CHEMISTRY 102 FALL 2009 EXAM 2 FORM B SECTION 501 DR. KEENEY-KENNICUTT PART 1

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1 NAME CHEMISTRY 102 FALL 2009 EXAM 2 FORM B SECTION 501 DR. KEENEY-KENNICUTT Directions: (1) Put your name on PART 1 and your name and signature on PART 2 of the exam where indicated. (2) Sign the Aggie Code on PART 2 of this exam. (3) Each multiple choice question is actually 2 questions on your scanning sheet. If you are sure of an answer, put the same answer down for both questions for 5 pts. If you cannot decide between two answers, put your best answer down for the first (odd) question and the other answer down for the second (even) question. If you get the first one correct you'll get 3 pts; if you get the second one correct you ll get 2 pts. If there is an ambiguous multiple choice question, use the last page to explain your answer. (4) Do NOT write on the envelope. (5) When finished, put everything in the envelope and wait to be excused. At the table, take everything out of the envelope. You can pick up the multiple choice part with the answers outside my office after 3pm. (6) There are a total of 35 questions (19 actual questions). PART 1 1&2. At room temperature, A and B react according to the net overall equation A + B --> C + D. If the overall order of the reaction and the order of reaction with respect to A are both 1, what is the rate-law expression for this reaction? (a) Rate = k[a][b] (b) Rate = k[a] 2 [B] (c) Rate = k[a] (d) Rate = k[b] 2 (e) Rate = k[a] 2 [B] 2 3&4. Here is an equilibrium expression: K c = [X][Y]2 3 [Z]. The equilibrium consistent with this expression is: (a) X(g) + Y(g) Z(g) (b) X(g) 2Y(g) + 3Z(g) (c) 2X(g) + Y(g) 3Z(g) (d) 3Z(g) X(g) + 2Y(g) (e) X(g) + 2Y(g) 3Z(g) 5&6. If it is known for a particular reaction that the units of the rate constant, k, is 1/M s, then it is known that the overall order of the reaction is: (a) zero (b) one (c) two (d) three (e) four 7&8. Which of the following statements concerning catalysts is FALSE? (a) Adding a catalyst to a reaction at equilibrium will change the equilibrium constant. (b) The catalyst can be classified as homogeneous or heterogeneous. (c) A catalyst will lower the activation energy of both the forward and reverse reaction. (d) The mechanism of a reaction always changes when a catalyst speeds up the reaction. Keeney-Kennicutt, 2009 B1

2 (e) Catalysts participate in a reaction. Keeney-Kennicutt, 2009 B2

3 9&10. For a certain reaction, the ΔH = ΔE is +50 kj. The activation energy for the reverse reaction is 20 kj. Therefore, the activation energy for the forward reaction is. (It may be useful to draw the potential energy diagram.) (a) -30 kj (b) +30 kj (c) -70 kj (d) +70 kj (e) +20 kj A 11&12. For a reaction where ΔH is +170 kj/mol rxn and ΔS = 80 J/K. Free response: When is this reaction spontaneous? Multiple Choice: (a) the reaction is spontaneous only at temperatures above a certain value. (b) the reaction is spontaneous only at temperatures below a certain value. (c) the reaction is spontaneous at all temperatures. (d) the reaction is nonspontaneous at all temperatures (e) It is impossible to tell if the reaction is or is not spontaneous. B Briefly explain when a reaction is spontaneous if the reaction has ΔH = +170 kj/mol rxn and ΔS = 80 J/K. For which set of values of ΔH and ΔS will a reaction be spontaneous (product-favored) at all temperatures? (a) ΔH = +10 kj, ΔS = -5 J/K (b) ΔH = -10 kj, ΔS = -5 J/K (c) ΔH = -10 kj, ΔS = +5 J/K (d) ΔH = +10 kj, ΔS = +5 J/K (e) no such values exist 13&14. Which of the following factors can influence the rate of a chemical reaction? (1) the nature of the reactants (2) the reactant concentration (3) the temperature (4) the presence of a catalyst (a) 1, 3 and 4 (b) both 2 and 3 (c) 1, 2 and 3 (d) both 1 and 4 (e) all of them 15&16. Which sketch best represents the final equilibrium mixture for A(g) 2B(g) in which K << 1. A is represented by and B is represented by (a) (b) (c) Keeney-Kennicutt, 2009 B3

4 (d) (e) 17&18. The equilibrium constant for the following gas phase reaction is 81 at a certain temperature. A reaction is carried out in a 2.00 liter container at this temperature starting with moles of H 2 (g) and moles of F 2 (g). What will be the equilibrium concentration of HF? 2HF(g) H 2 (g) + F 2 (g) (a) M (b) M (c) M (d) M (e) M 19&20. The gas-phase reaction 2ClO 2 (aq) + 2OH (aq) ClO 3 (aq)+ ClO 2 (aq) + 2H 2 O(l) gives the following initial rates at 20 o C: Experiment [ClO 2 ] initial [OH ] initial Initial Rate of Reaction M M 3.88 x 10 4 M/s M M 1.55 x 10 3 M/s M M 7.76 x 10 4 M/s M M 3.11 x 10 3 M/s s (a) Rate = k[clo 2 ][OH ] (b) Rate = k[clo 2 ] 2 [OH ] (c) Rate = k[clo 2 ][OH ] 2 (d) Rate = k[clo 2 ] 2 [OH ] 2 (e) Rate = k[clo 2 ] 2 Keeney-Kennicutt, 2009 B4

