(print) NAME: __________________________
UIN #: ________________________
SECTION # ________________
CHEMISTRY 107
Sections 501--513 Final ExamVersion A December 10, 2007 Dr. Larry Brown
This is a 2-hour exam, and contains 11 problems. There should be 14 numbered pages, including this one. The last page is blank, and can be used if you need more space on any problems. 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, and a few others appear in some of the problems. 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 Avogadro's Number NA = 6.022 1023 mol1 Gas Constant R = 0.08206 L atm mol 1 K 1 = 62.364 L torr mol 1 K 1 = 8.314 J mol 1 K 1 Planck's Constant Speed of light h = 6.626 1034 J s c = 2.9979 108 m s 1 CONVERSION FACTORS 1 atm = 760 torr = 14.7 lb in-2 1 atm = 101,324 N m 2 0C = 273.15 K 1 m = 109 nm = 106 m 1 L = 1,000 mL 1 kJ = 1,000 J = 103 MJ
"On my honor, as an Aggie, I have neither given nor received unauthorized aid on this exam." SIGNATURE: ___________________________________________
Exam 2007 L.S. Brown
A1
(18 pts) 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! (For Grading) _____ (a). NaOH is a strong base. Therefore NaOH will dissociate in solution to produce Na+ and OH ions. _____ (b). Elements from the second column of the periodic table (Be, Mg, etc.) usually form ionic compounds. _____ (c). If an atom has a large positive electron affinity value, it is likely to form an anion. _____ (d). The best way to tell if two atoms can form an ionic bond is to compare their ionization energies. _____ (e). When a semiconductor material is called "n-type," that means it contains nitrogen. _____ (f). As the wavelength of light increases, the photon energy decreases. _____ (g). According to the kinetic theory of gases, a gas will behave most ideally near room temperature and pressure. _____ (h). The Arrhenius equation can be used to help predict the rate of a chemical reaction as a function of temperature. _____ (i). One reason why most chemical reactions are faster at higher temperatures is that activation energy decreases at higher temperatures.
Problem Score
1 (18) 2 (13) 3 (12) 4 (10) 5 (7) 6 (10) 7 (11) 8 (20) 9 (25) 10 (10) 11 (14) TOTAL
A2
2007 L.S. Brown
NAME:_________________________________ 2. Methyl methacrylate, with the structure shown below, is the monomer from which Plexiglas is made.
O
O
(3 pts) (a). In the space provided, write the correct molecular formula for this molecule.
Formula: ________________________ (3 pts) (b). Assuming your answer to (a) is correct, find the molar mass of the molecule. (A whole number answer is OK here.) Molar mass: ________________________ (3 pts) (c). How many molecules would be present in one gram of this compound?
# of molecules per gram: ________________________ (2 pts) (d). How many sigma bonds are present in this molecule? (Count carefully this is tricky!) # of sigma bonds: ________________________ (2 pts) (e). How many pi bonds are present in this molecule? # of pi bonds: ________________________
2007 L.S. Brown
A3
3. Consider the elements in the second and third rows of the periodic table (lithium through argon). Answer the following questions by writing the symbols of the elements you choose in the blanks provided. (2 pts) (a). Which one of these elements has the smallest atomic radius? _________ (2 pts) (b). Which one of these elements has the greatest electronegativity? _________ (2 pts) (c). Which one of these elements has the highest ionization energy? _________ (2 pts) (d). Which one of these elements has an electron configuration of 2s2 2p3 ? _________ (2 pts) (e). Which two of these elements could be the element `X' in an ionic compound with the formula XF? _________ (2 pts) (f). Which two of these elements could be the element `X' in a compound with the formula XH4 ? _________
A4
2007 L.S. Brown
NAME:_________________________________ 4. Dr. Brown's new house will need to use propane (C3 H8 ) gas to fuel the range, because natural gas service is not available at the location. The propane is stored as a liquid, and the gas to be burned is produced as the liquid evaporates. (10 pts) The propane gas consumed by a typical range burner at high power in 1 hour would occupy roughly 165 L at 25C and 1 atm, and the range will have 6 burners. If the liquid propane were to be stored in a tank that holds 500-gallons, how many hours would it take for the range to an consume entire tankful of propane? The density of liquid propane is 0.5077 kg/L. 1 gallon = 3.7854 L.
2007 L.S. Brown
A5
(7 pts)
5. Dissociation of O2 molecules into oxygen atoms is an important step in the formation and destruction of ozone in the atmosphere. The energy needed for the dissociation is provided by photons of ultraviolet light. The bond energy for the O2 molecule is 495 kJ/mol. Use this to find the longest wavelength of light for which a single photon is capable of dissociating an O2 molecule.
