AP Chemistry
Unit 10 Practice Test
Briefly explain why the answer is correct in the space provided.
Questions 1-10. Consider the equilibrium: 2 A(g)  B(g) + C(g).
1. Which is the correct equilibrium expression?
(A) K = [B][C]/[A]2
(B) K = [A]2/[B][C]
(C) K = [B][C] – [A]
(D) K = [A]2 + [B][C]
2.
Calculate Kc at 25oC given the equilibrium concentrations:
[A] = 0.50 M, [B] = 1.0 M, and [C] = 4.0 M.
(A) 8.0
(B) 0.013 (C) 200
(D) 16
3.
Calculate Kp for the equilibrium at 25oC.
(A) 4.5
(B) 2000
(C) 8.0
(D) 16
4.
Calculate Go for the equilibrium at 25oC when all the
species are at 1 atm partial pressure.
(A) -(0.0821)(298)lnK
(B) -(8.31)(298)lnK
(C) (8.31)(298)lnK
(D) -(8.31)(25)lnK
5.
In another experiment 1.0 mol of B and 1.0 mol of C are
placed in an evacuated 1.0-L vessel at 25oC. What is the
concentration of A after equilibrium is established?
(A) 0.050 M (B) 0.11 M (C) 0.22 M (D) 0.35 M
6.
1.0 M A is placed in a vessel at 100oC. At equilibrium, [C]E
= 0.40 M. What is the equilibrium concentration of A?
(A) 0.20 M (B) 0.55 M (C) 0.40 M (D) 0.90 M
7.
What is Kc at 100oC?
(A) 0.25
(B) 4.0
8.
9.
(C) 16
(D) 2.0
Which of the following is correct concerning this equilibrium?
(A) The forward reaction is exothermic because raising
temperature favors the exothermic reaction.
(B) The forward reaction is endothermic because raising
temperature favors the exothermic reaction.
(C) The forward reaction is exothermic because raising
temperature favors the endothermic reaction.
(D) The forward reaction is endothermic because raising
temperature favors the endothermic reaction.
Which is correct concerning this equilibrium?
(A) Increase [A] shifts the equilibrium to the left.
(B) Increase [B] shifts the equilibrium to the right.
(C) Decrease [C] shifts the equilibrium to the left.
(D) Increasing the volume has no effect on the equilibrium.
14. Cu + 2 Ag+  Cu2+ + 2 Ag
If the equilibrium constant is 3.7 x 1015, which of the
following correctly describes the standard voltage, Eo, and
the standard free energy change, Go, for this reaction?
(A) Eo > 0 and Go < 0
(B) Eo < 0 and Go > 0
(C) Eo > 0 and Go > 0
(D) Eo < 0 and Go < 0
15. For the reaction A(g)  B(g) + C(g), Kp, = 2 x 10-4 at 25oC.
A mixture of the three gases at 25oC is placed in a flask
and the initial pressures are PA = 2 atm, PB = 0.5 atm, and
PC = 1 atm. Which is true at the instant of mixing?
(A) G < 0 (B) G > 0 (C) S = 0 (D) Go = 0
16.
17. 2 SO3(g)  2 SO2(g) + O2(g)
After equilibrium is established, some pure O2(g) is injected
into the reaction vessel at constant temperature. After
equilibrium is reestablished, which of the following has a
lower value compared to its original equilibrium value?
(A) Keq for the reaction
(B) The amount of SO3(g) in the reaction vessel
(C) The amount of O2(g) in the reaction vessel
(D) The amount of SO2(g) in the reaction vessel
18. 2 NO(g) + O2(g)  2 NO2(g)
H < 0
Which of the following changes alone would cause a
decrease in the value of Keq for the reaction above?
(A) Decreasing the temperature
(B) Increasing the temperature
(C) Decreasing the volume of the reaction vessel
(D) Increasing the volume of the reaction vessel
19. 2 SO2(g) + O2(g)  2 SO3(g)
When 0.40 mole of SO2 and 0.60 mole of O2 are placed in
an evacuated 1.00 L flask. After reaching equilibrium the
flask contains 0.30 mole of SO3. Kc, is
(A) (0.30)2/(0.45)(0.10)2 (B) (0.30)2/(0.60)(0.40)2
(C) (0.60)/(0.45)(0.20)
(D) (0.30)/(0.45)(0.10)
Practice Free Response
1.
Given:
2 NO(g) + Br2(g)  2 NOBr(g)
Kc = 2.0
2 NO(g)  N2(g) + O2(g)
Kc = 2.1 x 1030
What is Kc for the following equilibrium?
N2(g) + O2(g) + Br2(g)  2 NOBr(g)
2.
The equilibrium constant for the reaction is Kc = 2.19 x 10-10.
COCl2(g)  CO(g) + Cl2(g)
Is a mixture containing 2.0 x 10-3 M COCl2, 3.3 x 10-6 M CO,
and 6.6 x 10-6 M Cl2 at equilibrium? If not, what direction will
the reaction proceed to achieve equilibrium?
3.
Consider the equilibrium at 25oC: A(g) + B(g)  2 C(g).
a. Write the equilibrium expression for this equilibrium.
