AP Chemistry Unit 7: Equilibrium Practice Exam

Assessment for Unit 7: Equilibrium

Estimated Time: 60 minutes

Exam Details

Questions: 30 total (30 multiple choice)
Time: 60 minutes (recommended)
Topics Covered: Unit 7: Equilibrium
Format: Multiple Choice questions matching real exam difficulty
Scoring: Instant results with detailed explanations

A. Multiple-Choice Questions (30 Questions)

Select the one best answer for each question.

Refer to the figure below.

1. [Skill: 4.A | Topic: 7.1] A student studies the reaction $N_2O_4(g) \rightleftharpoons 2NO_2(g)$ in a sealed container at constant temperature. A graph of concentration versus time is shown. Which statement best describes what is happening at time $t_2$? [Image Cue]: Line graph, "Concentration vs. Time for $N_2O_4(g) \rightleftharpoons 2NO_2(g)$". x-axis: Time (labeled $t_0$, $t_1$, $t_2$, $t_3$). y-axis: Concentration (M). Two curves: $[N_2O_4]$ starts high and decreases, leveling off by about $t_2$; $[NO_2]$ starts near 0 and increases, leveling off by about $t_2$. After $t_2$ both curves are horizontal (constant but not equal).

(A)The reaction has stopped because the concentrations are constant.(B)The system is at equilibrium because the forward and reverse reaction rates are equal, resulting in constant concentrations.(C)The system is at equilibrium only if $[N_2O_4]=[NO_2]$ at $t_2$.(D)The reaction is at equilibrium because all $N_2O_4$ has been converted to $NO_2$ by $t_2$.

2. [Skill: 4.A | Topic: 7.1] A student monitors the reaction $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$ in a sealed flask at constant temperature and records the partial pressures over time. | Time (min) | $P_{H_2}$ (atm) | $P_{I_2}$ (atm) | $P_{HI}$ (atm) | |---|---:|---:|---:| | 0 | 0.80 | 0.80 | 0.00 | | 2 | 0.60 | 0.60 | 0.40 | | 4 | 0.52 | 0.52 | 0.56 | | 6 | 0.52 | 0.52 | 0.56 | | 8 | 0.52 | 0.52 | 0.56 | Which conclusion is best supported by the data?

(A)Equilibrium is reached at 6 min because the partial pressures stop changing after 6 min.(B)Equilibrium is reached at 4 min because the partial pressures are constant from 4 min onward.(C)Equilibrium is reached at 2 min because the partial pressures of $H_2$ and $I_2$ are equal at that time.(D)Equilibrium cannot be reached in a sealed flask because gases continue reacting until one reactant is used up.

3. [Skill: 6.A | Topic: 7.1] A student prepares a beaker of water and adds solid $NaCl(s)$ while stirring until some undissolved solid remains at the bottom. After the beaker sits undisturbed for several minutes at constant temperature, the student observes that the amount of undissolved solid and the appearance of the solution remain unchanged. Which statement best explains these observations?

(A)The system is at equilibrium because the rate of dissolution equals the rate of crystallization, so the concentration of dissolved ions remains constant.(B)The system is not at equilibrium because the undissolved solid indicates the reaction has not started.(C)The system is at equilibrium because dissolution stops once the solution becomes saturated, so ions no longer move between solid and solution.(D)The system is at equilibrium only if all of the $NaCl(s)$ dissolves and the beaker becomes clear.

4. **1. [Skill: 2.A | Topic: 7.10]** A closed container at a constant temperature contains the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ At a particular moment, the concentrations are measured to be: - $[N_2] = 0.20\ \text{M}$ - $[H_2] = 0.30\ \text{M}$ - $[NH_3] = 0.40\ \text{M}$ At this temperature, $K_c = 6.0$. Which of the following best predicts how the system will proceed to reach equilibrium?

(A)The system is at equilibrium because Q = K.(B)The reaction will proceed in the forward direction (toward products) because Q < K.(C)The reaction will proceed in the reverse direction (toward reactants) because Q > K.(D)The reaction will proceed in the reverse direction (toward reactants) because Q < K.

