Chemical Equilibrium Free Response Practice
PSI Chemistry
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1. Consider the following reaction at some temperature:
Some molecules of H2O and CO are placed in a 1.0-L container as shown below.
When equilibrium is reached, how many molecules of H2O, CO, H2, and CO2 are present? Do
this problem by trial and error— that is, if two molecules of CO react, is this equilibrium; if three
molecules of CO react, is this equilibrium; and so on.
The Equilibrium Constant
2. Write the equilibrium expression (K) for each of the following gas-phase reactions
3.
At a given temperature, K =1.3 x 10-2 for the reaction
Calculate values of K for the following reactions at this temperature
a.
b.
c.
d.
½ N2(g) + 3/2H2(g) <=. NH3(g)
2 NH3(g)  N2(g) + 3H2(g)
NH3(g)  1/2N2(g) + 3/2H2(g)
2N2(g) + 6H2(g)  4NH3(g)
4. For the reaction
2NO(g) + 2H2(g)  N2(g) + 2H2O (g)
It is determined that, at equilibrium at a particular temperature, the concentrations are as
follows: [NO (g)] = 8.1 x 10-3M, [H2(g)] =4.1 x 10-5 m, [N2(g)] = 8.1 x 10-3 M, and [H2O (g)] = 2.9 X 103
m. calculate the value of K for the reaction at this temperature.
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At a particular temperature, a 3.0L flask contains 2.4 mol Cl2, 1mol NOCl, and 4.5 x 10-3
mol NO. Calculate k at this temperature for the following reaction:
2NOCl(g)  2NO(g) + Cl2(g)
6. The following equilibrium pressures at a certain temperature were observed for the
reaction
5.
Calculate the value for the equilibrium constant Kp at this temperature.
7. If PN2 = 0.525 atm, PNH3 = 0.0167 atm, and PH2 = 0.00761 atm, does this represent a
system at equilibrium?
8. At 327OC, the equilibrium concentrations are [CH3OH] = 0.15M, [CO]=0.24M, and [H2] =
1.1 M for the reaction
CH3OH(g)  CO(g) + 2H2(g)
Calculate Kp at this temperature.
9. Write expressions for K and Kp for the following reactions.
a. 2NH3(g) + CO2(g)  N2CH4O(s) + H2O(g)
b. 2NBr3(s)  N2(g) +3Br2(g)
c. 2KClO3(s)  2KCl(s) + 3O2(g)
d. CuO(s) + H2(g)  Cu(l) + H2O(g)
10. For which reactions in question (9) is kp equal to K?
11. Consider the following reaction at a certain temperature:
An equilibrium mixture contains 1mol Fe, 1.0 x 10-3 mol O2 and 2.0 mol Fe2O3 all in a 2.0
L container. Calculate the value of K for this reaction.
Equilibrium Calculations
12. The equilibrium constant is 0.0900 at 25OC for the reaction
H2O(g) + Cl2O(g)  2HOCl(g)
For which of the following sets of conditions is the system at equilibrium? For those which
are not at equilibrium, in which direction will the equilibrium shift?
a. A .0 L flask containing 1.0 mol HOCl, 0.10 mol Cl2O, and 0.10 mol H2O.
b. A 2.0 L flask contains 0.084 mol HOCl, 0.080 mol Cl2O, and 0.98 mol H2O.
c. A 3.0L flask contains 0.25 mol HOCl, 0.0010 mol Cl2O, and 0.56 mol H2O.
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13. For the reaction
2H2O(g)  2H2(g) + O2(g)
K = 2.4 x 10-3 at a given temperature. At equilibrium it is found that [H2O(g)] = 1.1 x 10-1 M
and [H2(g)] = 1.9 x 10-2 M. What is the concentration of O2(g) under these conditions?
14. A 1.00-L flask was filled with 2.00 mol gaseous SO2 and 2.00 mol gaseous NO2 and
heated. After equilibrium was reached, it was found that 1.30 mol gaseous NO was
present. Assume that
the reaction occurs under these conditions. Calculate the value of the equilibrium
constant, K, for this reaction.
15. At a particular temperature, 12.0 mol SO3 is placed into a 3.0-L rigid container, and the
SO3 dissociates by the reaction At equilibrium, 3.0 mol SO2 is present. Calculate K for
this reaction.
16. An initial mixture of nitrogen gas and hydrogen gas is reacted in a rigid container at a
certain temperature by the reaction
At equilibrium, the concentrations are [H2] = 5.0 M, [N2] = 8.0 M, and [NH3] = 4.0 M.
What were the concentrations of nitrogen gas and hydrogen gas that were reacted
initially?
17. At a particular temperature, K = 3.75 for the reaction
If all four gases had initial concentrations of 0.800 M, calculate the equilibrium
concentrations of the gases.
18. At 2200OC, Kp = 0.050 for the reaction
N2(g) + O2(g)  2NO(g)
What is the partial pressure of NO in equilibrium with N2 and O2 that were placed in a
flask at initial pressures of 0.80 and 0.20 atm, respectively?
