CHEMISTRY 107 Section 501 Exam #3 Version A November 16, 2016 Dr. Larry Brown

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1 NAME: (print) UIN #: CHEMISTRY 107 Section 501 Exam #3 Version A November 16, 2016 Dr. Larry Brown This is a 50-minute exam, and contains 7 problems. There should be 10 numbered pages, including this one. (The last page is blank.) 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 = 10 6 µm Planck s Constant h = J s 1 L = 1,000 ml Speed of light c = m s 1 1 kj = 1,000 J 1 kg = 1,000 g "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 (12 pts) 1. For (a) (f), fill in the blanks. (Please enter just one word in each blank. You may use the same word more than once if appropriate. 2 points for each blank.) (a). If a solid material has a very large band gap, then the material will be an electrical. (b). If a carbon atom has a steric number of, it will exhibit sp 2 hybridization. (c). According to the law of thermodynamics, the entropy of the universe is always increasing. (d). The heat of formation of a(n) in its most stable form at room temperature and pressure is always zero. (e). is a measure of molecular disorder. (f). A calorimetry experiment performed at constant would be a good way to measure H for a (For Grading) Problem Score 1 (12) 2 (10) 3 (18) 4 (8) 5 (22) 6 (12) 7 (18) TOTAL chemical reaction. (10 pts) 2. Consider each of the following thermodynamic quantities, and decide whether it should be positive, negative, or zero. (Enter either +,, or 0 in the blank. 2 points each.) (a). S for liquid water at 25 C and 1 atm (b). H for the vaporization of liquid nitrogen (N2(!) N2(g)) (c). G for the melting of ice at 25 C (d). DH for the formation of gaseous F2 from F atoms (F(g) + F(g) F2(g)) (e). DS for the formation of gaseous F2 from F atoms (F(g) + F(g) F2(g)) A L.S. Brown

3 NAME: 3. The molecule shown below is responsible for the smell of popcorn. (Be careful here, as some of these are trickier than they seem. Realize that not all of the atoms are shown in the drawing. Lone pairs are also not shown here, but the structure obeys the octet rule.) O #1 N #2 (2 pts) (a). What type of orbital hybridization would be expected for the carbon atom labeled as #1? (2 pts) (b). What type of orbital hybridization would be expected for the nitrogen atom labeled as #2? (2 pts) (c). The molecule contains a total of 6 carbon-carbon bonds, with 2 different bond lengths. How many of the shorter bonds are present? (3 pts) (d). How many sigma bonds are present in the molecule? (3 pts) (e). How many pi bonds are present in the molecule? (6 pts) (f.) Consider each of the following structures, and write the correct shape for each molecule in the space below its Lewis structures. F F Br F F F O F Shape of BrF4: Shape of OF2: 2016 L.S. Brown A3

4 4. Band diagrams for 4 different materials are shown below. Be sure to read and follow the directions for where to put your answers! (4 pts) (a). In the blanks ABOVE the diagram, identify each diagram as a metal, an insulator, a pure elemental semiconductor, a p-type semiconductor, or an n-type semiconductor. (4 pts) (b). From the list below, choose the most likely identity for each of the substances represented above. Then write your choices in the blanks BELOW the band diagrams. Silicon Silicon doped with aluminum Diamond Silicon doped with phosphorous Potassium A L.S. Brown

5 NAME: 5. The space shuttle orbiter used the oxidation of methyl hydrazine, CH6N2, by dinitrogentetroxide for propulsion. The reaction is shown below, along with some thermodynamic data. 4 CH6N2(!) + 5 N2O4(g) 9 N2(g) + 12 H2O(g) + 4 CO2(g) DH = 4,738 kj Substance CH6N2(!) N2O4(g) N2(g) H2O(g) O2(g) H2(g) C(graphite) CO2(g) S (J K 1 mol 1 ) (4 pts) (a). Write a balanced chemical equation for the formation reaction for liquid CH6N2(!). (Hydrogen, nitrogen, and oxygen are all diatomic gases under standard state conditions, and the standard state of carbon is graphite.) (6 pts) (b). Find DS for the reaction above between CH6N2 and N2O L.S. Brown A5

6 Problem #5, continued. (The reaction is shown again here for your convenience.) 4 CH6N2(!) + 5 N2O4(g) 9 N2(g) + 12 H2O(g) + 4 CO2(g) DH = 4738 kj (6 pts) (c). How much energy would be released if 2.00 kg of CH6N2 reacts with excess N2O4 at constant pressure? (6 pts) (d). Find DHf for CH6N2(!). Some of the following data may be helpful. DH = kj for H2(g) + 2 O2(g) 2 H2O(g) DH = 9.16 kj for N2(g) + 2 O2(g) N2O4(g) DHf for CO2(g) = kj/mol A L.S. Brown

7 NAME: (12 pts) 6. A piece of lead and a piece of zinc have a combined mass of 50.0 g. The two pieces of metal are heated to 85.0 C, and then both are added to the same thermos bottle containing g of water at 22.2 C. If the final temperature of the water and metals is 23.9 C, what was the mass of the lead? (The heat capacities of the metals are given below, and the specific heat of water is J g 1 C 1. Assume that no heat is absorbed by the thermos or lost to the surroundings.) Metal Heat Capacity (J mol 1 K 1 ) Lead (Pb) 26.4 Zinc (Zn) L.S. Brown A7

8 7. The graph below shows the variation of G with temperature for the following reaction: 2 NO(g) + O2(g) 2NO2(g) ΔG 0 T (J K 1 ) (8 pts) (a). Use data from the graph to estimate H for the reaction. (Pay attention to the quantities on both axes here and note that the graph is linear. Make sure that you clearly explain how you have found your H value.) 1 T (K 1 ) A L.S. Brown

9 NAME: Problem #7, continued. (Note that it is possible to answer parts (b) and (c) even if you can t get (a).) (4 pts) (b). What is the sign of S for this reaction? (For credit you must also explain briefly how you can tell what the correct sign is.) (6 pts) (c). At what temperatures is the reaction spontaneous? 2016 L.S. Brown A9

10 A L.S. Brown