Practice Questions for EXAM III

Transcription

1 1 Chem 13 Fall 014 Practice Questions for EXAM III 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 WAT TE CORRECTION ANSWERS ARE FOR QUESTIONS WERE YOU CAN CECK ANSWERS EASILY BY LOOKING IT UP IN TE TEXTBOOK OR LECTURE NOTES. 1. PE Reaction Coordinate Indicate in the graph above where the following is: (a) Activation energy of the rate determining step. (b) intermediate(s) if any (c) transition state(s) if any (d) of the overall reaction (e) Is this an endothermic or exothermic reaction? (f) In the reverse reaction, which step is rate determining?. The destruction of ozone occurs via a -step mechanism, where X can be any of several species: (1) X O 3 XO O (slow) () XO O X O (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. ( 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 Rxn Coordinate Rxn Coordinate Rxn Coordinate Rxn Coordinate A B C D

2 Molar Concen 4. In the PE Diagram below, which is the rate determining step? PE A. Step 1 B. Step Rxn Coordinate CAPTER 15: CEMICAL EQUILIBRIUM 5. A. Which curve represents the reactant? 1 or 1 B. Which does the equilibrium favor? Reactant or Product C. Indicate on the graph when equilibrium has been reached. D. True or False: At equilibrium, the reactant stops converting to product time 6. Write the equilibrium constant expression for the following reaction: NO (g) (g) N (g) O (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? C 4 (g) (g) C 6 (g) K c = 1.3 x 10 1 A. Yes B. No 10. For the reaction stated below, what the effect of decreasing the volume would be on the amount of product. SO (g) O (g) SO 3 (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) CO (g) CO (g) A. increase B. decrease C. no effect For the next 3 questions consider the reaction, I (g) purple gas, and the reaction is exothermic. (g) I (g). I and are colorless gases, but I is a 1. What will happen if some of the 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

3 15. The equilibrium constant for the reaction, (g) I (g) I (g) is 54.9 at K. what is the equilibrium constant for 4I (g) (g) I (g) under the same conditions? Show your work. Ans. 3.3x Given the two reactions shown with their equilibrium constants, PCl 3 (g) Cl (g) PCl 5 (g) K 1 NO (g) Cl (g) NOCl (g) K What is the equilibrium constant for the reaction shown below in terms of K 1 and K? Show your work. PCl 5 (g) NO (g) PCl 3 (g) NOCl (g) Ans. K /K For the reaction, SO (g) O (g) SO 3 (g) at K the equilibrium constant, K c, has the value of Calculate the value of K P at the same temperature. Show your work. Ans atm The equilibrium law for the system, CaO (s) CO (g) CaCO 3 (s) is A. K c = [CO ] B. K c = [CaCO 3 ] / ([CaO] [CO ]) C. K c = 1/[CO ] D. K c = [CaCO 3 ]/[CaO] E. K c = ([CaO] [CO ])/[CaCO 3 ] 19. The equilibrium law for the system, CaO (s) CO (g) CaCO 3 (s) is 0. Consider the reaction of (g) F (g) F (g) which has an equilibrium constant of 1.15x10 at a certain temperature. If we start with 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. 1. Gaseous NOCl decomposes to form the gases NO and Cl. At 35 C the equilibrium constant is 1.6x10 5. In an experiment in which 1.0 mol NOCl is placed in a.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. NOCl (g) NO (g) Cl (g) Ans. [NOCl] = 0.48 M [NO] = 0.00 M [Cl ] = M. In the reaction shown below, specify which is the acid, the base, the conjugate acid and the conjugate base: SO 4 S S SO 4 acid = base = conjugate acid = conjugate base = 3. 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 _ O: N O : N 3 Label: 4. The conjugate acid of AsO 4 is A. AsO 4 B. 3 AsO 4 C. AsO 4 D. AsO 4 e. AsO For the system N O C 3 N 3 C 3 N N 3 O the position of the equilibrium lies to the left. Which is the strongest acid in the system? A. N O B. C 3 N 3 C. C 3 N D. N 3 O 6. Which is the most acidic? A. ClO B. ClO C. ClO 3 D. ClO 4

