CHEMISTRY 107 Section 501 Final Exam Version A December 12, 2016 Dr. Larry Brown

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1 NAME: (print) UIN #: CHEMISTRY 107 Section 501 Final Exam Version A December 12, 2016 Dr. Larry Brown This is a 2-hour exam, and contains 11 problems. 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 do not need to turn that page in. Point values for the different questions are as indicated. Some constants and conversion factors you might need are listed below. Please show ALL of your work 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 might be 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 = mol 1 1 atm = 760 torr = 14.7 lb in 2 Gas Constant R = L atm mol 1 K 1 1 atm = 101,324 N m 2 = J mol 1 K 1 0 C = K = L torr mol 1 K 1 1 m = 10 9 nm = pm Planck s Constant h = J s 1 L = 1,000 ml Speed of light c = 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 2016 L.S. Brown A1

2 1. True or False? (Please enter T or F in the blank in front of each statement. 2 points each.) BE SURE THAT YOU READ EACH STATEMENT VERY CAREFULLY! (a). When a chemical equation is properly balanced, the number of molecules on each side must be equal. (b). The nitrogen and oxygen molecules in this room right now have the same average kinetic energies but slightly different average speeds. (c). Wavelength is one way to specify the color of visible light. (d). A 3p orbital has quantum numbers of n = 3 and l = 2. (e). The common monatomic ions all have electron configurations that resemble those of the noble gases. (f). A Cs + ion should have a smaller radius than a neutral Xe atom. (g). Metals have higher ionization energies than nonmetals. (h). The O H bonds in a water molecule are classified as ionic. (i). Of all the elements in the 4 th row of the periodic table (K through Kr), bromine (Br) has the highest electronegativity. (j). Doping germanium (Ge) with traces of arsenic (As) would produce a p-type semiconductor. (For Grading) Problem 1 (20) 2 (18) 3 (8) 4 (8) 5 (18) 6 (15) 7 (12) 8 (20) 9 (9) 10 (12) 11 (10) TOTAL (150) Score A L.S. Brown

3 NAME: 2. Answer each of the following questions, and write your answer neatly on the line provided. Any calculations required here should be simple, and these will be graded as either right or wrong. You do not need to show your work for these, but you can certainly use the space below to work them out where necessary. No more than 2 significant figures here, please! (3 pts) (a). How many molecules are there in 88 g of propane (C 3 H 8 )? (3 pts) (b). How many moles of carbon atoms are there in 2 moles of benzene (C 6 H 6 )? (3 pts) (c). How many moles of NO 3 ions are there in 200 ml of 0.50 M Al(NO 3 ) 3? (3 pts) (d). What is the pressure in a 10.0-L balloon inflated with 0.5 mol He at 25 C? (3 pts) (e). If 125 ml of 12 M HCl is diluted to a final volume of 1.0 L, what is the molarity of the resulting solution? (3 pts) (f). If 200 ml of water at 30 C is added to 100 ml of water at 90 C in an insulated container, what should the temperature of the resulting mixture be? 2016 L.S. Brown A3

4 (8 pts) 3. The energies of high-energy photons are often given in units of electron volts, or ev. (1 ev = 96,485 J/mol) Gamma rays with an energy of ev are used in radiation treatment for some cancers. Find the wavelength of these gamma rays, and express your answer in picometers. (1 pm = m) (8 pts) 4. Sketch an energy level diagram in the box below, showing all of the orbitals up through the 3p subshell. Label each subshell. Then use arrows in your diagram to show the electron configuration of silicon E A L.S. Brown

5 NAME: 5. Consider the following Lewis structures, and assume that they are all drawn correctly. Then use the structures to answer the questions that follow. Structure A Structure B Structure C Cl P Cl Cl F F S F F Structure D O N O Structure E O H C O Please answer in the space provided. For (a) (c), write the appropriate letter or letters (i.e., A, B, etc.) to indicate your answers, and be sure to include all of the structures that fit the description. (These will be graded as simply right or wrong, so be careful!) (3 pts) (a). Which structure(s) represent ions? (3 pts) (b). For which structure(s) should additional resonance forms be drawn? (3 pts) (c). Which structure(s) represent planar species? (Meaning all of the atoms must be coplanar.) (3 pts) (d). How many sigma bonds would be present in structure A? (3 pts) (e). How many pi bonds would be present in structure A? (3 pts) (f). What is the correct shape for the species represented by structure C? 2016 L.S. Brown A5

