Changes on the System (continued)

• 3. Pressure
• a. Addition of inert gas does not affect the
  equilibrium position.
• b. Decreasing the volume shifts the
  equilibrium toward the side with fewer moles.

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Le Châteliers Principle

• Change in Reactant or Product Concentrations
• Consider the Haber process
• If H2 is added while the system is at
  equilibrium, the system must respond to
  counteract the added H2 (by Le Châtelier).
• That is, the system must consume the H2 and
  produce products until a new equilibrium is
  established.
• Therefore, H2 and N2 will decrease and NH3
  increases.

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Le Châteliers Principle
Change in Reactant or Product Concentrations
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Le Châteliers Principle
The Haber Process
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Le Châteliers Principle
The Haber Process for producing NH3

• N2 and H2 are pumped into a chamber.
• The pre-heated gases are passed through a heating
  coil to the catalyst bed.
• The catalyst bed is kept at 460 - 550 ?C under
  high pressure.
• The product gas stream (containing N2, H2 and
  NH3) is passed over a cooler to a refrigeration
  unit.
• In the refrigeration unit, ammonia liquefies but
  not N2 or H2.
• The unreacted nitrogen and hydrogen are recycled
  with the new N2 and H2 feed gas.
• The equilibrium amount of ammonia is optimized