4 4 7. Which is the most acidic? A. O B. S C. Se D. Te 8. Which anion is the most basic, assuming the difference in electronegativity is negligible? A. the larger anion B. the smaller anion 9. Which of the following cations is most likely to act as Lewis acids? A. Sr B. K C. Al 3 D. none of the above 30. Complete and balance the following equations to show how Al(O) 3 is amphoteric. Include physical states. Al(O) 3 (s) (aq) Reactant is acting as a Brønsted base. Al(O) 3 (s) O (aq) Al 3 is acting as a Lewis acid. 31. Write the equation for the hydration of Co 3 ion: Co 3 6 O Is Co 3 acting as an acid? base? neither? 3. Write the equation showing how the hydrated Cr 3 ion reacts as a Brønsted acid: Cr( O) 3 6 O 33. Explain why this is not likely to happen: Na 6 O Na( O) 6 (int: 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: Cl (g) NaO (aq) O (l) NaCl (aq) Note: Physical states need to be considered. 35. If the O ion concentration in an aqueous solution at 5.0 C is measured as 3.4x10 3 M, then the p is A..47 B. 7. C. 8.4 D E Calculate the p of a 0.00 M solution of Ca(O) whose temperature is 5.0 C. A B C D E For a solution with p = 3.40, the hydronium ion concentration is A..5x10 3 B..51x10 3 C. 4.0x10 4 D. 3.98x Explain why the p of a solution that is 1.0 x M Cl cannot be and state what you expect it to be instead. Answer in one to three sentences. 39. OCl has a pka of 9.1 and that of N 3 is Which is the stronger acid? A. OCl B. N 3 C. not enough information is given 40. a) Write the chemical equation showing the acid dissociation of C 6 5 O in water. b) Write the K a expression for the dissociation. c) Identify the two acid/conjugate base pairs. 41. a) Write the chemical equation showing the ionization of the base, ON. int: This is a base. b) Write the Kb expression for the dissociation. c) Write the Ka expression of its conjugate acid. 4. What is Kb of CN? Which table do you need to determine this value? 43. CN has a K b of 1.6 x a) Write the equilibrium expression corresponding to this K b. b) Write the equilibrium expression for the corresponding K a. c) Calculate K a. Show your work. d) What is the pka? Show your work.

5 The questions below are tentatively to be covered on Wed s lecture (10/9/14). 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 C 5 5 N. The p of a 0.10 M solution of protonated pyridine, C 5 5 N is.93. What is the K b of pyridine. Ans, 1.5x A 0.15 M solution of a weak acid is 3.0% dissociated. Calculate its Ka. Ans. 1.4x What is the percent ionization of 0.0 M solution of NO? 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 CN (K a = 4.9 x ) in a solution that has initial concentrations of 0.05 M in CN and M in NaCN. Show your work clearly and watch your sig. fig. Ans. 4.9x10 6 % ionized Ans. 1.x10 9 M, 0.05M CN, M CN 48. Examine the titration curve on the right. p a) What kind of acid is in the titration? b) Where is the buffer region located? 9 c) ow might you obtain the pka of the acid from this graph? 5 Volume of NaO added 49. Write chemical equations to show why the p of water is affected by CO 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 M NaO to 45 ml of a buffer that contained M C 3 O and 0.30 M C 3 O. a) What is the change in concentration of C 3 O and C 3 O after the reaction and the system had time to reach an equilibrium? Ans M C 3 O M C 3 O b) What is the p of the final solution? Ans What would you expect the p to be for a solution prepared by dissolving KCN in water? A. p < 7 B. p = 7 C. p > 7 5. The pka of F is 3.17 and pka of CN is 9.1. Which is the stronger Brønsted base? A. F B. CN 53.Consider the buffer containing 0.1M OCN and 0.1M NaOCN. OCN O 3 O OCN What will happen when Br is added? Assume the volume does not change appreciably. A. [OCN ] will increase. B. [OCN] will increase. C. [OCN] will decrease. D. [OCN] will not be affected. Questions beyond #53 have been deleted as we did not get that far in lecture.