Dr. D. DeCoste T.A (80 pts.) 21 (30 pts.) 22 (25 pts.) 23 (25 pts.) Total (160 pts)

Transcription

1 CHEMISTRY 202 Hour Exam III December 6, 2018 Dr. D. DeCoste Name KEY Signature T.A. This exam contains 23 questions on 13 numbered pages. Check now to make sure you have a complete exam. You have two hours to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and provide complete answers to questions 21, 22, and (80 pts.) 21 (30 pts.) 22 (25 pts.) 23 (25 pts.) Total (160 pts) Useful Information: PV = nrt R = J/Kmol = Latm/molK k = Ae Ea/RT k 2 E ln( ) = a 1 [ k1 R T 1 1 T ] 2

2 Hour Exam III Page No If we have access to the following values: The first and second ionization energies for Na. The ionization energy for Mg 2+. The third electron affinity value for P. The electron affinity value for O. How many of the following could be determined? I. The ionization energy of Na +. II. The electron affinity of Na. III. The lattice energy for Na3P. IV. The electron affinity value of Mg +. V. The electron affinity value of Mg 2+. VI. The ionization energy of O. a) 0 b) 1 c) 3 d) 4 e) 5 2. Which of the following best explains the ratio of 2 1 nd st ionization energy ionization energy for Ne and F? a) The ratio for Ne is much greater than that for F because Ne has more protons than F. b) The ratio for Ne is much greater than that for F because Ne has a filled outer shell of electrons. c) The ratio for Ne is much less than that for F because the 1 st ionization energy for Ne is greater than the 1 st ionization energy of F. d) The ratio for Ne is much less than that for F because Ne has a filled outer shell of electrons. e) The ratio for Ne is about the same as that for F since all electrons are removed from the same energy level. 3. Hydrogen cyanide gas (HCN) can react with hydrogen gas to produce methylamine (CH3NH2) gas. Use the bond energies in Table 13.6 (on the page with the periodic table) to estimate ΔH for the production of one mole of methylamine from hydrogen cyanide and hydrogen gas. a) 158 kj b) 590 kj c) 590 kj d) 744 kj e) 744 kj 4. Consider the Lewis structure for a phosphate ion that obeys the octet rule. What is the formal charge on phosphorus? a) 3 b) 1 c) 0 d) +1 e) Which of the following pairs of molecules can be described with the same geometry (electron-pair arrangement) although they have different shapes (molecular structures)? a) H2O and SO2 b) OCl2 and OF2 c) NH3 and CH4 d) SF4 and CF4 e) At least two of the above (a-d) have the same geometry but different shapes.

3 Hour Exam III Page No. 2 6, 7. The following is a skeletal structure of the amino acid histidine (as seen in the practice exam!). Complete the Lewis structure such that all atoms have a formal charge of zero, and answer the following questions. 6. How many lone pairs of electrons are present in the completed Lewis structure? a) 3 b) 5 c) 6 d) 7 e) 8 7. Which of the following best describes the geometry (electron-pair arrangement) around the carbon atom labeled 1 and the nitrogen atom labeled 2? Carbon atom (#1) Nitrogen atom (#2) a) tetrahedral tetrahedral b) trigonal pyramid trigonal planar c) trigonal planar trigonal pyramid d) trigonal planar tetrahedral e) trigonal pyramid trigonal pyramid How many of the following statements is/are true? I. Ethanol (CH3CH2OH) has a higher boiling point than dimethyl ether (CH3OCH3). II. Propane (C3H8) has a higher boiling point than dimethyl ether (CH3OCH3). III. Cl2 is a more ideal gas than Br2. IV. HCl has a higher vapor pressure than HF at 0 C. V. NaCl has a higher melting point than HCl. a) 0 b) 1 c) 2 d) 3 e) 4 9. Consider a reaction of the type aa Products. You monitor the time it takes for 20.0% of A to react but neglect to record the original or final concentrations of A. Can we determine the rate constant, k? a) Yes, if the rate law is zero order in A. b) Yes, if the rate law is first order in A. c) Yes, if the rate law is second order in A. d) You could determine the value of k for at least two of the above (a-c). e) You cannot determine the value of k for any of the above (a-c).

