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AP Chemistry
Equilibrium Practice 2
The Equilibrium Constant and LeChatlier’s Principle
The Equilibrium Constant
Answers
1. For the following system at equilibrium:
H2 (g) + I2 (g) ↔ 2 HI (g)
a. Predict the shift in equilibrium when more HI(g) is added to the system.
b. How will the concentration of I2 change?
a. Adding something on the product side of the reaction will cause equilibrium to shift to the reactant
side. In other words, the reverse reaction will be favored, or you could say that equilibrium shifts to
the left.
b. The concentration of I2 will increase.
2. For the reaction below, predict the direction the equilibrium will shift given the following changes.
Temperature and volume are held constant.
2 NO2 (g) + 7 H2 (g) ↔ 2 NH3 (g) + 4 H2O(g)
a. addition of ammonia
b. removal of nitrogen
dioxide
c. removal of water vapour
d. addition of hydrogen
reverse reaction is favored to use up the additional
ammonia
reverse reaction is favored to replace the lost
nitrogen dioxide
forward reaction is favored to replace the lost water
vapor
forward reaction is favored to use up the additional
hydrogen
3. At a particular temperature, the following reaction has an equilibrium constant, Keq of 0.18
PCl3 (g) + Cl2 (g) ↔ PCl5 (g)
More PCl3 is added to the system. Will the value of Keq increase, decrease, or remain the same?
Changing the concentration of a reaction participant will not change the value of Keq. Keq will still equal
0.18 after equilibrium becomes re-established.
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Le Châtelier's Principle - Temperature & Catalysts
Answers
4. For each of the following equilibria, predict whether the system will shift in the forward or reverse
directions. Note the energy changes involved and assume that the volume remains constant.
a. heat is removed from: A  B
ΔH° = +40.0 kJ
Removing heat favors the exothermic direction to replace the lost heat. Thus the
reverse direction will be favored.
b. heat is removed from: A + B  2 C
ΔH° = -25.5 kJ
In this case the forward direction is the exothermic direction, so the forward
reaction is favored.
c. heat is added to: A + 2B  3 C
ΔH° = -32.0 kJ
The reverse reaction is endothermic, so it will be favored to remove the excess heat.
5. In each of the following equilibria, would you increase or decrease the temperature to force the reaction in
the forward direction?
a. H2 (g) + CO2 (g)  H2O(g) + CO(g)
ΔH° = +41.0 kJ
Answer: Increase temperature, because the forward direction is endothermic and will
use up the additional heat.
b. 2 SO2 (g) + O2 (g)  2 SO3 (g)
ΔH° = -198 kJ
Answer: Decrease, because the forward reaction will produce heat removed by making
the system cooler.
6. For each of the equilibria in Question 2 will the value for Keq increase or decrease if the temperature is
raised?
a. H2 (g) + CO2 (g)  H2O(g) + CO(g)
ΔH° = +41.0 kJ
Answer: Keq will increase. Increasing the temperature will favor the products.
b. 2 SO2 (g) + O2 (g) 2 SO3 (g)
ΔH° = -198 kJ
Answer: Keq will decrease. Increasing the temperature will favor the reactants.
7. Explain the effect of using a platinum catalyst in the equilibrium reaction of ammonia with oxygen:
4 NH3 (g) + 5 O2 (g)  4 NO(g) + 6 H2O(g) + heat
Answer:
The addition of a catalyst will cause the system to reach equilibrium sooner, but it will have no effect on
equilibrium once it has been reached.
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Le Châtelier's Principle - Volume & Pressure
Answers
8. The pressure on each of the following systems is increased by decreasing the volume of the container.
Explain whether each system would shift in the forward direction, the reverse direction, or stay the same.
a. 2 SO2 (g) + O2 (g)  2 SO3 (g)
b. H2 (g) + I2 (g)  2 HI(g)
c. CaCO3 (s)  CaO(s) + CO2 (g)
d. AgCl(s)  Ag+(aq) + Cl-(aq)
forward reaction is favored; there are fewer
moles of gas on the product side.
No change - same number of moles on each
side
reverse reaction is favored; there are no moles
of gas on the reactants side vs. one mole of
gas on the product side.
No change - no gases present
9. List three ways that the following equilibrium reaction could be forced to shift to the right:
2 NO2 (g)  2 NO(g) + O2 (g)
Solution:

add more NO2 (g)

remove NO(g) or O2 (g)

decrease the pressure

increase the volume
10. Given the following equilibrium reaction:
2 C(s) + O2 (g)  2 CO(g)
What will be the effect of the following disturbances to the system:
a. adding CO (at constant volume and
temperature)
b. addition of O2 (at constant volume and
temperature)
reverse reaction is favored. Adding a substance
on one side of the equation forces the reaction to
the other side of the equation.
The forward reaction is favored to use up the
additional O2
c. addition of solid carbon (at constant
temperature)
No effect. Adding more of a solid does not
change it's concentration. The reaction may go
on longer, but equilibrium is not affected.
d. decreasing the volume of the container
Count moles of GAS on both sides of the
equation. There is 1 mole of gas on the reactant
side, and 2 moles on the product side.
Decreasing volume will favor the side with the
fewest moles of gas - the reverse reaction will be
favored.
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