5. **2. [Skill: 4.A | Topic: 7.10]** A sealed rigid flask at constant temperature contains the equilibrium system $CO(g) + Cl_2(g) \rightleftharpoons COCl_2(g)$ At equilibrium, a student injects additional $COCl_2(g)$ into the flask (the volume and temperature remain constant). Which of the following statements is correct about the reaction quotient immediately after the addition and the subsequent shift as equilibrium is re-established?

(A)Q decreases immediately, so the system shifts right to form more COCl2.(B)Q increases immediately, so the system shifts left to form more CO and Cl2.(C)K increases immediately, so the system shifts right to form more COCl2.(D)K decreases immediately, so the system shifts left to form more CO and Cl2.

6. **3. [Skill: 6.A | Topic: 7.10]** Consider the equilibrium system in a closed container: $N_2O_4(g) \rightleftharpoons 2NO_2(g) \qquad \Delta H > 0$ The system is at equilibrium at temperature $T_1$. The temperature is then increased to $T_2$ while the volume remains constant. Which of the following best describes what happens immediately after the temperature increase and what shift occurs as the system reaches the new equilibrium?

(A)Immediately after the temperature increase, Q increases; then the system shifts left because K decreases.(B)Immediately after the temperature increase, Q is unchanged; then the system shifts right because K increases.(C)Immediately after the temperature increase, K is unchanged; then the system shifts right because Q decreases.(D)Immediately after the temperature increase, Q is unchanged; then the system shifts left because K decreases.

7. [Skill: 2A | Topic: 7.11] A student investigates the solubility of silver chromate, $Ag_2CrO_4(s)$, in pure water at 25°C. The dissolution equilibrium is shown. $Ag_2CrO_4(s) \rightleftharpoons 2Ag^+(aq) + CrO_4^{2-}(aq)$ The solubility-product constant is $K_{sp}=1.1\times 10^{-12}$. Which of the following is the best estimate of the molar solubility of $Ag_2CrO_4$ in pure water at 25°C?

(A)6.5 × 10^-5 M(B)1.1 × 10^-4 M(C)5.2 × 10^-7 M(D)1.4 × 10^-6 M

8. [Skill: 2A | Topic: 7.11] In an experiment at 25°C, a student determines that the molar solubility of calcium fluoride is $2.3\times 10^{-4}$ M in pure water. $CaF_2(s) \rightleftharpoons Ca^{2+}(aq) + 2F^-(aq)$ Based on the measured molar solubility, which of the following is the best estimate of $K_{sp}$ for $CaF_2$ at 25°C?

(A)4.9 × 10^-11(B)1.2 × 10^-11(C)2.3 × 10^-4(D)5.3 × 10^-8

9. [Skill: 4A | Topic: 7.11] Three salts are placed separately into pure water at 25°C until each solution is saturated. Their dissolution equilibria and $K_{sp}$ values are shown. Salt 1: $CaCO_3(s) \rightleftharpoons Ca^{2+}(aq) + CO_3^{2-}(aq)$, $K_{sp}=3.3\times10^{-9}$ Salt 2: $Mg(OH)_2(s) \rightleftharpoons Mg^{2+}(aq) + 2OH^-(aq)$, $K_{sp}=5.6\times10^{-12}$ Salt 3: $MZ(s) \rightleftharpoons M^+(aq) + Z^-(aq)$, $K_{sp}=2.5$ Which salt has the greatest molar solubility in pure water at 25°C?

(A)Salt 1 ($CaCO_3$)(B)Salt 2 ($Mg(OH)_2$)(C)Salt 3 ($MZ$)(D)Salt 1 and Salt 2 have equal molar solubility because each produces 1 cation

10. [Skill: 4.A | Topic: 7.12] A student investigates the dissolution of $AgCl(s)$ in a solution that already contains chloride ions. At 25°C, excess $AgCl(s)$ is added to 0.100 M $NaCl(aq)$ and stirred until equilibrium is reached. The student measures the equilibrium concentration of silver ions and finds $[Ag^+] = 1.8\times10^{-9}\,\text{M}$. Assuming the chloride concentration from $AgCl$ dissolution is negligible compared to 0.100 M, what is the best estimate of $K_{sp}$ for $AgCl(s)$ at 25°C?

