NAME ______________________________________________ © Keeney-Kennicutt, 2010 A1 CHEMISTRY 102 FALL 2010 EXAM 2 FORM A SECTION 501 DR. KEENEY-KENNICUTT Directions: (1) Put your name on PART 1 and 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. Which is the correct Kc expression for the equilibrium: N2H4(l) →← N2(g) + 2H2(g)? (a) Kc = [N2]⋅2[H2][N2H4] (b) Kc = [N2][2H2]2[N2H4] (c) Kc = [N2][H2]2[N2H4] (d) Kc = [N2][2H2]2 (e) Kc = [N2][H2]2 3&4. The rate law for the chemical reaction, 5Br− + BrO3− + 6H+ → 3Br2 + 3H2O has been determined experimentally to be: −Δ[BrO3−] Δt = k[Br−][BrO3−][H+]2 The reaction order with respect to the hydrogen ion is (a) 1 (b) 2 (c) 6 (d) k[H+]2 (e) k[Br−][BrO3−][H+]2 5&6. If a reaction has a highly negative value of ΔG, such as −300 kJ, the reaction will occur: (a) slowly. (b) rapidly. (c) with a high activation energy. (d) with a low activation energy. (e) It is impossible to know from the ΔG information.© Keeney-Kennicutt, 2010 A2 7&8. This sketch represents which of the following situations where : is X and is Y? (a) Y2 →← X K << 1 (b) 2X →← Y K >> 1 (c) X2 →← Y K << 1 (d) Y →← X2 K << 1 (e) Y →← 2X K >> 1 9&10. Most reactions are more rapid at high temperatures than at low temperatures. This is consistent with: (1) An increase in the percentage of “high energy” collisions with increasing temperature. (2) An increase in the activation energy with increasing temperature (3) An increase in the rate constant with increasing temperatures (a) only (1) (b) only (2) (c) only (3) (d) only (2) and (3) (e) only (1) and (3) 11&12. The ΔH (=ΔE) for a reaction is +70 kJ. The activation energy for the reverse reaction is 30 kJ . What is the activation energy for the forward reaction? It might be helpful to sketch the potential energy diagram. (a) +100 kJ (b) +70 kJ (c) −100 kJ (d) −70 kJ (e) +40 kJ 13&14. Which of the following statements is/are TRUE concerning the action of catalysts? (1) Their presence always changes the mechanism of the reaction. (2) Catalysts do not participate in the reaction. (3) The activation energy of the rate-determining step is lowered and the reaction speeds up. (a) 1, 3 only (b) 2, 3 only (c) 2 only (d) 1, 2, 3 (e) 1, 2 only 15&16. Which CANNOT be true for the 1 step reaction: C≡O + O=N-O Æ O=C=O + N=O? (a) As temperature increases, the rate of the reaction increases. (b) Rate = k[CO][NO2] (c) The oxygen atom in CO must collide with the O in NO2 for an effective collision. (d) The rate determining step is a bimolecular collision. (e) All the statements are true.© Keeney-Kennicutt, 2010 A3 17&18. Rate data were collected for the following reaction at a particular temperature. What is the rate law expression? 2X(g) + Y(g) → Z(g) Experiment [X]initial [Y]initial Initial Rate of Reaction 1 0.10 M 0.10 M 0.040 M/s 2 0.10 M 0.20 M 0.16 M/s 3 0.30 M 0.20 M 0.48 M/s (a) Rate = k[X][Y] (b) Rate = k[X]2[Y] (c) Rate = k[X]2[Y]2 (d) Rate = k[X][Y]2 (e) Rate = k[X]2[Y]3 19&20. At 460oC, the value for Kc for the following reaction is 51: H2(g) + I2(g) →← 2HI(g) A mixture of H2, I2 and HI in a vessel has the following concentrations: [H2] = 1.0 M [I2] = 3.0 M [HI] = 0.10 M Which one of the following statements concerning the reaction quotient, Qc, is TRUE for the above system? (a) Qc = Kc; and the system is at equilibrium. (b) Qc = 0.0033; more HI will be produced as the system proceeds to equilibrium. (c) Qc = 0.0033; more H2 and I2 will be produced as the system proceeds to equilibrium. (d) Qc = 300; more HI will be produced as the system proceeds to equilibrium. (e) Qc = 300; more H2 and I2 will be produced as the system proceeds to equilibrium© Keeney-Kennicutt, 2010 A4 21&22. Estimate the sublimation temperature of solid iodine, I2, given the following data from thermodynamic tables. Assume that ΔHo f 298 and So do not change with temperature: I2(s) →← I2(g) ΔHo f 298 (kJ/mol) So (J/mol⋅K) ΔGo f 298 (kJ/mol) I2(s) 0 116.1 0 I2(g) 62.4 260.6 19.4 (a) 273 oC (b) 317 oC (c) 159 oC (d) 425 oC (e) 0.45 oC 23&24. Consider the following equilibrium at 300oC: A + B →← 2C, with a Kc of 64.0. Initially, there is 3.00 M of C in the container. What is the concentration of C in the container after the system has reached equilibrium? (a) 1.6 M (b) 2.4 M (c) 2.8 M (d) 0.35 M (e) 0.70 M© Keeney-Kennicutt, 2010 A5 25&26. Consider the reaction: A → B. What is the activation energy for the forward reaction (in kJ) when the rate constant at 20oC is 0.0850 min-1 and the rate constant at 50oC is 1.34 min-1? ln⎝⎛⎠⎞k2k1 = – EaR ⎝⎛⎠⎞1T2 - 1T1 or ln⎝⎛⎠⎞k2k1 = EaR ⎝⎛⎠⎞T2 - T1T1T2 or ln⎝⎛⎠⎞k2k1 = EaR ⎝⎛⎠⎞1T1 - 1T2 R = 8.314 J/mol·K (a) 764 J (b) 39.4 kJ (c) 22.9 kJ (d) 72.3 kJ (e) 61.5 kJ 27&28. The decomposition of N2O5, dinitrogen pentoxide, is a first order reaction. If the half-life of the decomposition is 17.3 min at 80oC, what percentage of N2O5 will be converted to products after 4,000 seconds? N2O5 → O2 + N2O3 (a) 39% (b) 61% (c) 93% (d) 7.2% (e) 3x10–70 %© Keeney-Kennicutt, 2010 A6 29&30. Calculate ΔGo for the following reaction at 25oC