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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.
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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.
N2O4 (g)
2 NO2 (g)
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Notes on Equilibrium Expressions (EE)
K=
[C]l[D]m
[A]j[B]k
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
For
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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)
K=
[C]l[D]m
[A]j[B]k
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.
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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)
Equilibrium 2.00
6.00
[4.00]
K
[6.0][ 2.0]

FeSCN2+(aq)
4.00
K  0.33
Complete sample problems 6 & 7 for more practice using ICE
charts.
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Equilibrium Constant in Terms of
Pressure


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Equilibria involving gases can be described in terms
of either pressure or concentrations.
The relationship can be seen using the Ideal Gas
Law, PV=nRT.
n
Rearranging this equation gives P  V RT  CRT
n/V represents concentration in M.
P
C
or P  CRT
RT
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)
Q
HF
H2
0
0
2
F2
0
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Predicting the Direction of a Reaction
Using Reaction Quotient
If Q > 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 < 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:
Initial:10.0 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



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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 Châtelier’s 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 Châtelier’s Principle

Consider the production of ammonia
N2(g) + 3H2(g)




2NH3(g)
As the pressure increases, the amount of ammonia
present at equilibrium increases.
As the temperature decreases, the amount of ammonia
at equilibrium increases. C
Can this be predicted?
Le Châtelier’s 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 Châtelier’s Principle
C 
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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 Châtelier’s Principle

Change in Reactant or Product Concentrations

Consider the Haber process



N2(g) + 3H2(g)
2NH3(g)
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 Châtelier’s Principle
Change in Reactant or Product Concentrations
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Le Châtelier’s Principle
The Haber Process
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Le Châtelier’s 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
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