(A)1.8×10^-8(B)1.8×10^-10(C)3.6×10^-18(D)5.6×10^-19

11. [Skill: 5.A | Topic: 7.12] A solution is prepared by dissolving $NaF(s)$ in water to make 0.200 M $F^-(aq)$. Solid $CaF_2$ is then added until the solution becomes saturated. At 25°C, $K_{sp}$ for $CaF_2(s)$ is $3.9\times10^{-11}$. What is the molar solubility of $CaF_2$ in the 0.200 M $F^-$ solution? Assume the change in $[F^-]$ due to dissolution of $CaF_2$ is negligible.

(A)9.8×10^-10 M(B)2.0×10^-6 M(C)1.0×10^-5 M(D)1.6×10^-4 M

12. [Skill: 6.A | Topic: 7.12] A student compares the solubility of $PbI_2(s)$ in pure water and in a solution containing a common ion. At 25°C, $K_{sp}$ for $PbI_2(s)$ is $7.9\times10^{-9}$. Which statement best compares the molar solubility of $PbI_2$ in pure water to its molar solubility in 0.050 M $KI(aq)$? Assume activity effects are negligible.

(A)The molar solubility is the same in both solutions because $K_{sp}$ depends only on temperature.(B)The molar solubility is greater in 0.050 M $KI$ because added $I^-$ increases the rate of dissolution.(C)The molar solubility is lower in 0.050 M $KI$ by about a factor of 400 due to the common-ion effect.(D)The molar solubility is lower in 0.050 M $KI$ by about a factor of 2 because $PbI_2$ produces 2 iodide ions.

Refer to the figure below.

13. **1.** [Skill: 2.A | Topic: 7.2] A student studies the reversible reaction $A \rightleftharpoons B$ at constant temperature in a closed container. Immediately after $A$ is introduced, the student measures the forward and reverse reaction rates over time. > [Image Cue]: Line graph, "Forward and Reverse Reaction Rates vs. Time", x-axis: Time, y-axis: Rate. Forward rate starts high and decreases over time; reverse rate starts near zero and increases over time; the two curves intersect at time $t = t^*$ and then remain equal and constant. Which statement best describes the system at times before and after $t^*$ ?

(A)Before $t^*$, there is a net conversion of $A$ to $B$; at and after $t^*$, the system is at equilibrium because the forward and reverse rates are equal.(B)Before $t^*$, the system is at equilibrium because both $A$ and $B$ are present; after $t^*$, there is a net conversion of $B$ to $A$ because the reverse rate is increasing.(C)At $t^*$, the concentrations of $A$ and $B$ must be equal because the forward and reverse rates are equal.(D)After $t^*$, the reaction stops because the forward and reverse rates have become equal.

14. **2.** [Skill: 4.A | Topic: 7.2] For the reaction $2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$ at a particular temperature, a student determines that at a certain moment the forward reaction rate is greater than the reverse reaction rate. Which change is expected to occur in the system during the next short interval of time (before a new equilibrium is reached)?

(A)The amount of $SO_3(g)$ increases, while the amounts of $SO_2(g)$ and $O_2(g)$ decrease.(B)The amount of $SO_3(g)$ decreases, while the amounts of $SO_2(g)$ and $O_2(g)$ increase.(C)The amounts of all three gases remain constant because both the forward and reverse reactions are occurring.(D)The amounts of all three gases increase because the forward and reverse reactions occur simultaneously.

15. **3.** [Skill: 2.A | Topic: 7.2] A reversible reaction $X(g) \rightleftharpoons Y(g)$ is studied at constant temperature. For three different trials, a student prepares different initial mixtures of $X$ and $Y$ in identical containers and immediately measures the initial forward and reverse rates. The results are shown below. | Trial | Initial forward rate (M/s) | Initial reverse rate (M/s) | |------:|----------------------------:|----------------------------:| | 1 | $8.0 \times 10^{-3}$ | $2.0 \times 10^{-3}$ | | 2 | $4.5 \times 10^{-3}$ | $4.5 \times 10^{-3}$ | | 3 | $1.0 \times 10^{-3}$ | $6.0 \times 10^{-3}$ | In which trial is the system at equilibrium at the moment the rates are measured, and what is the net direction of change (if any) for the other trials immediately after the measurement?