4 Hour Exam III Page No The activation energy for the reaction H2O(g) + CO(g) H2(g) + CO2(g) is 35 kj/mol and ΔE for the reaction is 94 kj/mol. Determine the activation energy for the reaction: H2(g) + CO2(g) H2O(g) + CO(g) a) 35 kj/mol b) 59 kj/mol c) 59 kj/mol d) 129 kj/mol e) 129 kj/mol 11. You run the ferroin lab in Chemistry 203 at 25 C and then again at 55 C. You find that at the higher temperature, the reaction runs about three times as fast as at the lower temperature. From these data, what is the approximate activation energy? Note: no claim is made that these data will give you the same results as you found in lab. a) 290 J/mol b) 30. J/mol c) 30. kj/mol d) 420 J/mol e) 420 kj/mol 12. The reaction A + 2B C has the following proposed mechanism. Determine the rate law. Step 1: A + B D (fast equilibrium) Step 2: D + A E (slow) Step 3: E + B C + A (fast) a) k[a] b) k[a][b] c) k[a] 2 [B] d) k[a] 2 [B] 2 e) k[a][b] Consider the following reaction: The following mechanism is proposed: 2NO(g) + H2(g) N2O(g) + H2O(g) 1. 2NO(g) N2O2(g) 2. N2O2(g) + H2(g) N2O(g) + H2O(g) d [H ] Using rate = 2, use the steady-state approximation and determine the apparent rate dt law for the proposed mechanism when the concentration of [H2] is very high. Note: the k terms in the rate laws takes into account the rate law constants for the elementary steps and are not necessarily equivalent in the choices below. a) rate = k[no] b) rate = k[no][h2] c) rate = k[no][h2] 2 d) rate = k[no] 2 [H2] e) rate = k[no] 2

5 Hour Exam III Page No How many of the following can change the activation energy of a given reaction? Increasing the concentrations of the reactants. Increasing the temperature. Adding a catalyst. Decreasing the volume (by increasing the pressure) for a gaseous reaction taking place in a container fitted with a piston. Stirring the reaction vessel. a) 0 b) 1 c) 2 d) 3 e) You run the iodine clock reaction (as seen in lecture), once at 25 C and again at 45 C. The first reaction (at 25 C) takes 50.0 sec. If the activation energy for the iodine clock reaction is 31.4 kj/mol, determine the time it takes for the second reaction (at 45 C). a) 8.41 sec b) 22.5 sec c) 38.9 sec d) 50.0 sec e) 111 sec 16. Consider a reaction of the type aa Products in which [A] = 5.00 M and the rate constant, k, has a value of The reaction proceeds until 40.0% of A has reacted. Which of the following statements best describes how the order of the rate law affects its rate? a) If the rate law is second order in A, the rate to react 40.0% of A is 3.83 times as fast as if the rate law was first order. b) If the rate law is second order in A, the rate to react 40.0% of A is 3.05 times as fast as if the rate law was first order. c) If the rate law is first order in A, the rate to react 40.0% of A is 3.83 times as fast as if the rate law was second order. d) If the rate law is first order in A, the rate to react 40.0% of A is 3.05 times as fast as if the rate law was second order. e) The rates are the same whether the rate law is first or second order in A. 17. Consider a reaction of the type aa Products. How many of the following statements apply to zero order, first order, and second order rate laws as the reaction proceeds? [A] does not change over time. The rate law constant, k, does not change over time. The rate of the reaction does not change over time. The half-life of the reaction does not change over time. The activation energy does not change over time. Zero order First order Second order a) b) c) d) e) 3 1 4

6 Hour Exam III Page No A reaction of the type aa products in which [A]0 = M is 40.0% complete 50.0 minutes after it is started, and 57.1% complete minutes after it is started. How long after it is started does it take until it is 90.0% complete? a) 126 min b) 226 min c) 438 min d) 675 min e) 942 min , 20. Choose the best graph for the plots described below. a) b) c) d) e) 19. k vs T (K) for a typical chemical reaction. A 20. t ½ vs [A] for a reaction type aa products which is first order in A. C