NAME: (print) UIN #: CHEMISTRY 107 Section 501 Final Exam Version A December 11, 2017 Dr. Larry Brown This is a 2-hour exam and contains 21 multiple-choice questions and 5 free response problems. Point values for the different questions are as indicated. There should be 14 numbered pages, including this one. There is also a periodic table attached to the back of the exam; you may tear the periodic table page off, and you can use the grid on the back of it to keep a record of your multiple choice answers if you d like. Questions 1 21 should be answered on the scantron provided to you. Problems 22 26 should be answered on the exam paper, with ALL of your work shown as clearly as possible. This will help us award partial credit. Answers without supporting work may not receive credit. You may use a calculator for this exam, but you may NOT retrieve or use any alphanumeric information or algorithms that you might have stored in your calculator s memory. Please PRINT your name and UIN number above and SIGN the honor code statement below. Also, please put your name on every page of the exam, in case a page gets detached from the exam. Potentially Useful Information PHYSICAL CONSTANTS CONVERSION FACTORS Avogadro s Number N A = 6.022 10 23 mol 1 1 atm = 760 torr = 14.7 lb in 2 Gas Constant R = 0.08206 L atm mol 1 K 1 1 atm = 101,324 N m 2 = 8.314 J mol 1 K 1 0 C = 273.15 K = 62.364 L torr mol 1 K 1 1 m = 10 9 nm = 10 12 pm Planck s Constant h = 6.626 10 34 J s 1 L = 1,000 ml Speed of light c = 2.9979 10 8 m s 1 1 kj = 1,000 J "On my honor, as an Aggie, I have neither given nor received unauthorized aid on this exam." SIGNATURE: Exam 2017 L.S. Brown A1
(4 pts) 1. Write a chemical equation for the complete combustion of hexane (C6H14), and balance it using the smallest possible whole number coefficients. How many moles of O2 appear in the equation? (A) 10 (B) 9 (C) 13 (D) 17 (E) 19 (4 pts) 2. What is the correct molar mass of the molecule whose structure is shown below? (A) 107 g mol 1 (B) 136 g mol 1 (C) 135 g mol 1 (D) 134 g mol 1 (E) 129 g mol 1 (3 pts) 3. How many of the following statements are true? (i) (ii) (iii) (iv) A material with a large band gap will act as an electrical insulator. Doping silicon with phosphorus produces a p-type semiconductor. The conductivity of pure silicon increases at higher temperatures. The conduction band lies at higher energies than the valence band. (A) 0 (B) 1 (C) 2 (D) 3 (E) 4 (6 pts) 4. HCl and CaCl2 are both strong electrolytes. Suppose 125 ml of 0.120 M HCl and 250 ml of 0.240 M CaCl2 are mixed. Assuming a final volume of 375 ml, what is the concentration of Cl ions in the resulting solution? (A) 0.360 M (B) 0.200 M (C) 0.180 M (D) 0.240 M (E) 0.300 M A2 2017 L.S. Brown
NAME: For questions 5 8, consider the atoms in the 2 nd through 4 th rows of the periodic table (lithium through krypton, Z = 3 36). (3 pts) 5. Which one of these elements would have the largest radius? (A) Li (B) Ne (C) K (D) Kr (E) Zn (3 pts) 6. Which one of these elements would have the largest ionization energy? (A) N (B) Ne (C) K (D) Kr (E) Zn (3 pts) 7. Which one of these elements would have the largest electronegativity? (A) F (B) Ne (C) K (D) Kr (E) Zn (3 pts) 8. Which two of these elements would be most likely to be the element X in a trigonal bipyramidal compound with the formula XCl5? (A) S, P (B) N, P (C) Br, Kr (D) P, As (E) As, Se (6 pts) 9. A typical human eye must be exposed to at least 4.0 10 17 J of radiant energy in order to detect light. For green light with a wavelength of 532 nm, what is the minimum number of photons the eye could detect? (A) 107 (B) 45 (C) 89 (D) 134 (E) 6 2017 L.S. Brown A3