(A)Trial 2 is at equilibrium; Trial 1 has net $X \to Y$; Trial 3 has net $Y \to X$.(B)Trial 2 is at equilibrium; Trial 1 has net $Y \to X$; Trial 3 has net $X \to Y$.(C)Trial 1 is at equilibrium because its forward rate is the largest; Trial 2 has net $X \to Y$; Trial 3 has net $Y \to X$.(D)Trial 3 is at equilibrium because it has more reverse reaction; Trial 1 has net $Y \to X$; Trial 2 has net $X \to Y$.

16. [Skill: 3.A | Topic: 7.3] A student investigates the heterogeneous equilibrium reaction $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$ Which of the following correctly represents the reaction quotient, $Q_p$, for the system at a given moment?

(A)$Q_p = P_{CO_2}$(B)$Q_p = \dfrac{P_{CO_2}}{[CaCO_3][CaO]}$(C)$Q_p = \dfrac{[CaO][CO_2]}{[CaCO_3]}$(D)$Q_p = \dfrac{1}{P_{CO_2}}$

17. [Skill: 2.A | Topic: 7.3] For the reaction $Fe^{3+}(aq) + SCN^-(aq) \rightleftharpoons FeSCN^{2+}(aq)$ At a particular time, a student measures the concentrations shown. $[Fe^{3+}] = 0.10\ \text{M}$ $[SCN^-] = 0.0020\ \text{M}$ $[FeSCN^{2+}] = 0.020\ \text{M}$ At the same temperature, $K_c = 890$. Which of the following statements is correct?

(A)$Q_c = 100$, so the system is at equilibrium.(B)$Q_c = 100$, so the system is not at equilibrium because $Q_c \ne K_c$.(C)$Q_c = 0.0010$, so the system is not at equilibrium because $Q_c \ne K_c$.(D)$Q_c = 1000$, so the system is not at equilibrium because $Q_c \ne K_c$.

18. [Skill: 2.A | Topic: 7.3] A closed container at constant temperature contains a mixture of gases involved in the equilibrium $2NO_2(g) \rightleftharpoons N_2O_4(g)$ At a particular moment, the measured partial pressures are $P_{NO_2} = 0.50\ \text{atm}$ $P_{N_2O_4} = 0.20\ \text{atm}$ At this temperature, $K_p = 6.9$. Which of the following correctly gives $Q_p$ and a correct conclusion about whether the system is at equilibrium at that moment?

(A)$Q_p = \dfrac{(0.50)^2}{0.20} = 1.25$, so the system is at equilibrium because $Q_p < K_p$.(B)$Q_p = \dfrac{0.20}{(0.50)^2} = 0.80$, so the system is not at equilibrium because $Q_p \ne K_p$.(C)$Q_p = \dfrac{0.20}{0.50} = 0.40$, so the system is not at equilibrium because $Q_p \ne K_p$.(D)$Q_p = \dfrac{0.20}{(0.50)^2} = 0.80$, so the system is at equilibrium because $Q_p < K_p$.

19. [Skill: 5.B | Topic: 7.4] A student studies the equilibrium system shown below in a rigid 1.00 L container at constant temperature. $N_2O_4(g) \rightleftharpoons 2NO_2(g)$ After the system reaches equilibrium, the student determines the equilibrium concentrations to be: - $[NO_2]_{eq} = 0.120\,\text{M}$ - $[N_2O_4]_{eq} = 0.0600\,\text{M}$ Which of the following is the correct value of $K_c$ for the reaction at this temperature?

(A)0.240(B)0.500(C)4.17(D)0.120

20. [Skill: 5.B | Topic: 7.4] A chemistry class investigates the equilibrium reaction at a constant temperature in a sealed container. $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ At equilibrium, the partial pressures are measured and recorded in the table below. - $P_{NH_3} = 1.20\,\text{atm}$ - $P_{N_2} = 0.300\,\text{atm}$ - $P_{H_2} = 0.900\,\text{atm}$ Which of the following is the correct value of $K_p$ for the reaction at this temperature?

