AP Chemistry: Equilibrium Student Notes
Objectives
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What is Equilibrium
The Equilibrium Constant
Equilibrium and Pressure
Calculations involving equilibrium
Le Chatelier’s Principle
What is Equilibrium
An Analogy
A Dance
Who is present at a dance?
_______________
_______________
What happens at a high school dance?
Lets Graph this:
Amount
Time
So we can summarize this by saying
_______________ + ________________  __________________
What is the purpose of the 
Definition of Equilibrium
-1-
Equilibrium is Dynamic
=
The Equilibrium Constant
Law of Mass Action
For the reaction:
jA + kB  nC + mD
Kc=
This value is constant (hence the name) at a specified ______________
The Meaning of K
K>1
K<1
K≈1
Equilibrium and Pressure
Many reactions are easier to measure using partial pressures. So for the reaction:
N2(g) + 3H2(g)  2NH3(g)
Kp=
Relationship between Kc and Kp
Q=
-2-
Calculations involving equilibrium
How to solve Equilibrium Problems:
1. Start with a balanced Chemical Equation
2. Write down the amounts (either concentration or pressure units) in an ICE table.
3. Shift the equilibrium by subtracting and adding x to either side to the equation.
4. Solve for x using your solver on your calculator
5. Sometimes the equations must be solved in reverse….
Examples
Example 1
The equilibrium constant for the following reaction is 0.0900 at 25ْC:
 2HOCl (g)
H2O(g) + Cl2O (g) 
For which of the following situations is the system at equilibrium? For those which are not at equilibrium,
in which direction will they shift?
a. PH 2 O  200.torr, PCl 2 O  49.8torr, PHOCl  21.0torr
b. PH 2 O  1.00atm, PCl 2 O  1.00atm, PHOCl  1.00atm
c. PH 2 O  296.torr , PCl 2 O  15.0torr , PHOCl  20.0torr
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Example 2:
The value of the equilibrium constant for the following reaction is 1.5 x 10-5 at 298K.
 H2(g) + F2(g)
2HF(g) 
If the [HF] = 2.0M. What are the final concentrations of all species once equilibrium is established?
Example 3:
The value of the equilibrium constant for the following reaction at 500K is 4.35:
 2NH3(g)
N2(g) + H2 (g) 
If the initial [N2] = 2.3 M and the [H2]=1.25M. What are the total concentrations of all species once
equilibrium is established?
-4-
Example 4:
Phosphorus pentachloride, at initial partial pressure of 189.2 torr was placed into an empty flask and
allowed to decompose to PCl3 and Cl2 by the following reaction at 557K.
 PCl3(g) + Cl2(g)
PCl5(g) 
At equilibrium the total pressure inside the flask was observed to be 358.7 torr. Calculate the partial
pressure of each gas at equilibrium and the value of Kp at 557K
1995 A
CO2(g) + H2(g) H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2,000 K, equilibrium is achieved according to the equation above. In one
experiment, the following equilibrium concentrations were measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
(a) What is the mole fraction of CO(g) in the equilibrium mixture?
(b) Using the equilibrium concentrations given above, calculate the value of Kc, the equilibrium constant for the
reaction.
(c) Determine Kp in terms of Kc for this system.
(d) When the system is cooled from 2,000 K to a lower temperature, 30.0 percent of the CO(g) is converted back to
CO2(g). Calculate the value of Kc at this lower temperature.
(e) In a different experiment, 0.50 mole of H2(g) is mixed with 0.50 mole of CO2(g) in a 3.0-L reaction vessel at
2,000 K. Calculate the equilibrium concentration, in moles per L, of CO(g) at this temperature.
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Le Chatelier’s Principle
What is Le Chatelier’s Principle?
When a system at equilibrium is placed under stress it will _________ to relive that stress. Stressors
of equilibrium are:
 ________________
 ________________
 ________________
Stressors do not include adding an ___________ _____________ or a ____________
The only way to change the value of the equilibrium constant is to change the ____________
Adding and Removing Chemicals in the reaction
 Adding chemicals in the reaction will shift the reaction _________ from that side of the reaction
 Removing chemicals in the reaction will shift the reaction _________ from that side of the reaction
Adding Chemicals other than those in the reaction
 Adding chemicals other than those in the reaction will usually have _________ affect on the
equilibrium.
