chemical equilibrium packet

AP Chemistry Chemical Equilibrium Packet

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USE THIS SPACE FOR SCRATCHWORK. Section I: Multiple Choice A calculator may NOT be used for Questions 1−29. Questions 1−4

(A) Kp

(B) Ka

(C) Kw

(D) Ksp

1. This equilibrium constant uses partial pressures of gases.

2. This equilibrium constant always has a value of 1 × 10−14 at 25°C.

3. This equilibrium constant is used for the dissociation of an acid.

4. The expression for this equilibrium constant does not contain a denominator.

Questions 5−8 refer to the following equilibrium: 2 SO3(g) ⇌ 2 SO2(g) + O2(g) ΔH° = 197 kJ

[SO3] [SO2] [O2] Keq

(A) increase decrease decrease −

(B) decrease increase increase increase

(C) increase decrease increase −

(D) increase increase increase −

Assume that the above reaction is initially at equilibrium. Determine how the following changes will affect the above quantities once a new equilibrium is established.

5. Pure O2 is added to the reaction vessel.

6. The temperature is increased from 25°C to 75°C.

7. The pressure of the reaction vessel is increased by compression to a smaller volume at constant temperature.

8. Pure SO3 is added to the reaction vessel.


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USE THIS SPACE FOR SCRATCHWORK. 9. For a particular salt, the solution process is endothermic. As the

temperature at which the salt is dissolved increases, which of the following will occur?

(A) Ksp will increase, and the salt will become more soluble.

(B) Ksp will decrease, and the salt will become more soluble.

(C) Ksp will increase, and the salt will become less soluble.

(D) Ksp will decrease, and the salt will become less soluble.

10. 2 HI(g) + Cl2(g) ⇌ 2 HCl(g) + I2(g) + energy A gaseous reaction occurs and comes to equilibrium, as shown above. Which of the following changes to the system will serve to increase the number of moles of I2 present at equilibrium?

(A) Decreasing the volume at constant temperature

(B) Adding a mole of inert gas at constant volume

(C) Increasing the temperature at constant volume

(D) Decreasing the temperature at constant volume

11. A sealed isothermal container initially contained 2 moles of CO gas and 3 moles of H2 gas. The following reversible reaction occurred:

CO(g) + 2 H2(g) ⇌ CH3OH(g)

At equilibrium, there is 1 mol of CH3OH in the container. What is the total number of moles of gas present in the container at equilibrium?

(A) 2

(B) 3

(C) 4

(D) 5


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USE THIS SPACE FOR SCRATCHWORK. 12. 4 NH3(g) + 3 O2(g) ⇌ 2 N2(g) + 6 H2O(g) + energy

Which of the following changes to the system shown above at equilibrium would cause the concentration of H2O to increase?

(A) The volume of the system is decreased at constant temperature.

(B) The temperature of the system is increased at constant volume.

(C) N2 is removed from the system.

(D) O2 is removed from the system.

13. A sample of solid potassium nitrate is placed in water. The solid

potassium nitrate comes to equilibrium with its dissolved ions by the endothermic process shown below.

KNO3(s) + energy ⇌ K+(aq) + NO3−(aq)

Which of the following changes to the system would increase the concentration of K+ ions at equilibrium?

(A) The volume of the solution is increased.

(B) The volume of the solution is decreased.

(C) Additional solid KNO3 is added to the solution.

(D) The temperature of the solution is increased.

14. Citric acid, H3C6H5O7, can give up 3 hydrogen ions in solution. The 3 dissociation reactions are as follows: H3C6H5O7 ⇌ H+ + H2C6H5O7

− K1 = x

H2C6H5O7− ⇌ H+ + HC6H5O7

2− K2 = y

HC6H5O72− ⇌ H+ + C6H5O7

3− K3 = z

Which of the following expressions gives the equilibrium constant for the reaction shown below?

