Name
Chem 163 Section: ______ Team Number: ______
ALE 10. Le Châtelier’s Principle
(Reference: 17.6 Silberberg 5th edition)
How do systems at equilibrium respond to stress?
The Model: Le Châtelier’s Principle
Le Châtelier’s Principle says: If a system at equilibrium undergoes a stress, the amounts of the reactants
and products will change in such a way so that the stress will be relieved.
Sources of stress that a system at equilibrium might encounter include: a change in the amount of a reactant
or product; a change in the pressure of the system in a container of variable volume; or a change in the
temperature of the system.
Key Questions
1. Consider the synthesis of ammonia from its elements (eqn 1).
N2(g) + 3 H2(g)
2 NH3(g)
(1)
Let us say that the system is at equilibrium having concentrations of [H2]eq, [N2]eq, and [NH3]eq at
certain values.
a. Laws of mass action involving the equilibrium
2
[NH3]eq
constant and the reaction quotient for the reaction Kc = 
3
[H2]eq [N2]eq
are shown on the right. How is a reaction quotient
different from an equilibrium constant?
[NH3]2
Q = 
[H2]3 [N2]
b. If suddenly additional nitrogen is injected to the system, how will the new concentration of N2 (i.e.,
[N2]new) compare to previous equilibrium concentration of N2 (i.e., [N2]old,eq)?
[N2]new ____ [N2]old,eq
(Place a <, =, or > sign in the blank.)
c. If suddenly additional nitrogen is injected to the system, how will the reaction quotient compare to
the equilibrium constant for the reaction?
Q ____ Kc
(Place a <, =, or > sign in the blank.)
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d. After the additional nitrogen has been injected to the system, the amount of each gas will change
until Q = Kc. Which of the following will be observed? (Circle your answer.)
i. [N2] will decrease to the amount it was before when the system was at equilibrium;
[H2] will decrease and [NH3] will increase stoichiometrically.
ii. [N2] will decrease, but not back to the amount it was before when the system was at
equilibrium; [H2] will decrease and [NH3] will increase stoichiometrically.
iii. Since [N2] was increased, by stoichiometry [H2] must also be increased three times as much;
[NH3] decreases twice as much as [N2] was increased.
2. Consider the general reversible one-phase reaction:
aA + bB
dD + eE
The system starts off at equilibrium. Then the system encounters the stress that more of A is added to
the reaction mixture. Which of the following does Le Châtelier’s Principle say will occur? (Circle your
answer.)
i. The concentrations of A and B will decrease (although [A] will still be higher than what it had been
before additional A was added) and as a result the concentrations D and E will increase.
ii. The concentrations of D and E will decrease and as a result the concentrations of both A and B will
increase.
3. When ammonia is synthesized industrially (see eq 1 in Question 1), the gas mixture is pumped through
a condenser. The condenser is at a temperature sufficient to liquefy ammonia, but not hydrogen or
nitrogen.
a. As a result of passing the gases through the condenser, how does the new concentration of NH3 (i.e.,
[NH3]new) in the gaseous reaction mixture compare to previous equilibrium concentration of NH3
(i.e., [NH3]old,eq)?
[NH3]new ____ [NH3]old,eq
(Place a <, =, or > sign in the blank
b. If suddenly ammonia is removed from the gaseous system, how will the reaction quotient compare
to the equilibrium constant for the reaction?
Q ____ Kc
(Place a <, =, or > sign in the blank.)
c. After the ammonia has been removed from the gaseous system, the amount of each gas will change
until Q = Kc. Which of the following will be observed? (Circle your answer.)
i. [NH3] decreases because [H2] and [N2] were increased stoichiometrically.
ii. [H2] and [N2] decrease in stoichiometric proportions in order to partially replenish the lost NH3.
iii. [H2] and [N2] decrease in stoichiometric proportions in order to fully replenish the lost NH3,
making [NH3] as high as it had been before NH3 was removed from the reaction mixture.
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4. Stated generally, Le Châtelier’s Principle says that when a species is removed from a system at
equilibrium, the system shifts: (Circle your answer.)
i.
to the side with that removed species.
ii. away from the side with that removed species.