 However, sometimes adding a chemical will ___________ ___________ a chemical in the reaction
and thus taking that chemical _________ from the reaction.
Temperature shifts:
When heat is added to a system the reaction will shift __________ from the side of the equation where
energy is:
 For an exothermic reaction: Increasing temperature will shift the reaction to the ________
 For an endothermic reaction: Increasing temperature will shift the reaction to the ________
Pressure Shifts
 When pressure is increased, the reaction will shift toward the side of the reaction with the
__________ number of __________ of gas
 When pressure is decreased, the reaction will shift toward the side of the reaction with the
__________ number of __________ of gas
Adding Inert Gases
 Adding inert gases shifts the equilibrium to the _____________
Adding a Catalyst:
 Adding a catalyst will shift the equilibrium to the ______________
Solids:
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Example 1
Hydrogen for use in ammonia production is produced by the reaction:
Ni , 750C
 CO(g) + 3H2(g)
CH4(g) + H2O(g) 
What will be happen to the reaction mixture if:
a. H2O(g) was removed?
b. The temperature is increased?
c. Helium is added?
d. CO(g) is removed?
e. The volume of the container is tripled?
Example 2
Predict the effect of a decrease in pressure on the following chemical equilibria:
a. 2 NOCl (g)
2 NO (g) + Cl2 (g)
b. 2 SO2 (g) + O2 (g)
c. H2 (g) + Cl2 (g)
2 SO3 (g)
2 HCl (g)
d. CH4 (g) + H2O (g)
CO (g) + 3 H2 (g)
e. NO (g) + CO2 (g)
NO2 (g) + CO2 (g)
1980 D: NH4Cl(s)  NH3(g) + HCl(g) H = +42.1 kilocalories (+ H means it is Endothermic)
Suppose the substances in the reaction above are at equilibrium at 600K in volume V and at pressure P. State whether
the partial pressure of NH3(g) will have increased, decreased, or remained the same when equilibrium is reestablished
after each of the following disturbances of the original system. Some solid NH4Cl remains in the flask at all times.
Justify each answer with a one–or–two sentence explanation.
(a) A small quantity of NH4Cl is added.
(b) The temperature of the system is increased.
(c) The volume of the system is increased.
(d) A quantity of gaseous HCl is added.
(e) A quantity of gaseous NH3 is added.
1988 D
NH4HS(s) NH3(g) + H2S(g)
H = +93 kilojoules
The equilibrium above is established by placing solid NH4HS in an evacuated container at 25°C. At equilibrium, some
solid NH4HS remains in the container. Predict and explain each of the following.
(a) The effect on the equilibrium partial pressure of NH3 gas when additional solid NH4HS is introduced into the
container
(b) The effect on the equilibrium partial pressure of NH3 gas when additional solid H2S is introduced into the
container
(c) The effect on the mass of solid NH4HS present when the volume of the container is decreased
(d) The effect on the mass of solid NH4HS present when the temperature is increased.
-7-
Equilibrium Practice
Multiple Choice
Identify the choice that best completes the statement or answers the question.
____ 1.
When the system A + B
C + D is at equilibrium,
a. the forward reaction has stopped.
b. the reverse reaction has stopped.
c. both the forward and the reverse reactions have stopped.
d. neither the forward nor the reverse reaction has stopped.
e. the sum of the concentrations of A and B must equal the sum of the concentrations of C and D.
____ 2.
Consider the gas phase system below at a high temperature. The form of the expression for the
equilibrium constant, Kc, ____.
4NH3 + 5O2
4NO + 6H2O
a. cannot be determined without rate data
b.
is Kc =
c.
is Kc =
d.
is Kc =
e.
is Kc =
____ 3.
What is the value of Kc for the reaction 2A(g) + 3B(g)
2C(g) + D(g) if at equilibrium [A] =
0.60 M, [B] = 0.30 M, [C] = 0.10 M and [D] = 0.50 M?
a. 1.9
b. 0.15
c. 2.4
d. 0.51
e. 0.088
____ 4.
What is the value of Kc for the reaction CH4(g) + H2O(g)
CO(g) + 3H2(g) if at equilibrium
[CH4] = 0.20 M, [H2O] = 0.20 M, [CO] = 0.50 M and [H2] = 1.50 M?
a. 19
b. 0.24
c. 0.053
d. 42
e. 16
____ 5.
In a 1.0-liter container there are, at equilibrium, 0.20 mole of I2, 0.30 mole of H2, and 0.20 mole of
HI. What is the value of Kc for the reaction? H2(g) + I2(g)
2HI(g)
a. 0.33
b. 0.67
c. 2.7
d. 1.3
e. 1.5
____ 7.
Given the following reaction and its equilibrium constant at a certain temperature,
N2(g) + 3H2(g)
2NH3(g) Kc = 3.6  108
calculate the numerical value of the equilibrium constant for the following reaction at the same temperature.