H3C6H5O7 ⇌ 3 H+ + C6H5O7

3−

(A)

€

xyz

(B)

€

xyz

(C)

€

xyz

(D)

€

zxy


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USE THIS SPACE FOR SCRATCHWORK. 15. H2(g) + I2(g) ⇌ 2 HI(g)

At 450°C, the equilibrium constant Kc for the reaction shown above has a value of 50. Which of the following is true of the reaction at equilibrium?

(A) The rate of the forward reaction is greater than the rate of

the reverse reaction.

(B) The rate of the forward reaction is less than the rate of the reverse reaction.

(C) The rate of the forward reaction is equal to the rate of the reverse reaction.

(D) An increase in the volume of the system will cause an increase in the value of Kc.

16. 2 NOBr(g) ⇌ 2 NO(g) + Br2(g) The reaction above came to equilibrium at a temperature of 100°C. At equilibrium, the partial pressure due to NOBr is 4 atm, the partial pressure due to NO is 4 atm, and the partial pressure due to Br2 is 2 atm. What is the equilibrium constant Kp for this reaction at 100°C?

(A)

€

14

(B)

€

12

(C) 2

(D) 4

17. HCrO4

− (aq) + Ca2+(aq) ⇌ H+(aq) + CaCrO4(s) If the acid dissociation constant for HCrO4

− is Ka and the solubility product constant for CaCrO4 is Ksp, which of the following gives the equilibrium expression for the reaction above?

(A)

€

Ka Ksp

(B)

€

Ka

Ksp

(C)

€

Ksp

Ka

(D)

€

Ka Ksp

2


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USE THIS SPACE FOR SCRATCHWORK. 18. Br2(g) + I2(g) ⇌ 2 IBr(g)

At 150°C, the equilibrium constant Kc for the reaction shown above has a value of 300. This reaction was allowed to reach equilibrium in a sealed container and the partial pressure due to IBr(g) was found to be 3 atm. Which of the following could be the partial pressures due to Br2(g) and I2(g) in the container?

Br2(g) I2(g)

(A) 0.1 atm 0.3 atm

(B) 0.3 atm 1 atm

(C) 1 atm 1 atm

(D) 1 atm 3 atm

19. H2(g) + CO2(g) ⇌ H2O(g) + CO(g) Initially, a sealed vessel contained only H2(g) with a partial pressure of 6 atm and CO2(g) with a partial pressure of 4 atm. The reaction above was allowed to come to equilibrium at a temperature of 700 K. At equilibrium, the partial pressure due to CO(g) was found to be 2 atm. What is the value of the equilibrium constant Kp for the reaction?

(A)

€

16

(B)

€

14

(C)

€

13

(D)

€

12


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USE THIS SPACE FOR SCRATCHWORK. 20. C(s) + H2O(g) ⇌ CO(g) + H2(g) ΔH > 0

The equilibrium mixture shown above is placed in a sealed container at 150°C. The amount of the products may be increased by which of the following changes?

I. Raising the temperature of the container

II. Increasing the volume of the container III. Adding 1 mol of C(s) to the container

(A) I and II only

(B) I only

(C) II and III only

(D) III only

21. C2H4(g) + 3 O2(g) ⇌ 2 CO2(g) + 2 H2O(g) Consider the reaction shown above. An equal number of moles of each of the reactants are sealed in a container and allowed to come to the equilibrium. Which of the following must be true?

I. [CO2] must equal [H2O]

II. [O2] must be less than [C2H4] III. [CO2] must be greater than [C2H4]

(A) II and III only

(B) I only

(C) III only

(D) I and II only

22. CH4(g) + CO2(g) ⇌ 2 CO(g) + 2 H2(g) A 1.00-L flask is filled with 0.30 mol of CH4 and 0.40 mol of CO2, and allowed to come to equilibrium. At equilibrium, there are 0.20 mol of CO in the flask. What is the value of Kc, the equilibrium constant, for the reaction?

(A) 1.2

(B) 0.027

(C) 0.30

(D) 3.0


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USE THIS SPACE FOR SCRATCHWORK. 23. 2 CH4(g) + O2(g) ⇌ 2 CO(g) + 4 H2(g) ΔH < 0

In order to increase the value of the equilibrium constant, K, which of the following changes must be done to the above equilibrium?