A6
2007 L.S. Brown
NAME:_________________________________ (10 pts) 6. Diborane (B2 H6 ) can be produced by the following reaction. 2NaBH4 (s) + 2H3 PO4 (aq) B2 H6 (g) + 2NaH2 PO4 (aq) + 2H2 (g) Suppose that 18.9 g of solid NaBH4 is allowed to react with 250 mL of 0.400 M H3 PO4 . If all of the B2 H6 produced is collected at 25C and 760 torr, what volume would it occupy?
2007 L.S. Brown
A7
7. Consider the following Lewis structure, where E represents an unknown element.
O E O O
(4 pts) (a). List at least 2 possible identities for the element E, and give a BRIEF explanation of how you made your choice.
(3 pts) (b). What shape would this ion be?
(4 pts) (c). Consider the two possible structures shown below for the ozone molecule (O3 ): a bent geometry (with two resonance forms) and a triangular ring.
O
O
O O O O
O
O
O
The bent structure is actually the correct one. Briefly explain why the bent structure is better than the triangular ring. (Hint: It may help to think about the expected hybridization of the central Oatom(s).)
A8
2007 L.S. Brown
NAME:_________________________________ 8. Kinetic studies show that the reaction below proceeds at high temperatures and follows a first order rate law. 2 N2 O5 (g) 4NO2 (g) + O2 (g) (6 pts) (a). Suppose a sample of pure N2 O5 is introduced into an empty container and allowed to react at a constant temperature. Sketch the expected results on the axes below. (This is a qualitative question no calculations should be needed. But be sure that your sketches are clear enough to show that you know how these graphs would look.)
[N2 O5 ]
ln[ N2 O5 ]
time (s)
time (s)
(4 pts) (b). Assume that you had accurate versions of the graphs you sketched. How could you find the rate constant for the reaction from those data? PLEASE BE SPECIFIC BUT BRIEF.
(2 pts) (c). Will the rate constant for this reaction be positive or negative? (4 pts) (d). What will the correct units for the rate constant be?
__________
(4 pts) (e). Would these graphs allow you to find the activation energy for the reaction? If so, how? If not, what additional experiment could you do to make it possible to find the activation energy? Again, PLEASE BE CLEAR BUT BRIEF.
2007 L.S. Brown
A9
9. The reaction shown below is involved in the refining of iron. 2Fe2 O3 (s) + 3C(s, graphite) 4Fe(s) + 3CO2 (g) Compound Fe2 O3 (s) C(s, graphite) Fe(s) CO2 (g) Hf (kJ/mol) S (J/mol/K) Gf (kJ/mol) 824.2 0 0 393.5 ? 5.740 27.3 213.6 742.2 0 0 394.4
(6 pts) (a). Use data from the table above as needed to find H for the reaction.
(6 pts) (b). S for the reaction above is 557.98 J/K. Find S for Fe2 O3 (s).
A10
2007 L.S. Brown
NAME:_________________________________ (Problem #9, continued) (6 pts) (c). Calculate G for the reaction at the standard temperature of 298 K. (There are at least 2 possible ways that you could do this.)
(7 pts) (d). At what temperatures would this reaction be thermodynamically spontaneous?
2007 L.S. Brown
A11
(10 pts) 10. Consider a self-heating coffee can. The can is equipped with an outer insulating jacket, and the bottom of the can contains a pellet of calcium oxide (CaO) and some water. Pushing a button on the bottom of the can breaks a foil seal and allows the CaO to dissolve in the water. CaO(s) + H2 O() Ca(OH)2 (aq) H = 81.5 kJ The heat released in the chemical reaction warms the coffee. The can contains 10 oz. of coffee. The heating unit contains 35.0 g of CaO, and 35.0 g of water. If the can and the coffee are initially at room temperature (23C), what is the highest temperature that the coffee can reach? Assume that the Ca(OH)2 solution and the coffee both have the same heat capacity and density as pure water: 4.184 J g-1 C-1 and 1 g/mL, and that the can has a heat capacity of 40 J C-1. 1 oz. = 29.57 mL. (HINT: Think of this like a calorimetry problem. Be sure to account for ALL of the things that will be getting hot.) A can like this was actually marketed for a while under the Wolfgang Puck brand name, but it seems to have been discontinued.
10 oz. of coffee 35 g H2 O
35 g CaO
A12
2007 L.S. Brown
NAME:_________________________________ 11. Nitrogen and oxygen do not react appreciably at room temperature, as illustrated by our atmosphere. But at high temperatures, the reaction below can proceed to a measurable extent. N2 (g) + O2 (g) 2NO(g) (4 pts) (a). Write an expression for the equilibrium constant for this reaction in terms of the concentrations of the various substances.
(10 pts) (b). At 3000 K, Keq = 0.0153. If 0.200 mol of pure NO is injected into an evacuated 2.0-L container and heated to 3000K, what will be the equilibrium concentrations of all three substances?
2007 L.S. Brown
A13
A14
2007 L.S. Brown