10. What is Kc at 25oC for the equilibrium A(g)  ½ B(g) + ½ C(g)?
(A) 4.0
(B) 0.063 (C) 8.0
(D) 260
11. Which must be true for an equilibrium initially at standard
state that proceeds spontaneously in the forward direction?
(A) Go > 0 and Keq > 1 (B) Go > 0 and Keq < 1
(C) Go < 0 and Keq > 1 (D) Go < 0 and Keq < 1
12. The equilibrium constant for a reaction is 200. What is K
for the reverse reaction at the same temperature?
(A) -200
(B) -0.005 (C) 0.002 (D) 0.005
13. 4 HCI(g) + O2(g)  2 Cl2(g) + 2 H2O(g)
Equal moles of HCI and O2 are added to an evacuated
vessel. Which must be true for the system at equilibrium?
(A) [HCI] < [Cl2]
(B) [HCI] > [Cl2]
(C) [HCl] > [O2].
(D) [Cl2] = [H2O]
CuO + H2(g)  Cu + H2O
H = -2.0 kJ
The equilibrium can be shifted to favor the products by
(A) increasing the volume at constant temperature.
(B) increasing the pressure by adding an inert gas.
(C) decreasing the temperature.
(D) releasing H2(g) at constant pressure and temperature.
b.
Calculate Kc at 25oC given [A]E = 0.025 M,
[B]E = 0.100 M, and [C]E = 0.500 M.
c.
Calculate Kp for the equilibrium at 25oC.
d.
State which direction the equilibrium will shift (, )
to relieve the following stresses on the system.
Increase V
Increase P
Increase [A]

4.
5.
e.
What is Kc at 25oC for C(g)  ½ A(g) + ½ B(g)?
f.
Calculate Go for the equilibrium at 25oC when all the
species are at 1 atm partial pressure.
g.
In another experiment 1.00 mol of A and 1.00 mol of
B are placed in an evacuated 1.00-L vessel at 25oC.
What are the concentrations of the three gases after
equilibrium is established?
h.
A mixture of 1.00 M A and 0.50 M B are placed in a
vessel at 100oC. At equilibrium, [C] = 0.90 M. What
are the equilibrium concentrations of A and B?
i.
What is Kc at 100oC?
j.
Is the forward reaction endothermic or exothermic.
Explain your reasoning.
N2(g) + 3 H2(g)  2 NH3(g) Kp = 1.45 x 10-5
What is the partial pressure of NH3 if the partial pressures
of N2 and H2 are 0.432 atm and 0.928 atm respectively?
H2(g) + CO2(g)  H2O(g) + CO(g)
When H2 is mixed with CO2 at 2,000 K, equilibrium is
achieved according to the equation. In one experiment,
the following equilibrium concentrations were measured.
[H2] = 0.20 M, [CO2] = 0.30 M, [H2O] = [CO] = 0.55 M
a. Calculate the value of Kc.
b.
Determine Kp in terms of Kc for this system.
c.
When the system is cooled to a lower temperature,
30.0 percent of the CO is converted back to CO2.
(1) Calculate the change in [CO]
(2) Calculate Kc at this lower temperature.
d.
In a different experiment, 0.500 mole of H2 is mixed with
0.500 mole of CO2 in a 3.00-liter vessel at 2,000 K.
(1) Calculate the initial concentration of H2 and CO2.
(2) Calculate the equilibrium concentrations.
6.
Answer the following questions regarding the
decomposition of arsenic pentafluoride, AsF5(g).
a. A 55.8 g sample of AsF5(g) is introduced into an
evacuated 10.5 L container at 105oC.
(1) What is the initial molar concentration of AsF5(g)?
(2) What is the initial pressure, in atmospheres, of the
AsF5(g) in the container?
At 105oC, AsF5(g) decomposes into AsF3(g) and F2(g)
according to the following chemical equation.
AsF5(g)  AsF3(g) + F2(g)
b.
In terms of molar concentrations, write the equilibriumconstant expression for the decomposition of AsF5(g).
c.
When equilibrium is established, 27.7 percent of the
original number of moles of AsF5(g) has decomposed.
(1) Calculate the molar concentration of AsF5(g) at
equilibrium.
(2)
d.
7.
Using molar concentrations, calculate the value
of the equilibrium constant, Keq, at 105oC.
Calculate the mole fraction of F2(g) in the container at
equilibrium.
Consider bromine chloride. It is formed by the reaction
between red-orange bromine vapor and yellow chlorine
gas; BrCl is itself a gas. The reaction is endothermic.
a. Write the chemical equation for the formation of BrCl,
using simplest whole-number coefficients.
b.
Write the equilibrium expression for Kc.
c.
At 400oC, after the reaction reached equilibrium, the
mixture contained 0.82 M BrCl, 0.20 M Br2 and 0.48 M
Cl2. Calculate Kc for the reaction.
d.
What is Kp at 400oC?
e.
Initially, a 2.00-L flask contains Cl2 with partial
pressure 0.51 atm and Br2 with partial pressure 0.34
atm. After equilibrium is established, the partial
pressure of BrCl is 0.46 atm. Calculate the equilibrium
pressures of Cl2 and Br2.
f.
Initially, a 1.00-L flask contains 0.15 mol of each gas.
What are the equilibrium concentrations of each gas at
400oC?
g. In what direction will the system shift if at equilibrium?
the volume is increased?
helium gas is added?
the temperature is increased?
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