NH3(g)
N2(g) + H2(g)
a. 2.8  10-9
c. 1.3  107
e.7.7  10-18
4
-5
b. 1.9  10
d. 5.3  10
-8-
____ 8.
Which of the following statements about the reaction quotient, Q is false?
a. The value of Q can be used to predict equilibrium concentrations.
b. It has the same expression as Kc.
c. Its value is calculated using nonequilibrium concentrations.
d. If Q > Kc, the reaction must move to equilibrium by forming more reactants.
e. If Q < Kc, the reaction must move to equilibrium by forming more products.
____ 9.
For the following reaction, Kc is 144 at 200°C. If 0.400 mol of both A and B are placed in a 2.00-liter
container at that temperature, what will be the concentration of C at equilibrium?
A(g) + B(g)
C(g) + D(g)
a. 0.015 M
b. 1.64 M
c. 0.200 M
d. 0.185 M
e. 1.13 M
____ 10.
For the following reaction, Kc is 144 at 200°C. If 0.400 mol of both A and B are placed in a 2.00-liter
container at that temperature, what will be the concentration of B at equilibrium?
A(g) + B(g)
C(g) + D(g)
a. 0.015 M
b. 1.64 M
c. 0.200 M
d. 0.185 M
e. 1.13 M
____ 11.
Ammonium carbamate decomposes into NH3 and CO2 when heated.
NH4(NH2CO2)(s)
2NH3(g) + CO2(g)
If 25.0 g of ammonium carbamate is placed in an empty chamber and heated to 100°C, what are the equilibrium
concentrations of NH3 and CO2? Kc = 58.3 at 100°C.
a. [NH3] = 2.44 M, [CO2] = 4.88 M
b. [NH3] = 4.88 M, [CO2] = 2.44 M
c. [NH3] = 29.2 M, [CO2] = 14.6 M
d. [NH3] = 15.3 M, [CO2] = 7.64 M
e. [NH3] = 10.8 M, [CO2] = 5.40 M
____ 12.
Exactly one mol/L of each of the four gases involved in the following reaction is placed in a reactor at
200°C and allowed to reach equilibrium. What is the equilibrium concentration of H2O? Kc = 2.38 @ 200°C.
CO(g) + H2O(g)
CO2(g) + H2(g)
a. 1.21 M
b. 0.213 M
c. 0.787 M
d. 2.38 M
e. 1.0 M
____ 13.
For the following system, the equilibrium constant at 445°C is 51.0. If a reaction is initiated with the
following initial concentrations, [H2] = 2.06  10-2 M, [I2] = 1.45  10-2 M, and [HI] = 0, what will be the
equilibrium concentration of HI?
H2(g) + I2(g)
2HI(g)
a. 1.8  10-2 M
b. 1.4  10-1 M
c. 2.7  10-1 M
d. 2.6  10-2 M
e. 3.7  10-3 M
____ 14.
A reaction begins with 0.600 mole of A and 0.200 mole of B in a 2.00-L container at a certain
temperature. What will be the equilibrium concentration of C?
A(g) + B(g)
C(g) Kc = 23.5
a. 0.0684 M
c, 0.044 M
e. 0.105 M
b. 0.0200 M
d. 0.0836 M
-9-
____ 15.
Considering the following reaction at equilibrium, identify which of the stresses below would cause
more products to form. CO(g) + Cl2(g)
COCl2(g) Ho = -113 kJ/mol
a. Increase the pressure.
b. Increase the [Cl2].
c. Increase the temperature.
d. Both (a) and (b) are correct.
e. All three are correct.
____ 16.
Suppose we let the reaction below come to equilibrium. Then we decrease the total pressure, by
increasing the volume of the container. What will be the effect on the net amount of SO3(g) present?
2SO2(g) + O2(g)
2SO3(g)
a. It increases.
b. It decreases.
c. It does not change.
d. The question cannot be answered without knowing the value of K.
e. The question cannot be answered without knowing the value of H0.