(A)0.152(B)2.19(C)6.59(D)19.8

21. **1. [Skill: 4.A | Topic: 7.5]** A researcher studies three different reversible reactions at the same temperature. | Reaction | Equilibrium Constant, K | |---|---| | I | $3.2\times10^{-7}$ | | II | $0.85$ | | III | $4.5\times10^{6}$ | Which reaction is expected to have the most comparable (most similar) equilibrium amounts of reactants and products?

(A)Reaction I(B)Reaction II(C)Reaction III(D)All three reactions will have comparable amounts because equilibrium means equal amounts of reactants and products

22. **2. [Skill: 6.A | Topic: 7.5]** A student investigates the equilibrium system $N_2O_4(g) \rightleftharpoons 2NO_2(g)$ in a 1.0 L sealed container at constant temperature. The student starts with 1.0 mol of $N_2O_4(g)$ and no $NO_2(g)$. After equilibrium is reached, analysis shows that 0.20 mol of $N_2O_4(g)$ remains. Which statement best describes the magnitude of K for this reaction at this temperature and what it implies about the equilibrium mixture?

(A)K is much less than 1; reactants are strongly favored, so very little $NO_2(g)$ forms at equilibrium(B)K is approximately 1; reactants and products are present in roughly equal amounts at equilibrium(C)K is greater than 1; products are favored, but a significant amount of $N_2O_4(g)$ remains at equilibrium(D)K is extremely large; the reaction proceeds essentially to completion so virtually no $N_2O_4(g)$ remains at equilibrium

23. [Skill: 2.A | Topic: 7.6] A student is given the following set of equilibrium reactions and their equilibrium constants at the same temperature. Reaction 1: $N_2(g)+O_2(g)\;\rightleftharpoons\;2NO(g)$ $K_1=4.0\times10^{-31}$ Reaction 2: $2NO(g)+O_2(g)\;\rightleftharpoons\;2NO_2(g)$ $K_2=1.2\times10^{13}$ Reaction 3: $2NO_2(g)\;\rightleftharpoons\;N_2O_4(g)$ $K_3=6.9\times10^{2}$ Which of the following is the value of $K$ for the overall reaction $N_2(g)+2O_2(g)\;\rightleftharpoons\;N_2O_4(g)$?

(A)$3.3\times10^{-15}$(B)$2.1\times10^{-20}$(C)$3.3\times10^{-46}$(D)$6.9\times10^{-2}$

24. [Skill: 2.A | Topic: 7.6] For the reaction $H_2(g)+I_2(g)\;\rightleftharpoons\;2HI(g)$, a student measures the following partial pressures in a reaction vessel: $P_{H_2}=0.20\ \text{atm}$ $P_{I_2}=0.10\ \text{atm}$ $P_{HI}=0.40\ \text{atm}$ Based on these measurements, what is the value of the reaction quotient $Q$ for the reaction $4HI(g)\;\rightleftharpoons\;2H_2(g)+2I_2(g)$?

(A)64(B)8.0(C)0.125(D)0.016

25. [Skill: 2.B | Topic: 7.7] A sealed 1.0 L container at 700 K contains the following concentrations: $H_2(g) + I_2(g) \rightleftharpoons 2HI(g)$, with $K_c = 50$ Initial concentrations: - $[H_2]_0 = 0.20\,\text{M}$ - $[I_2]_0 = 0.20\,\text{M}$ - $[HI]_0 = 0.10\,\text{M}$ Which of the following sets of concentrations is closest to the equilibrium concentrations?

(A)$[H_2] = 0.055\,\text{M}$, $[I_2] = 0.055\,\text{M}$, $[HI] = 0.390\,\text{M}$(B)$[H_2] = 0.145\,\text{M}$, $[I_2] = 0.145\,\text{M}$, $[HI] = 0.290\,\text{M}$(C)$[H_2] = 0.345\,\text{M}$, $[I_2] = 0.345\,\text{M}$, $[HI] = 0.010\,\text{M}$(D)$[H_2] = 0.200\,\text{M}$, $[I_2] = 0.200\,\text{M}$, $[HI] = 0.100\,\text{M}$

26. [Skill: 2.B | Topic: 7.7] A student studies the equilibrium system in a rigid container at constant temperature: $SO_2Cl_2(g) \rightleftharpoons SO_2(g) + Cl_2(g)$, with $K_p = 0.040$ Initially, only $SO_2Cl_2(g)$ is present at a partial pressure of 1.00 atm. Which of the following is closest to the equilibrium partial pressures?