6 6. The reaction shown below is involved in the refining of lead from its ores. 2 PbS(s) + 3 O 2 (g) 2 PbO(s) + 2 SO 2 (g) ΔH = kj Compound H f (kj/mol) S (J/mol/K) G f (kj/mol) PbS(s) 100.4? 98.7 O 2 (g) PbO(s) SO 2 (g) (5 pts) (a). S for the reaction above is J/K. Find S for PbS(s). (5 pts) (b). Calculate G for the reaction above at the standard temperature of 298 K. (There are at least 2 possible ways that you could do this with the information given above.) A L.S. Brown

7 (Problem #6, continued) NAME: (5 pts) (c). At what temperatures would this reaction be thermodynamically spontaneous? 2016 L.S. Brown A7

8 (12 pts) 7. Although it is extremely toxic, hydrogen cyanide (HCN) is an important industrial chemical, used in the manufacture of several common plastics. HCN can be produced by the gas phase reaction of methane (CH 4 ) and ammonia (NH 3 ), with hydrogen gas also being formed as a byproduct. All four substances involved are gases, and above 700 C, this reaction goes to completion. A 1,000. L tank is evacuated, and then filled with 2.00 kg of CH 4 and 1.10 kg of NH 3. The mixture is heated to 845 C, and held at that temperature. Find the total pressure in the tank when the reaction is complete. (This is NOT a simple problem. Be sure to show all of your work!) A L.S. Brown

9 NAME: 8. The following two-step mechanism has been proposed for a reaction. (Step #1 & Step #2 are both elementary reactions.) Step #1: NO(g) + O 2 (g) Á OONO(g) (fast equilibrium) Step #2: NO(g) + OONO(g) 2 NO 2 (g) (slow) (3 pts) (a). Write a balanced equation for the overall reaction that would result from this mechanism. (3 pts) (b). Which step(s) in the mechanism should be rate-determining? (Circle your choice.) Step #1 Step #2 Both Neither (3 pts) (c). Assuming the proposed mechanism is correct, which of the rate laws below would be expected? (Circle your choice.) Rate = k[no][o 2 ] Rate = k[no] 2 [O 2 ] Rate = k[no][o 2 ] 2 Rate = k[no] 2 [O 2 ] 2 Rate = k[no] 1/2 [O 2 ] 2016 L.S. Brown A9

10 9. The decomposition of N 2 O 5 was studied at 356 K, and the data were plotted to give the graphs below. Concentrations are in moles/liter and time is in minutes. 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) [N 2 O 5 ] time (min) ln [N 2 O 5 ] time (min) /[N 2 O 5 ] A time (min) 2016 L.S. Brown

11 NAME: (Problem #9, continued) (4 pts) (a). What is the order of reaction for the decomposition of N 2 O 5? Explain VERY BRIEFLY how you have chosen your answer. (4 pts) (b). Estimate the numerical value of the rate constant at the temperature of the experiment, including the correct units. Explain VERY BRIEFLY how you have found your value. (4 pts) (c). Estimate the half-life for the reaction at this temperature, and explain VERY BRIEFLY how you got your value L.S. Brown A11

12 (Problem #9, continued) (8 pts) (d). The experiment was repeated at various temperatures, producing the data shown in the graph below. Use the graph below to estimate for the activation energy for the reaction and express your result in kj/mol. 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) ln k /T (K 1 ) A L.S. Brown

13 NAME: 10. Nitrous acid (HNO 2 ) reacts as a weak acid when dissolved in water. HNO 2 (aq) + H 2 O(l) Á H 3 O + (aq) + NO 2 (aq) (4 pts) (a). Write the expression for K a for nitrous acid in terms of the concentrations of the relevant species. (8 pts) (b). In a 0.10 M solution of HNO 2, the acid is 6.56% dissociated. Find the value of K a for HNO L.S. Brown A13

14 (10 pts) 11. A reaction mixture is prepared so that it contains equal concentrations of H 2 and Cl 2, and then heated to 2800 C. When the equilibrium shown below is established, the concentration of HCl is M. What were the initial concentrations of the H 2 and Cl 2? (The initial mixture contains only H 2 and Cl 2.) Be sure to set up your work in an organized fashion so that you may receive some credit if you fail to solve the problem! H 2 (g) + Cl 2 (g) Á 2 HCl(g) K eq = 193 at 2800 C A L.S. Brown