____ 17.
Suppose we let the reaction below come to equilibrium. Then we decrease the total pressure, by
increasing the volume of the container. What will be the effect on the numerical value of the equilibrium constant,
K? 2SO2(g) + O2(g)
2SO3(g) + heat
a. It increases.
b. It decreases.
c. It does not change.
d. The question cannot be answered without knowing the initial value of K.
e. The question cannot be answered without knowing the value of H0.
____ 19.
Suppose we let this reaction come to equilibrium. Then we increase the temperature of the reaction
mixture. What will be the effect on the net amount of SO3(g) present?
2SO2(g) + O2(g)
2SO3(g) + heat
a. It increases.
b. It decreases.
c. It does not change.
d. The question cannot be answered without knowing the value of K.
e. The question cannot be answered without knowing the value of H0.
____ 21.
At equilibrium a 1.0-liter container was found to contain 0.20 mol of A, 0.20 mol of B, 0.40 mol of C,
and 0.40 mol of D. If 0.10 mol of A and 0.10 mol of B are added to this system, which of the following statements
is true?
A(g) + B(g)
C(g) + D(g)
a. The system remains in equilibrium.
b. Q > Kc
c. More reactants are formed to increase the value of Q.
d. The value of Kc decreases.
e. More products are formed to increase the value of Q
____ 22.
Which of the following will require the least time for a reaction to reach equilibrium?
a. Kc is a very small number.
b. Kc is a very large number.
c. Kc is approximately one.
d. Cannot tell without knowing the value of Kc.
e. Cannot tell, since the time required to reach equilibrium does not depend on Kc.
____ 30.
For the following reaction at equilibrium at 445°C the partial pressures were found to be [H2] = 0.45
atm, [I2] = 0.10 atm and [HI] = 1.53 atm. Calculate Kp for this reaction. H2(g) + I2(g)
2HI(g)
a. 150
b. 34
c. 52
d. 76
e. 4.4
- 10 -
____ 34.
Given: PCl5(g)
PCl3(g) + Cl2(g)
At 250.°C a sample of PCl5 was placed in a 24-liter evacuated reaction vessel and allowed to come to equilibrium.
Analysis showed that at equilibrium 0.42 mole of PCl5, 0.64 mole of PCl3, and 0.64 mole of Cl2 were present in the
vessel. Calculate Kp for the reaction at 250.°C.
a. 1.2
b. 1.3
c. 1.5
d. 1.8
e. 2.2
____ 35.
Certain amounts of the hypothetical substances A2 and B are mixed in a 3.00-liter container at 300. K.
When equilibrium is established for the reaction the following amounts are present: 0.200 mol of A2, 0.400 mol of
B, 0.200 mol of D, and 0.100 mol of E. What is Kp, the equilibrium constant in terms of partial pressures, for this
reaction? A2(g) + 3B(g)
2D(g) + E(g)
a. 16.4
b. 0.084
c. 3.81  10-2
d. 1.42  10-2
e. 2.50  10-1
____ 36.
For the reaction below, Kp = 6.70  10-3 at 25°C.
COCl2(g)
CO(g) + Cl2(g)
A sample of COCl2 is placed in a closed 15.0-liter vessel at 25°C, and it exerts a pressure of 4.65 atm before
decomposition begins. What will be the partial pressure of Cl2 at equilibrium?
a. 0.173 atm
b. 0.206 atm
c. 0.566 atm
d. 2.14 atm
e. 1.16 atm
____ 37.
The equilibrium constant, Kp, for the following reaction is 280. at 150.°C. Suppose that a quantity of
IBr is placed in a closed reaction vessel and the system is allowed to come to equilibrium at 150.°C. When
equilibrium is established, the pressure of IBr is 0.200 atm. What is the pressure of I2 at equilibrium?
I2(g) + Br2(g)
2IBr(g) + 11.7 kJ
a. 0.168 atm
b. 0.096 atm
c. 0.067 atm
d. 0.012 atm
e. 0.00014 atm
____ 38.
Given the equilibrium reaction ZnCO3(s)
ZnO(s) + CO2(g). Which one of the following
statements is true?
a. Equal concentrations of ZnO(s) and CO2(g) would result from the decomposition of a
given amount of ZnCO3(s).
b. The same equilibrium condition would result if we started with only pure ZnCO3(s) in a
closed container as if we started with only pure ZnO(s) in a closed container.
c. Introducing 1.0 atm pressure of N2(g) into the system at equilibrium in a closed
container would result in more ZnCO3(s) being formed.
d. Decreasing the volume of the closed system initially at equilibrium, at constant
temperature, would result in more ZnCO3(s) being formed.
e. At equilibrium conditions, the forward and reverse reactions have stopped.