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a. Suppose you are on a spaceship. Within the spaceship there is a breathable gas mixture at a
“comfortable” 1 atm of pressure. Outside the spaceship is the vacuum of space (pressure just
slightly greater than 0 atm). Suddenly, a meteorite strikes the spaceship, punching a hole in the
spaceship’s hull! What happens to the air within the spaceship?
b. From this illustration, we can infer that: (Circle your answer.)
i. gases prefer low-pressure conditions.
ii. gases prefer high-pressure conditions.
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a. Suppose there is a chemical reaction involving gaseous reactants and products in a reaction vessel of
variable volume (e.g., a cylinder with a piston that can move in and out). If the volume is increased
at constant temperature, the stress that the gas mixture encounters is (Hint: Recall Boyle’s law.):
(Circle your answer.)
i. the pressure decreases.
ii. the pressure increases.
b. To overcome this stress, Le Châtelier’s Principle says that the reaction will shift in order to
(Hint: Look at your answer to Question 5b.): (Circle your answer.)
i. decrease the number of moles of gas.
ii.
7. When ammonia is synthesized industrially: N2(g) + 3 H2(g)
pressure of several hundred atmospheres. Briefly explain why.
increase the number of moles of gas.
2 NH3(g) it is typically done at a
8. Since exothermic reactions are more common than endothermic reactions, we’ll consider the
exothermic reversible reaction:
2A
A2 + Heat
(Molecular interpretation: When two molecules of A collide to form the dimer A2, a covalent bond
forms between the monomers. When a covalent bond forms, heat energy is released.)
a. But we may also think of the reverse reaction. The A—A bond in A2 can be broken, and this
is likely to occur when A2 is “bathed” in: (Circle your answer.)
i. less heat.
ii. more heat.
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b. The change in heat from part (a) may be as a result of the reaction being exposed to a: (Circle your
answer.)
i. lower temperature.
ii.
higher temperature.
9. In general, Le Châtelier’s Principle says that a decrease in temperature will cause a reaction to shift
(Hint: Think of it as “a desire to replenish the removed heat”.): (Circle your answer.)
i. to the side on which heat may be written.
ii. away from the side on which heat may be written.
Exercises
10. Consider the following reversible reaction.
W(s) + 6 HCl(g)
WCl6(g) + 3 H2(g) H = 60 kJ
For each of the following changes, write in each blank box whether the amount of the corresponding
species will “decrease”, “increase”, or remain “unchanged”.
Stress
Additional W is added to the
a. reaction vessel (Assume the addition of
HCl
WCl6
H2
the tungsten does not change the total
pressure within the reaction vessel.)
Some WCl6 is liquefied and
withdrawn from the mixture.
The volume of the reaction vessel
c.
decreases at constant temperature.
The temperature of the reaction
d.
mixture is decreased.
b.
decrease
11. Determine the direction of the shift in equilibrium when these external changes are made to the
EXOTHERMIC equilibrium: (Circle your answers.)
4 NH3(g) + 5 O2(g)
4 NO(g) + 6 H2O(g)
a) If the system is heated the equilibrium will shift to the: i. Right
ii. Left iii. Remains unchanged.
b) A decrease in pressure due to increasing the volume of the system at equilibrium will cause the
equilibrium will shift to the: i. Right ii. Left iii. Remains unchanged.
c) An increase in pressure due to pumping air into the system at equilibrium will cause the equilibrium
will shift to the: i. Right ii. Left iii. Remains unchanged.
d) An increase in pressure due to pumping helium gas into the system at equilibrium will cause the
equilibrium will shift to the: i. Right ii. Left iii. Remains unchanged. (circle one and explain below)
e) Removal of water vapor from the system at equilibrium will cause the equilibrium will shift to the:
i. Right ii. Left iii. Remains unchanged.
f) Addition of hydrogen chloride gas, HCl(g), to the system at equilibrium will cause the equilibrium
will shift to the: i. Right ii. Left iii. Remains unchanged.
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Application Question
12. Substance X is a product of a gaseous reaction, which results in an equilibrium mixture being formed.
The percentage of X in the equilibrium mixture under different conditions is shown in the table below:
550oC
650oC
750oC
850oC
1 Atm
0.77
0.032
0.016
0.009
100 Atm
6.70
3.02
1.54
0.87
200 Atm
11.9
5.71
2.99
1.68
Use the data to answer and justify each of the following questions.
a.) Is the production of X exothermic or endothermic? Explain.
b.) Does the production of X involve an increase or decrease in the number of moles of gas present?
Explain.
c.) What are the best conditions of temperature and pressure to obtain the greatest yield of X? Explain.
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