(A)$P_{SO_2Cl_2} = 0.819\,\text{atm}$, $P_{SO_2} = 0.181\,\text{atm}$, $P_{Cl_2} = 0.181\,\text{atm}$(B)$P_{SO_2Cl_2} = 0.961\,\text{atm}$, $P_{SO_2} = 0.039\,\text{atm}$, $P_{Cl_2} = 0.039\,\text{atm}$(C)$P_{SO_2Cl_2} = 0.667\,\text{atm}$, $P_{SO_2} = 0.333\,\text{atm}$, $P_{Cl_2} = 0.333\,\text{atm}$(D)$P_{SO_2Cl_2} = 0.710\,\text{atm}$, $P_{SO_2} = 0.290\,\text{atm}$, $P_{Cl_2} = 0.290\,\text{atm}$

Refer to the figure below.

27. [Skill: 1A | Topic: 7.8] A student studies the reversible reaction in a sealed container: $A_2(g) + B_2(g) \rightleftharpoons 2AB(g)$ The student draws particulate diagrams of the container contents at four different times after the reaction begins (all diagrams represent the same container volume). Based on the particulate diagrams, at which time has the system FIRST reached equilibrium?

(A)Time 1(B)Time 2(C)Time 3(D)Time 0

Refer to the figure below.

28. [Skill: 5A | Topic: 7.8] A sealed container at a constant temperature contains a mixture of $N_2O_4(g)$ and $NO_2(g)$ at equilibrium for the reaction $N_2O_4(g) \rightleftharpoons 2NO_2(g)$ A particulate diagram of the equilibrium mixture shows 3 particles of $N_2O_4$ and 8 particles of $NO_2$ in the container (all particles are in the gas phase and the diagram represents a single fixed volume). Which of the following is the best estimate of the equilibrium constant $K_c$ for the reaction?

(A)0.047(B)2.7(C)21(D)64

29. [Skill: 3.A | Topic: 7.9] A sealed, rigid flask contains the equilibrium system $2NO_2(g) \rightleftharpoons N_2O_4(g)$ The forward reaction is exothermic. $NO_2(g)$ is brown, and $N_2O_4(g)$ is colorless. At 25°C the gas mixture is light brown. The flask is then placed in a hot-water bath until it reaches 60°C, and the gas mixture becomes darker brown. Which of the following best describes the response of the system after the temperature increase?

(A)The equilibrium shifts left, producing more $NO_2(g)$, and the equilibrium constant $K$ decreases.(B)The equilibrium shifts right, producing more $N_2O_4(g)$, and the equilibrium constant $K$ increases.(C)The equilibrium shifts left, producing more $NO_2(g)$, and the equilibrium constant $K$ increases.(D)The equilibrium shifts right, producing more $N_2O_4(g)$, and the equilibrium constant $K$ decreases.

30. [Skill: 4.A | Topic: 7.9] A student investigates the equilibrium below in a test tube at constant temperature: $Fe^{3+}(aq) + SCN^-(aq) \rightleftharpoons FeSCN^{2+}(aq)$ $FeSCN^{2+}(aq)$ is red, while $Fe^{3+}(aq)$ and $SCN^-(aq)$ solutions are much less intensely colored. The student allows the mixture to reach equilibrium and observes a deep red solution. The student then adds a large amount of distilled water to the test tube, and the solution becomes noticeably less red. Which of the following best explains the observed color change?

(A)Dilution shifts the equilibrium left because the left side has more dissolved particles; the concentration of $FeSCN^{2+}$ decreases, so the red color fades.(B)Dilution shifts the equilibrium right because the right side has fewer dissolved particles; the concentration of $FeSCN^{2+}$ increases, so the red color fades.(C)Dilution does not change the equilibrium position because $K$ is constant at a given temperature; the color fades only because the total volume increases.(D)Dilution increases the value of $K$, causing the equilibrium to shift left and the red color to fade.

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