Chemistry 1211 Exam #3 Thursday, 4/17/2003

Transcription

1 Chemistry 1211 Exam #3 Thursday, 4/17/2003 Prof. Dan Schwartz 7:00 8:30 P.M. There is a single correct choice for each question. Answer all questions on the Scantron sheet by filling in the proper bubble with a #2 pencil. If you change an answer, erase the undesired mark thoroughly. Be sure to fill in the boxes for your student number and name; then correctly fill in the corresponding bubbles beneath them. A periodic table and other useful information are attached to the back of the exam. There are 5 exam pages and 25 questions. Check to be sure you have answered them all! 1. (5 points) Stainless steel is an example of a solution. (a) gas/solid (b) liquid/liquid (c) solid/liquid (d) solid/solid 2. (5 points) Commercial cold packs often contain solid NH 4 NO 3 and a pouch of water. The temperature of the pack drops as the NH 4 NO 3 dissolves in water. Therefore, for the dissolving of NH 4 NO 3 in water (a) DH soln is negative and DS soln may be negative or positive. (b) DH soln is negative and DS soln is positive. (c) DH soln is positive and DS soln may be negative or positive. (d) DH soln is positive and DS soln is positive. 3. (5 points) Which should be least soluble in water? (a) butane, H 3 C-CH 2 -CH 2 -CH 3 (b) formic acid, HCOOH (c) methanol, H 3 C-OH (d) methylamine, H 3 C-NH 2 4. (5 points) In general, as the temperature increases, the solubility of gases in water and the solubility of most solids in water. (a) decreases, decreases (b) decreases, increases (c) increases, decreases (d) increases, increases Exam #3 Page 1

2 5. (5 points) A KCl solution is prepared by dissolving 40.0 g KCl in g of water at 25 C. What is the vapor pressure of the solution if the vapor pressure of water at 25 C is mm Hg? (a) 20.5 mm Hg (b) 22.1 mm Hg (c) 22.9 mm Hg (d) 24.7 mm Hg 6. (5 points) Which of the following pairs of aqueous solutions have roughly the same boiling point? (a) m C 6 H 12 O 6 (glucose) and m CuCl 2 (b) m NaCl and m C 6 H 12 O 6 (glucose) (c) m NaCl and m Na 2 SO 4 (d) m KCl and m MgBr 2 7. (5 points) The decomposition of dinitrogen pentoxide is described by the reaction below: 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) If the rate of disappearance of N 2 O 5 is equal to 1.40 mol/min, what is the rate of appearance of NO 2? (a) mol/min (b) 1.40 mol/min (c) 2.80 mol/min (d) 5.60 mol/min 8. (5 points) The following set of data was obtained by the method of initial rates for the reaction 2 HgCl 2 (aq) + C 2 O 4 2 (aq) 2 Cl (aq) + 2 CO 2 (g) + Hg 2 Cl 2 (s) [HgCl 2 ], M [C 2 O 2 4 ], M Rate, M/s x x x 10 6 What is the rate law for the reaction? (a) Rate = k [HgCl 2 ] [C 2 O 4 2 ] 2 (b) Rate = k [HgCl 2 ] [C 2 O 4 2 ] 1 (c) Rate = k [HgCl 2 ] 2 [C 2 O 4 2 ] (d) Rate = k [HgCl 2 ] [C 2 O 4 2 ] 2 9. (5 points) The first-order reaction, SO 2 Cl 2 SO 2 + Cl 2, has a half-life of 8.75 hours at 593 K. How long will it take for the concentration of SO 2 Cl 2 to fall to 12.5% of its inital value? (a) hr (b) 6.07 hr (c) 26.2 hr (d) 38.1 hr Exam #3 Page 2

3 10. (5 pts) With reference to the energy diagram below, what is the activation energy for the reaction of ozone with atomic oxygen to form molecular oxygen? (a) 14 kj (b) 392 kj (c) 406 kj (d) none of these E 406kJ O 3 + O 392 kj O 2 + O 2 rxn progress 11. (5 points) Consider a bimolecular reaction in the gas phase. Which one of the following changes in conditions will NOT cause an increase in the rate of the reaction? (a) add a catalyst (b) increase the temperature at constant volume (c) increase the volume at constant temperature (d) all of the above will increase the rate of reaction 12. (5 points) A mechanism for a naturally-occurring reaction that destroys ozone is: Step 1: O 3 (g) + HO (g) HO 2 (g) + O 2 (g) slow Step 2: HO 2 (g) + O (g) HO (g) + O 2 (g) fast Choose the answer that best fills in the blanks in the following sentence: is the catalyst and catalysis is occurring. is a reaction intermediate and the kinetics are order. (a) HO 2, homogeneous, HO, second (b) HO, heterogeneous, O, first (c) HO 2, heterogeneous, HO, first (d) HO, homogeneous, HO 2, second 13. (5 points) Which of the following statements is FALSE regarding the equilibrium constant K c? (a) K c for a reaction at a particular temperature always has the same value. (b) K c for the reverse reaction is the negative of K c for the forward reaction. (c) The numerical value of K c depends on the form of the balanced reaction equation. (d) When quoting K c it is customary to omit units. Exam #3 Page 3