19. At 1100 K, Kp = 0.25 for the reaction
2SO2(g) + O2(g)  2SO3(g)
Calculate the equilibrium partial pressure of SO2, O2, and SO3 produced from an initial
mixture in which Pso2 = Po2 =0.5 atm and Pso3 = 0.
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20. At 35OC, k = 1.6 x 10-5 for the reaction
2NOCl(g)  2NO(g) + Cl2(g)
Calculate the concentration of all the species at equilibrium for each of the following
original mixtures.
a. 2.0 mol pure NOCl in a 2.0 L flask
b. 1.0 mol NOCl and 1.0 mol NO in a 1.0 L flask
c. 2.0 mol NOCl and 1.0 mol Cl2 in a 1.0 L Flask
21. At a particular temperature, k = 2.0 X 10-6 for the reaction
2CO2(g)  2CO(g) + O2(g)
If 2.0 mol CO2 is initially placed into a 2.0 L vessel, calculate the equilibrium
concentrations of all species.
22. At 25OC, Kp = 2.9 x 10-3 for the reaction
NH4OCONH2(s)  2NH3(g) + CO2(g)
In an experiment carried out at 25OC, a certain amount of NH4OCONH2 is placed in an
evacuated rigid container and allowed to come to equilibrium. Calculate total pressure in
the container at equilibrium.
Le Châtelier’s Principle
23. Suppose the reaction system
UO2(s) + 4HF(g)  UF4(g) + 2H2O(g)
has already reached equilibrium. Predict the effect that each of the following changes
will have on the equilibrium position. Tell whether the equilibrium will shift to the right, will
shift to the left, or will not be affected.
a. Additional UO2(s) is added to the system.
b. The reaction is performed in a glass reaction vessel;
c. HF(g) attacks and reacts with glass.
d. Water vapor is removed.
24. An important reaction in the commercial production of hydrogen is
How will this system at equilibrium shift in each of the five following cases?
a. Gaseous carbon dioxide is removed.
b. Water vapor is added.
c. In a rigid reaction container, the pressure is increased by adding helium gas.
d. The temperature is increased (the reaction is exothermic).
e. The pressure is increased by decreasing the volume of the reaction container.
25. In which direction will the position of the equilibrium be shifted for each of the following
changes?
a. H2(g) is added.
b. I2(g) is removed.
c. HI(g) is removed.
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d. In a rigid reaction container, some Ar(g) is added.
e. The volume of the container is doubled.
f. The temperature is decreased (the reaction is exothermic).
26. Old-fashioned “smelling salts” consist of ammonium carbonate, (NH4)2CO3. The reaction
for the decomposition of ammonium carbonate
is endothermic. Would the smell of ammonia increase or decrease as the temperature is
increased?
27. Calculate a value for the equilibrium constant for the reaction
28. Chromium (VI) forms two different oxyanions, the orange dichromate ion, Cr2O7 2-, and
the yellow chromate ion, CrO4 2-. (See the following photos.) The equilibrium reaction
between the two ions is Explain why orange dichromate solutions turn yellow when
sodium hydroxide is added.
29. Suppose K =4.5 x 10 -3 at a certain temperature for the reaction
If it is found that the concentration of PCl5 is twice the concentration of PCl3, what must
be the concentration of Cl2 under these conditions?
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Answers:
1. 4 molecules H2O, 2 molecules CO, 4 molecules H2, and 4 molecules CO2 are present at
2.
equilibrium.
3. a. 0.11; b. 77; c. 8.8; d. 1.7 x 10-4
4. 4.0 x 106
5. 1.7 x 10-5
6. 6.3 x 10-13
8. 4.6 x 103
9.
10. only reaction d
11. 8.0 x 109
12. a. not at equilibrium; Q > K, shift left; b. at equilibrium; c. not at equilibrium; Q > K, shift left
13. 8.0 x 10-2 M
14. 3.4
15. 0.056
16. [N2]0 =10.0 M, [H2]0 =11.0 M
17. [SO3] = [NO] =1.06 M; [SO2] = [NO] = 0.54M
18. 7.8 x 10-2 atm
19.. Po2 = 0.44 atm; Pso2 =0.38 atm; Pso3 = 0.12atm
20. a. [NO] = 0.032 M, [Cl2] = 0.016 M, [NOCl] =1.0 M;
b. [NO] = [NOCl] = 1.0 M, [Cl2] = 1.6 x 10-5 M;
c. [NO] = 8.0 x 10-3 M, [Cl2] =1.0 M, [NOCl] =2.0 M
21. [CO2] =0.39 M, [CO]= 8.6 x 10-3 M, [O2] =4.3 x10-3 M
22. 0.27 atm
23. a. no effect; b. shifts left; c. shifts right
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24. a. right; b. right; c. no effect; d. left; e. no effect
25. a. left; b. right; c. left; d. no effect; e. no effect; f. right
26. increase
27. 2.6 x 1081
28. Added OH- reacts with H+ to produce H2O. As H+ is removed, the reaction shifts right to
produce more H+ and CrO4 2-. Because more CrO4 2- is produced, the solution turns yellow.
29. 9.0 x 10-3 M
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