Although their formulas look very similar, SeF4, GeF4, and XeF4 all have different molecular shapes. Draw Lewis structures for these three compounds and use your structures to answer questions 10 12. (4 pts) 10. What is the correct hybridization for the germanium atom in GeF4? (A) sp (B) sp 2 (C) sp 3 (D) s (E) p (4 pts) 11. What is the correct molecular geometry for SeF4? (A) tetrahedral (B) trigonal pyramidal (C) square planar (D) trigonal bipyramidal (E) see-saw (4 pts) 12. What is the approximate F Xe F bond angle in XeF4? (A) 90 (B) 109.5 (C) 120 (D) 60 (E) 45 (6 pts) 13. Suppose that a 58.25-g block of nickel at 145 C is placed in a thermos bottle containing 125 g of water at 21.0 C. Assuming that no heat is lost to or gained from the surroundings, what will be the final temperature of the nickel and water? (The specific heat of water is 4.184 J K 1 mol 1 and that of nickel is 0.460 J K 1 mol 1.) (A) 34.2 C (B) 27.0 C (C) 76.5 C (D) 31.3 C (E) 83.0 C A4 2017 L.S. Brown
NAME: Kinetic studies of the following reaction at constant temperature produced the data below. Use this information to answer questions 14 & 15. NO2(g) + CO(g) NO(g) + CO2(g) Experiment [NO2]0 (M) [CO] (M) Initial Rate (M/s) 1 0.12 0.11 5.0 10 3 2 0.48 0.33 8.0 10 2 3 0.12 0.27 5.0 10 3 4 0.29 0.81??? (5 pts) 14. What is the correct rate law for this reaction? (A) rate = k [NO2][CO] (B) rate = k [NO2] 2 [CO] (C) rate = k [NO2][CO] 2 (D) rate = k [NO2] 2 (E) rate = k [CO] (5 pts) 15. Find the missing value for the rate in Experiment #4 (indicated in the table by??? ). (A) 0.012 M/s (B) 0.029 M/s (C) 0.089 M/s (D) 0.0090 M/s (E) 0.037 M/s 2017 L.S. Brown A5
(4 pts) 16. A mixture of H2S, H2, and S2 is prepared and allowed to establish the following equilibrium. 2 H2S(g) Á 2 H2(g) + S2(g) The graph below shows the concentrations of all three substances as the system comes to equilibrium. Concentration Time 1 2 3 Which choice properly matches the labels on the three curves with the substances involved in the reaction? (A) Curve 1 = H2S, Curve 2 = H2, Curve 3 = S2 (B) Curve 1 = S2, Curve 2 = H2, Curve 3 = H2S (C) Curve 1 = H2S, Curve 2 = S2, Curve 3 = H2 (D) Curve 1 = H2, Curve 2 = H2S, Curve 3 = S2 (E) Curve 1 = H2, Curve 2 = S2, Curve 3 = H2S (5 pts) 17. Consider the following reaction mechanism. 2 NO(g) N2O2(g) (slow) N2O2(g) + O2(g) N2O4(g) (fast) N2O4(g) 2 NO2(g) (fast) Choose the correct overall reaction and rate law corresponding to this mechanism. (A) 2 NO(g) + O2(g) 2 NO2(g) Rate = k[no] 2 [O2] (B) 2 NO(g) 2 NO2(g) Rate = k[no] 2 (C) 2 NO(g) N2O2(g) Rate = k[no] 2 (D) 2 NO(g) + O2(g) 2 NO2(g) Rate = k[no] 2 (E) 2 NO(g) + O2(g) 2 NO2(g) Rate = k[no] 2 [N2O2][O][N2O4] A6 2017 L.S. Brown
NAME: Consider a reversible elementary reaction 2 A(g) Á B(g) + C(g) as described by the following potential energy diagram. Use the figure to answer questions 18 & 19. Potential energy (kj/mol) Reaction coordinate (4 pts) 18. What is the activation energy for the reaction? (A) 250 kj/mol (B) 50 kj/mol (C) 150 kj/mol (D) 200 kj/mol (E) The activation energy cannot be determined from the diagram (4 pts) 19. What is DE for the reaction? (A) 100 kj/mol (B) 50 kj/mol (C) 50 kj/mol (D) 200 kj/mol (E) 150 kj/mol 2017 L.S. Brown A7