4 14. (5 points) The gas phase equilibrium constant K p, equals 3.40 at 25 C for the isomerization reaction: cis-2-butene trans-2-butene If a flask initially contains 1.00 atm of each gas, what direction will the reaction shift to reach equilibrium? (a) It will shift left. (b) It will shift right. (c) The reaction is already at equilibrium (d) The reaction is at equilibrium initially and then will shift right. 15. (5 points) An equilibrium mixture of CO, O 2, and CO 2 at a certain temperature contains M CO 2 and M O 2. At this temperature, K c equals 1.4 x 10 2 for the reaction: 2 CO (g) + O 2 (g) 2 CO 2 (g) What is the equilibrium concentration of CO? (a) 4.8x10 6 M (b) 4.6x10 4 M (c) 2.2x10 3 M (d) 6.9x10 2 M 16. (5 points) At a certain temperature the equilibrium constant, K c, equals 0.11 for the reaction: 2 ICl (g) I 2 (g) + Cl 2 (g). What is the equilibrium concentration of ICl if mol of I 2 and mol of Cl 2 are initially mixed in a 2.00 L flask? (a) M (b) M (c) M (d) 1.50 M 17. (5 points) Ammonium carbamate can dissociate into gases at 25 C according to the reaction: NH 2 COONH 4 (s) 2 NH 3 (g) + CO 2 (g). If sufficient ammonium carbamate solid is sealed in a flask, the total pressure will be atm at equilibrium. What is the value of K p at 25 C? (a) 2.37 x 10 4 (b) 2.00 x 10 4 (c) 1.60 x 10 3 (d) 3.42 x (5 points) Which change in the system will drive equilibrium to the left in the reaction below? N 2 O 5 (g) NO 2 (g) + NO 3 (g) (a) Decrease the amount of NO 3. (b) Increase the amount of N 2 O 5. (c) Increase the pressure. (d) Increase the volume. Exam #3 Page 4

5 19. (5 points) In the following reaction, the ammonia acts as a BF 3 + NH 3 BF 3 NH 3 (a) Lewis acid (b) Lewis base (c) Bronsted-Lowry acid (d) Bronsted-Lowry base 20. (5 points) Calculate the ph of an aqueous solution of pyridine with a hydroxide ion concentration of 2.15 x 10 4 M. (a) 4.65 x (b) 2.15 x 10 4 (c) 3.67 (d) (5 points) What statement is most consistent for an acid with a ph = 3? (a) one one-hundredth as strong as an acid with a ph of 5. (b) half as strong as an acid with a ph of 5. (c) twice as strong as an acid with a ph of 5. (d) one hundred times as strong as an acid with a ph of (5 points) What is the ph of a solution made by mixing ml of 0.10 M HNO 3, 50.0 ml of 0.20 M HCl, and ml of water? Assume that the volumes are additive. (a) 0.30 (b) 0.82 (c) 1.00 (d) (5 points) Determine the acid dissociation constant for a M nitrous acid solution that has a ph of Nitrous acid is a weak monoprotic acid and the equilibrium equation of interest is HNO 2 (aq) + H 2 O (l) H 3 O + (aq) + NO 2 (aq) (a) 8.0 x 10 3 (b) 2.0 x 10 3 (c) 5.0 x 10 4 (d) 4.0 x (5 points) What is the ph of a M KBrO solution if the K a for hypobromous acid (HBrO) is 2.0 x 10 9? (a) 3.15 (b) 4.85 (c) 9.15 (d) (5 EXTRA CREDIT points) Drinking too much water can be toxic, resulting in a condition called hyponatremia. According to your instructor this is due to the phenomenon known as (a) Le Chatelier s principle (b) osmotic pressure (c) Raoult s Law (d) freezing point depression Exam #3 Page 5

6 Answers: 1. d 2. d 3. a 4. b 5. b 6. a 7. c 8. d 9. c 10. a 11. c 12. d 13. b 14. b 15. c 16. c 17. a 18. c 19. b 20. d 21. d 22. d 23. c 24. d 25. b Exam #3 Page 6