- 11 -
____ 39.
Consider the following heterogeneous reaction:
Mg(s) + 2H3O+(aq)
Mg2+(aq) + H2(g) + 2H2O( )
Which of the following statements is false?
a. The amount of magnesium present does not affect the equilibrium position.
b. Adding 1.0 atm of N2 to the system would cause the reverse reaction to be favored.
c. Increasing PH2 causes the forward reaction to be favored.
d. Increasing [H3O+] causes the forward reaction to be favored.
e. Decreasing [Mg2+] causes the forward reaction to be favored.
____ 41.
Calculate the thermodynamic equilibrium constant at 25°C for a reaction for which G0 = 11.3 kJ per
mol of reaction. R = 8.314 J/mol•K.
a. 0.995
b. 95.7
c. - 4.56
d. 1.04  10-2
e. 4.32  10-24
____ 45.
The van't Hoff equation relates two equilibrium constants at two different temperatures.
Consider a reaction where K298 = 16.3 and Ho = -116 kJ/mol. Which of the following statements is true?
a. The reverse reaction is favored at 298 K.
b. A decrease in temperature will cause the forward reaction to be favored more.
c. An increase in temperature will cause the forward reaction to be favored more.
d. The value of K would be larger at 398 K.
e. The reaction is endothermic.
____ 46.
The value of Kp at 390.°C for the reversible reaction given below is 7.95  10-3. Calculate the value of
Kp at 25°C. H0 = 162 kJ/mol, R = 8.314 J/mol•K
A(g) + B(g)
C(g)
a. 2.8  10-2
b. 1.8  10-18
c. 6.3  10-12
d. 3.0  10-14
e. 3.4  1013
____ 47.
If the Kp at 2000. K for the reaction given below is 2.1  10-3 and the Kp at 27°C is 5.0  10-30,
calculate H0 for this reaction.
A(g) + B(g)
C(g)
a. 2.6 kJ
b. 1.44 J
c. -179 kJ
d. 180 kJ
e. 13.9 kJ
____ 48.
If the Kp at 2000. K for the reaction given below is 2.1  10-3, at what temperature would the value of
-10
Kp = 5.0  10 ? H0 for this reaction is 180 kJ/mol.
A(g) + B(g)
C(g)
a. 433 K
b. 718°C
c. 557°C
d. 800 K
e. 2000 K
- 12 -
____ 49.
At 986°C the equilibrium constant, Kc, for the reaction:
H2(g) + CO2(g) H2O(g) + CO(g)
is 1.60. If the equilibrium concentration of H2 is 1.0 M, that of CO2 is 0.50 M, and that of H2O is 0.40 M, then the
equilibrium concentration of CO is
a. 0.40 M
b. 0.50 M
c. 1.3 M
d. 2.0 M
____ 50.
Kc = 0.040 for the system below at 450°C:
PCl5(g) PCl3(g) + Cl2(g)
Calculate KP for this reaction at 450°C.
a. 240
b. 140
c. 2.4
d. 6.7  10-4
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Basic Concepts
The Equilibrium Constant
The Equilibrium Constant
The Equilibrium Constant
The Equilibrium Constant
Variation of Kc with the Balanced Equation
The Reaction Quotient
Uses of the Equilibrium Constant, Kc
Uses of the Equilibrium Constant, Kc
Uses of the Equilibrium Constant, Kc
Uses of the Equilibrium Constant, Kc
Uses of the Equilibrium Constant, Kc
* Harder Question
Disturbing a System in Equilibrium
Disturbing a System in Equilibrium:
Disturbing a System in Equilibrium:
Disturbing a System in Equilibrium:
Disturbing a System in Equilibrium:
Disturbing a System in Equilibrium:
Partial Pressures and the EquiliConstant
Relationship Between Kp and Kc
Relationship Between Kp and Kc
Relationship Between Kp and Kc
Relationship Between Kp and Kc
Heterogeneous Equilibria
Heterogeneous Equilibria
TOP: Additional Questions