(A) Increase the temperature

(B) Increase the volume

(C) Decrease the temperature

(D) Add a catalyst

24. The Ksp for Mn(OH)2 is 1.6 × 10−13. What is the molar solubility of this compound in water?

(A)

€

4.0 ×10−143

(B)

€

1.6 ×10−13

(C)

€

1.6 ×10−133

(D)

€

4.0 ×10−14


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USE THIS SPACE FOR SCRATCHWORK. Questions 25−29 refer to the following.

PCl5(g) ⇄ PCl3(g) + Cl2(g) PCl5(g) decomposes into PCl3(g) and Cl2(g) according to the equation above. A pure sample of PCl5(g) is placed in a rigid, evacuated 1.00 L container. The initial pressure of the PCl5(g) is 1.00 atm. The temperature is held constant until the PCl5(g) reaches equilibrium with its decomposition products. The figures below show the initial and equilibrium conditions of the system. PCl5 PCl5, PCl3, and Cl2

Ptotal = 1.00 atm Ptotal = 1.40 atm Figure 1: Initial Figure 2: Equilibrium

25. Which of the following is the most likely cause for the increase in pressure observed in the container as the reaction reaches equilibrium?

(A) A decrease in the strength of intermolecular attractions among molecules in the flask

(B) An increase in the strength of intermolecular attractions among molecules in the flask

(C) An increase in the number of molecules, which increases the frequency of collisions with the walls of the container

(D) An increase in the speed of the molecules that then collide with the walls of the container with greater force

26. As the reaction progresses toward equilibrium, the rate of the forward reaction

(A) increases until it becomes the same as the reverse reaction rate at equilibrium

(B) stays constant before and after equilibrium is reached

(C) decreases to become a constant nonzero rate at equilibrium

(D) decreases to become zero at equilibrium

27. If the decomposition reaction were to go to completion, the total pressure in the container will be

(A) 1.4 atm

(B) 2.0 atm

(C) 2.8 atm

(D) 3.0 atm


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USE THIS SPACE FOR SCRATCHWORK. 28. Which of the following statements about Kp, the equilibrium

constant for the reaction, is correct?

(A) Kp > 1

(B) Kp < 1

(C) Kp = 1

(D) It cannot be determined whether Kp > 1, Kp < 1, or Kp = 1 without additional information.

29. Additional Cl2(g) is injected into the system at equilibrium. Which of the following graphs best shows the rate of the reverse reaction as a function of time? (Assume that the time for injection and mixing of the additional Cl2(g) is negligible.)

Exam Information

127

6.A.3 When a system is at equilibrium, all macroscopic variables, such as concentrations, partial pressures, and temperature, do not change over time. Equilibrium results from an equality between the rates of the forward and reverse reactions, at which point Q = K.2.2 !e student can apply mathematical routines to quantities that describe natural phenomena.6.5 !e student can, given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K.

15. Additional Cl2(g) is injected into the system at equilibrium. Which of the following graphs best shows the rate of the reverse reaction as a function of time? (Assume that the time for injection and mixing of the additional Cl2(g) is negligible.)(A)

(B)

(C)

(D)

Return to the Table of Contents© 2013 !e College Board.


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Section II: Free-Response A calculator may be used for Questions 30−32.

BaF2(s) ⇌ Ba2+(aq) + 2 F−(aq)

30. The value of the solubility product Ksp for the reaction above is 1.0 × 10−6 at 25.0°C.

(a) Write the Ksp expression for BaF2.

(b) What is the concentration of F− ions in a saturated solution of BaF2 at 25.0°C?


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(c) 500. mL of a 0.0060 M NaF solution is added to 400. mL of a 0.0060 M Ba(NO3)2 solution. Will there be a precipitate? Explain.

(d) What is the value of ΔG° for the dissociation of BaF2 at 25.0°C?