5 21&22. In a 1.0 liter container there are 0.10 mol of HgO(s), 0.30 mole Hg(l) and 0.20 mole O 2 (g) in the system at equilibrium. 2 Hg(l) + O 2 (g) 2HgO(s) What is the value of K c for this reaction? (a) 0.56 (b) 4.0 (c) 1.8 (d) 5.0 (e) &24. Consider the reaction below at 25 o C for which ΔH o = kj and ΔS o = J/K at 25 o C. 2O 3 (g) 3O 2 (g) What is the value of ΔG at 25 C for the reaction? (a) 223 kj (b) 3075 kj (c) 411 kj (d) 151 kj (e) 325 kj Keeney-Kennicutt, 2009 B5

6 25&26. Consider this equilibrium reaction: N 2 (g) + O 2 (g) 2NO(g) K c = 1.11 x 10 5 at 1500K What is the value of K c for the reaction: 2N 2 (g) + 2O 2 (g) 4NO(g)? (a) 3.33 x 10 3 (b) 1.23 x (c) 9.00 x 10 4 (d) 1.11 x 10 5 (e) 2.22 x &28. The activation energy for a reaction was determined to be 122 kj/mol rxn. If the rate constant is 5.00 x 10 2 s 1 at 25 o C, what would the rate constant be at 100 o C? ln k 2 k 1 = E a R T 2 - T 1 T 1 T 2 or ln k 2 k 1 = E a R 1 T T 2 R = J/mol K (a) 998 s 1 (b) 4.12 x 10 5 s 1 (c) 2.23 x 10 3 s 1 (d) 5.37 x 10 2 s 1 (e) 8.16 x 10 6 s 1 Keeney-Kennicutt, 2009 B6

7 29&30. Estimate the boiling point of nitric acid, HNO 3, given the following data from thermodynamic tables. Assume that ΔHf o 298 and S o do not change with temperature: HNO 3 (l) HNO 3 (g) ΔH o f 298 (kj/mol) S o (J/mol K) ΔG o f 298 (kj/mol) HNO 3 (g) HNO 3 (l) (a) 355 o C (b) 215 o C (c) 79.6 o C (d) 148 o C (e) 53.7 o C 31&32. Formic acid decomposes at 550 o C with first order kinetics with a half-life of 36 s. How many minutes are required to reduce a 4.0 g sample to 0.10% of its original mass? CHOOH(g) CO 2 (g) + H 2 (g) (a) 2.3 min (b) 130 min (c) 0.55 min (d) 6.0 min (e) 370 min Keeney-Kennicutt, 2009 B7

8 CHEMISTRY 102 EXAM 2 Form B FALL 2009 Section 501 NAME PART 2 (Please Block Print) Please read and sign: On my honor, as an Aggie, I have neither given nor received unauthorized aid on this exam. 33. Consider this equilibrium reaction: 2NO 2 (g) N 2 O 4 (g) K c = 215 at 22 o C (3 pts) (a) Calculate Q c when [NO 2 ] = M and [N 2 O 4 ] = 1.00 M. (4 pts) (b) What does this result mean? (3 pts) 34. Consider the equilibrium: X(g) 2Y(g) + Z(s) The reaction as written is strongly exothermic. Predict how the following changes will affect (i) the moles of Y in the container, (ii) the value of the equilibrium constant, and (iii) the activation energy, E a, for the forward reaction The possible answers are: increase (I), decrease (D), or remain unchanged (U) (a) Extra X is added to the container. (b) The temperature of the container is decreased. moles of Y K E a OVER Keeney-Kennicutt, 2009 B8

9 35. Consider the following 2 step mechanism for a gas phase reaction. In the first step, a molecule of A collides with a molecule of B to form a molecule of X. In the second step, a molecule of X collides with a molecule of B to form a molecule of A and two molecules of D. (3 pts) (a) Write out the mechanism and the overall reaction. (2 pts) (b) Identify the catalyst (if any): Identify the intermediate (if any): (2 pts) (c) Write the rate law expression for this mechanism if the first step is the slow step. (3 pts) (d) Write the rate law expression for this mechanism if the first step is a fast equilibrium step and the second step is the slow step. (Remember: show all your work to get full credit.) Keeney-Kennicutt, 2009 B9

10 SCRAP PAPER OR COMMENTS ON EXAM CHEMISTRY 102 EXAM 2 Form B Fall 2009 S 501 NAME Keeney-Kennicutt, 2009 B10