Chem 123Fall 2014 CCBC-Catonsville
This has been updated after Wednesday’s lecture JUST studying these questions is not sufficient preparation. There certainly will be questions that are not shown here.
Before you do these practice problems, I strongly recommend you first re-do the examples from the lectures, AND the
practice exercises mentioned in the PowerPoints. PLEASE DO NOT ASK ME WHAT THE CORRECTION ANSWERS
ARE FOR QUESTIONS WHERE YOU CAN CHECK ANSWERS EASILY BY LOOKING IT UP IN THE TEXTBOOK OR
LECTURE NOTES.
1. PE
Reaction Coordinate
(a) Activation energy of the rate determining step.
(b) intermediate(s) if any
(d) H of the overall reaction
(e) Is this an endothermic or exothermic reaction?
(f) In the reverse reaction, which step is rate determining?
2. The destruction of ozone occurs via a 2-step mechanism, where X can be any of several species:
(1) X + O3 XO + O2 (slow)
(2) XO + O X + O2 (fast)
a) Write the overall reaction.
(b) Write the rate law for each step.
(c) Are there any intermediates in this reaction?
(d) Is a catalyst involved in this reaction?
(e) What would you expect the experimental rate law to look like?
3. (2 pts) Can you tell from a potential energy diagram below whether the reaction is reversible or not? Which of the
following diagrams show a reversible reaction? Circle as many as applies: A B C D
PE PE PE PE
A B C D
2
4. In the PE Diagram below, which is the rate determining step?
PE
Rxn Coordinate
C. Indicate on the graph when equilibrium has been reached.
D. True or False: At equilibrium, the reactant stops converting to product
6. Write the equilibrium constant expression for the following reaction:
2NO (g) + 2H2 (g) N2 (g) + 2H2O (g)
7. Explain how the reaction quotient is different from the equilibrium constant.
8. For a specific reaction, which statement can be made about the equilibrium constant?
A. It can change with temperature.
B. It may be changed by addition of a catalyst.
C. It increases if the concentration of one of the products is increased.
D. It increases if the concentration of one of the products is decreased.
E. It always remains the same.
9. For the reaction stated below, would you expect nearly complete reaction at equilibrium?
C2H4 (g) + H2 (g) C2H6 (g) Kc = 1.3 x 1021
A. Yes B. No
10. For the reaction stated below, what the effect of decreasing the volume would be on the amount of product.
2SO2 (g) + O2 (g) 2SO3 (g)
A. increase B. decrease C. no effect
11. For the reaction stated below, what is the effect of decreasing the volume on the amount of product?
C(s) + CO2 (g) 2CO (g)
A. increase B. decrease C. no effect
For the next 3 questions consider the reaction, 2HI (g) H2 (g) + I2 (g). HI and H2 are colorless gases, but I2 is a
purple gas, and the reaction is exothermic.
12. What will happen if some of the H2 is removed from the equilibrium mixture?
A. the purple color will get more intense B. the purple color will get less intense C. no effect on the color.
13. What will happen to the equilibrium if Ar(g) is added?
A. shift to the right B. shift to the left C. no effect
14. What will happen to the equilibrium if the temperature is decreased?
A. shift to the right B. shift to the left C. no effect
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2
n
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15. The equilibrium constant for the reaction, H2 (g) + I2 (g) 2HI (g) is 54.9 at 699.0 K. what is the
equilibrium constant for 4HI (g) 2 H2 (g) + 2 I2 (g) under the same conditions? Show your work.
Ans. 3.32x10–4
16. Given the two reactions shown with their equilibrium constants,
PCl3 (g) + Cl2 (g) PCl5 (g) K1
2 NO (g) + Cl2 (g) 2 NOCl (g) K2
What is the equilibrium constant for the reaction shown below in terms of K1 and K2? Show your work.
PCl5 (g) + 2 NO (g) PCl3 (g) + 2 NOCl (g)
Ans. K2/K1
17. For the reaction, 2 SO2 (g) + O2 (g) 2 SO3 (g) at 900.0 K the equilibrium constant, Kc, has the value of 13.0.
Calculate the value of KP at the same temperature. Show your work.
Ans. 0.176 atm –1
18. The equilibrium law for the system, CaO (s) + CO2 (g) CaCO3 (s) is
A. Kc = [CO2]
C. Kc = 1/[CO2]
D. Kc = [CaCO3]/[CaO]
E. Kc = ([CaO] [CO2])/[CaCO3]
19. The equilibrium law for the system, CaO (s) + CO2 (g) CaCO3 (s) is
20. Consider the reaction of H2 (g) + F2 (g) 2HF (g) which has an equilibrium constant of 1.15x102 at a certain
temperature. If we start with 3.000 mol of each of the three components in a 1-L flask…
(a) Calculate the equilibrium quotient.
(b) Would the reaction proceed to the right or to the left to reach equilibrium? Explain briefly how you reach your
conclusion.
21. Gaseous NOCl decomposes to form the gases NO and Cl2. At 35°C the equilibrium constant is 1.6x10 5. In an
experiment in which 1.0 mol NOCl is placed in a 2.0-L flask, what are the equilibrium concentrations? Show your
work carefully. If you make any assumptions, be sure to show they are justified. Circle your final answers.
2NOCl (g) 2NO (g) + Cl2 (g)
Ans. [NOCl] = 0.48 M [NO] = 0.020 M [Cl2] = 0.010 M
22. In the reaction shown below, specify which is the acid, the base, the conjugate acid and the conjugate base:
HSO4 + HS H2S + SO4 2
acid = base = conjugate acid = conjugate base =
23. Using curved arrows to show the flow of electrons and how the following reactants are acting as Lewis acids and
bases. Label which is the acting as the Lewis acid and which is acting as the Lewis base
H O
H
H
24. The conjugate acid of H2AsO4 is
A. H2AsO4 B. H3AsO4 C. HAsO4 D. HAsO4 2 e. AsO4
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25. For the system NH2OH + CH3NH3 + CH3NH2 + NH3OH+
the position of the equilibrium lies to the left. Which is the strongest acid in the system?