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CaCO3(s) ⇌ Ca2+(aq) + CO32−(aq) Ksp = 2.8 × 10−9

CaSO4(s) ⇌ Ca2+(aq) + SO42−(aq) Ksp = 9.1 × 10−6

31. The values for the solubility products for the two reactions above are given at 25.0°C.

(a) What is the concentration of CO3

2− ions in a saturated 1.00-L solution of CaCO3 at 25.0°C?

(b) Excess CaSO4(s) is placed in the solution in part (a). Assume that the volume of the solution does not change.

(i) What is the concentration of the SO42− ion?

(ii) What is the concentration of the CO32− ion?


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(c) A 0.20-mol sample of CaCl2 is placed in the solution in part (b). Assume that the volume of the solution does not change.

(i) What is the concentration of the Ca2+ ion?

(ii) What is the concentration of the SO42− ion?

(iii) What is the concentration of the CO32− ion?


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32. A student performs an experiment in which the conductivity of a solution of Ba(OH)2 is monitored as the solution is titrated with 0.10 M H2SO4. The original volume of the Ba(OH)2 solution is 25.0 mL. A precipitate of BaSO4 (Ksp = 1.0 × 10−10) formed during the titration. The data collected from the experiment are plotted in the graph above.

(a) As the first 30.0 mL of 0.10 M H2SO4 are added to the Ba(OH)2 solution, two types of chemical reactions occur

simultaneously. On the lines provided below, write the balanced net-ionic equations for (i) the neutralization reaction and (ii) the precipitation reaction.

(i) Equation for neutralization reaction: _______________________________________________________

(ii) Equation for precipitation reaction: ________________________________________________________

(b) The conductivity of the Ba(OH)2 solution decreases as the volume of added 0.10 M H2SO4 changes from 0.0 mL to 30.0 mL.

(i) Identify the chemical species that enable the solution to conduct electricity as the first 30.0 mL of 0.10 M

H2SO4 are added. _____________________________________________________________________________________

(ii) On the basis of the equations you wrote in part (a), explain why the conductivity decreases.

_____________________________________________________________________________________

_____________________________________________________________________________________

_____________________________________________________________________________________

_____________________________________________________________________________________

Exam Information

137

Sample Free-Response Questions

Question 1 is a long constructed-response question that should require about 20 minutes to answer. Questions 2, 3, and 4 are short constructed-response questions that should require about 7 minutes each to answer. Read each question carefully and write your response in the space provided following each question. Your responses to these questions will be scored on the basis of the accuracy and relevance of the information cited. Explanations should be clear and well organized. Speci!c answers are preferable to broad, di"use responses. For calculations, clearly show the method used and the steps involved in arriving at your answers. It is to your advantage to do this, since you may obtain partial credit if you do and you will receive little or no credit if you do not.

1. A student performs an experiment in which the conductivity of a solution of Ba(OH)2 is monitored as the solution is titrated with 0.10 M H2SO4. #e original volume of the Ba(OH)2 solution is 25.0 mL. A precipitate of BaSO4 (Ksp = 1.0 × 10−10) formed during the titration. #e data collected from the experiment are plotted in the graph above.

(a) As the !rst 30.0 mL of 0.10 M H2SO4 are added to the Ba(OH)2 solution, two types of chemical reactions occur simultaneously. On the lines provided below, write the balanced net-ionic equations for (i) the neutralization reaction and (ii) the precipitation reaction.

(i) Equation for neutralization reaction:

(ii) Equation for precipitation reaction:

(b) #e conductivity of the Ba(OH)2 solution decreases as the volume of added 0.10 M H2SO4 changes from 0.0 mL to 30.0 mL.

Return to the Table of Contents© 2013 #e College Board.


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(c) Using the information in the graph, calculate the molarity of the original Ba(OH)2 solution.

(d) Calculate the concentration of Ba2+(aq) in the solution at the equivalence point (after exactly 30.0 mL of 0.10 M H2SO4 are added).

(e) The concentration of Ba2+(aq) in the solution decreases as the volume of added 0.10 M H2SO4 increases from 30.0 mL to 31.0 mL. Explain. __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________ __________________________________________________________________________________________

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