(5 pts) 20. The graph shown below was obtained in an experiment studying the following reaction at a particular temperature. NH4NCO(aq) (NH2)2CO(aq) Which choice gives the correct rate law and rate constant for the reaction at this temperature? 1/ [NH4NCO] 10 9 8 7 6 5 4 3 2 1 0 0 200 400 600 Time (min) (A) Rate = k [NH4NCO], k = 91 min 1 (B) Rate = k [NH4NCO], k = 0.011 min 1 (C) Rate = k [NH4NCO] 2, k = 91 M min 1 (D) Rate = k [NH4NCO] 2, k = 0.011 M min 1 (E) Rate = k [NH4NCO] 2, k = 0.011 M min 1 (5 pts) 21. Hypobromous acid (HOBr) reacts as a weak acid in aqueous solution. Which of the following choices correctly expresses Ka, the acid dissociation constant, for HOBr? (A) K a = [H 3 O+ ][OBr ] [HOBr] (B) K (C) [H 2 O][HOBr] a = [H 3 O + ][OBr ] K a = [H 3 O+ ][OBr ] [HOBr] (D) K a = [HOBr][H 2 O] [H 3 O + ][OBr ] (E) K a = [H 3 O + ] [HOBr] + [H 3 O + ] A8 2017 L.S. Brown
NAME: (10 pts) 22. At very high temperatures, Si3N4 can be produced by the direct reaction of solid silicon and nitrogen gas. 3 Si(s) + 2 N2(g) Si3N4 Suppose that 1.4 kg of silicon and 1.2 kg of nitrogen are sealed into a suitable reaction vessel and heated to a temperature where the reaction goes to completion. What mass of silicon nitride could be produced if the reaction yield is 79.7%? 2017 L.S. Brown A9
23. The reaction shown below is involved in the refining of titanium from its ores. TiCl4(g) + 2 Mg(s) 2 MgCl2(!) + Ti(s) DH = 440.0 kj S = 128.4 J/K Compound Hf (kj/mol) S (J/mol/K) Gf (kj/mol) TiCl4(g) 763.2 353.2 726.3 Mg(s) 0 32.5 0 Ti(s) 0 30.6 0 (6 pts) (a). Use the data above to find S for MgCl2(!). (8 pts) (b). Use the data above to calculate Gf for MgCl2(!). A10 2017 L.S. Brown
(Problem #23, continued) NAME: (6 pts) (c). At what temperatures would this reaction be thermodynamically spontaneous? 2017 L.S. Brown A11
(10 pts) 24. The rate constant for a particular first-order reaction is 1.60 10 4 s 1 at 152 C, and the activation energy for the reaction is 104 kj/mol. At what temperature would the rate constant be 9.60 10 4 s 1? A12 2017 L.S. Brown
NAME: (10 pts) 25. At high temperatures, sulfur trioxide begins to decompose to give sulfur dioxide and oxygen, according to the following equilibrium. 2 SO3(g) Á 2 SO2(g) + O2(g) A 0.664 g sample of SO3 is sealed into a 1.00 L flask, which was first evacuated so that it contains no air or other gases. The flask is heated to 1125 K, and when the system has come to equilibrium, the total pressure reaches 1.034 atm. Find the value of the equilibrium constant for the above reaction at 1125 K. (HINT: Start by finding the total number of moles of gas present at equilibrium.) 2017 L.S. Brown A13
(10 pts) 26. The composition of an iron-aluminum alloy can be determined chemically by reacting it with hydrochloric acid. 2 Al(s) + 6 HCl(aq) 2 AlCl3(aq) + 3 H2(g) Fe(s) + 2 HCl(aq) FeCl2(aq) + H2(g) When a 6.724-g sample of a particular iron-aluminum alloy was dissolved in excess hydrochloric acid, 3.76 L of H2(g) at 842 torr and 24 C were produced. What was the mass percentage of aluminum in the alloy? (HINT: The alloy contains only iron and aluminum. If you aren t sure how to approach this, try thinking about how the mass of H2 produced would change if the percentage of aluminum in the alloy increased or decreased.) (For Grading) Problem Score 22 (10) 23 (20) 24 (10) 25 (10) 26 (10) A14 TOTAL (60) 2017 L.S. Brown