A. NH2OH B. CH3NH3 + C. CH3NH2 D. NH3OH+
26. Which is the most acidic?
A. HClO B. HClO2 C. HClO3 D. HClO4
4
A. H2O B. H2S C. H2Se D. H2Te
28. Which anion is the most basic, assuming the difference in electronegativity is negligible?
A. the larger anion B. the smaller anion
29. Which of the following cations is most likely to act as Lewis acids?
A. Sr2+ B. K+ C. Al3+ D. none of the above
30. Complete and balance the following equations to show how Al(OH)3 is amphoteric. Include physical states.
Al(OH)3 (s) + H+ (aq) Reactant is acting as a Brønsted base.
Al(OH)3 (s) + OH (aq) Al3+ is acting as a Lewis acid.
31. Write the equation for the hydration of Co3+ ion:
Co3+ + 6 H2O Is Co3+ acting as an acid? base? neither?
32. Write the equation showing how the hydrated Cr3+ ion reacts as a Brønsted acid:
Cr(H2O)6 3+ + OH−
Na+ + 6 H2O Na(H2O)6 +
(Hint: It has something to do with the fact that Na+ is in Group IA.)
34. The following is a well-known Brønsted acid/base reaction. Use arrows to show that it can also be considered a
Lewis acid/base reaction:
Note: Physical states need to be considered.
35. If the OH ion concentration in an aqueous solution at 25.0°C is measured as 3.4x10 3 M, then the pH is
A. 2.47 B. 7.22 C. 8.24 D. 11.53 E. 16.47
36. Calculate the pH of a 0.020 M solution of Ca(OH)2 whose temperature is 25.0 °C.
A. 1.40 B. 0.040 C. 1.69 D. 12.60 E. 12.30
37. For a solution with pH = 3.40, the hydronium ion concentration is…
A. 2.5x103 B. 2.51x103 C. 4.0x10 4 D. 3.98x10 4
38. Explain why the pH of a solution that is 1.0 x 10 10 M HCl cannot be 10.00 and state what you expect it to be instead.
Answer in one to three sentences.
39. HOCl has a pKa of 9.21 and that of HN3 is 4.74. Which is the stronger acid?
A. HOCl B. HN3 C. not enough information is given
40. a) Write the chemical equation showing the acid dissociation of HC6H5O in water.
b) Write the Ka expression for the dissociation.
c) Identify the two acid/conjugate base pairs.
41. a) Write the chemical equation showing the ionization of the base, HONH2. Hint: This is a base.
b) Write the Kb expression for the dissociation.
c) Write the Ka expression of its conjugate acid.
42. What is Kb of CN−? Which table do you need to determine this value?
43. CN has a Kb of 1.62 x 10 5.
a) Write the equilibrium expression corresponding to this Kb.
b) Write the equilibrium expression for the corresponding Ka.
c) Calculate Ka. Show your work. d) What is the pKa? Show your work.
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The questions below are tentatively to be covered on Wed’s lecture (10/29/14). Some of the questions may be deleted
after Wed’s lecture if we did not manage to cover them. UPDATE WILL BE DONE WED EVENING BEFORE 8
PM.
44. Pyridine has the formula C5H5N. The pH of a 0.210 M solution of protonated pyridine, C5H5NH+ is 2.93. What is
the Kb of pyridine. Ans, 1.5x10 –9
45. A 0.15 M solution of a weak acid is 3.0% dissociated. Calculate its Ka. Ans. 1.4x10–4
46. What is the percent ionization of 0.20 M solution of HNO2? You may use the table of Ka. Ans.4.8%
47. Calculate the concentrations of all species present at equilibrium, and the percent ionization of HCN (Ka = 4.9 x
10 10) in a solution that has initial concentrations of 0.025 M in HCN and 0.010 M in NaCN. Show your work
clearly and watch your sig. fig. Ans. 4.9x10–6 % ionized
Ans. 1.2x10–9 M H+, 0.025M HCN, 0.010 M CN–
48. Examine the titration curve on the right. pH
a) What kind of acid is in the titration?
b) Where is the buffer region located? 9
c) How might you obtain the pKa of the acid
from this graph?
Volume of NaOH added
49. Write chemical equations to show why the pH of water is affected by CO2 dissolving in water. Include physical states.
Circle only the equations you want me to grade. Do not make me choose.
50.A student added 85.3 mL of 0.110 M NaOH to 245 ml of a buffer that contained 0.140 M HC2H3O2 and 0.230 M
C2H3O2 .
a) What is the change in concentration of HC2H3O2 and C2H3O2 after the reaction and the system had time to reach
an equilibrium? Ans. 0.0756 M HC2H3O2 0.199 M C2H3O2 –
b) What is the pH of the final solution? Ans. 5.16
51. What would you expect the pH to be for a solution prepared by dissolving KCN in water?
A. pH < 7 B. pH = 7 C. pH > 7
52. The pKa of HF is 3.17 and pKa of HCN is 9.21. Which is the stronger Brønsted base?
A. F B. CN
HOCN + H2O H3O + + OCN
What will happen when HBr is added? Assume the volume does not change appreciably.
A. [OCN ] will increase. B. [HOCN] will increase.
C. [HOCN] will decrease. D. [HOCN] will not be affected.