Chemistry 1A, Fall 2011 Midterm Exam #1 September 20, 2011 (90 min, closed book) Name:__________________________________ SID:___________________________________ GSI Name:________________ • The test consists of 4 short answer questions and 18 multiple choice questions. • Put your written answers in the boxes provided. Answers outside the boxes may not be considered in grading. • Write your name on every page of the exam. Question Page Points Score Multiple Choice (1-18) 2-7 55 Formic acid Short Answer 3 8 Hydrogen Storage Short Answer 3 4 Capsaicin Short Answer 5 4 Rubber Short Answer 7 4 Total 75 Useful Equations and Constants: PV = nRT Ptotal = PA + PB = XAPA° + XBPB° N0 = 6.02214 x 1023 mol-1 T (K) = T (°C) + 273.15 R = 0.0821 L atm K-1 mol-1 1 atm = 760 mm Hg = 760 torr ≈ 1 barName_____________________GSI__________ Page 2 of 7 HYDROGEN STORAGE Hydrogen, H2, can be reacted with oxygen to release energy to power vehicles. For this purpose, the hydrogen must be stored for subsequent use. Various methods for storing hydrogen are under consideration, including high pressures, low temperatures, and chemical compounds that reversibly release H2 upon heating. Electronegativity values: H (2.20), C (2.55), O (3.44), Mg (1.31) 1. Magnesium hydride, MgH2, can be heated to release hydrogen. The best description of the hydrogen in MgH2 is A) H+ B) H2 C) H− D) H 2. How many grams of hydrogen are there in 1 L of compressed hydrogen gas, H2 (g), in a steel tank at a pressure of 350 atm, 25°C? A) 7.1 g/L B) 14.3 g/L C) 28.6 g/L D) 57.2 g/L 3. Can you store a greater number of moles of ammonia, NH3, at 350 atm and 25°C in a steel tank compared with hydrogen, H2, under identical conditions? A) Yes, because of attractions between NH3 molecules at 350 atm B) No, because NH3 is a larger molecule and takes up more space C) No, because the volume, pressure, and temperature are the same D) No, because NH3 has a larger molar mass 4. Most hydrogen storage methods for powering vehicles have drawbacks. Consider hydrogen storage as elemental hydrogen or as diborane B2H6. Which statement is NOT true? A) If H2 (g) is not kept in a heavy pressurized steel cylinder there will not be enough to drive very far. B) A special cooling system is needed to store hydrogen as H2 (l) because the boiling point is extremely low. C) Diborane, B2H6, is very stable and does not release hydrogen readily. 5. Formic acid, CH2O2, can be decomposed to produce hydrogen. The Lewis structure for formic acid, CH2O2, is show below. The atoms are labeled A, B, C, D, E. Choose the atom that has the largest partial positive charge.Name_____________________GSI__________ Page 3 of 7 6. A second Lewis structure with the formula CH2O2 is shown below. Why is this not the preferred structure? A) There are too many electrons. B) There are two resonance structures. C) There is a negative formal charge on the C atom. D) The molecule is not linear. SHORT ANSWER: Formic acid, CH2O2, is a liquid at 25°C. It has a strong odor. Formamide, CH3NO, has similar characteristics. a) The Lewis structure for formic acid, CH2O2, is shown below. Redraw the structure to show the shape of the molecule. Specify all the bond angles. b) Consider the empirical formula: CH3NO. One isomer, formamide, has similar chemical properties to formic acid. Draw the Lewis structure of formamide. SHORT ANSWER: Sodium amide, NaNH2, is under consideration for hydrogen storage. This compound is prepared by reacting sodium, Na, with ammonia, NH3. In this reaction, the colorless NH3 gas is converted to the hard solid, NaNH2. Use Lewis structures to show the difference in bonding between NH3 and NaNH2.Name_____________________GSI__________ Page 4 of 7 SPICES Data concerning compounds classified as vanilloids (vanillin, eugenol, zingerone, and capsaicin) are provided in the table below. While these compounds have similar structures, they have distinctive flavors. Vanillin Eugenol Zingerone Capsaicin Flavor vanilla cloves ginger hot peppers Molecular formula C8H8O3 C10H12O2 C11H14O3 C18H27NO3 Structural formula Space-filling models Melting point 80°C -9°C 40°C 63°C Boiling point 285°C 256°C 290°C 512°C Water solubility 10 mg/mL < 1 mg/mL insoluble insoluble Vapor pressure @ 25°C 0.0022 mm Hg 0.0226 mm Hg 0.000143 mm Hg 4.41 x 10-11 mm Hg 7. There is an odd number of H atoms on the capsaicin molecule because of the presence of A) C=C B) both C=C and C=O C) one N atom D) one N atom and C=C 8. The hybridization on the N atom in capsaicin is A) sp B) sp2 C) sp3 D) none of theseName_____________________GSI__________ Page 5 of 7 9. The molecule with the weakest intermolecular attractions is A) vanillin B) eugenol C) zingerone D) capsaicin 10. The water solubility differences for these molecules is due mostly to A) molecular mass B) polarity C) shape D) hydrogen bonds 11. For relief from the burning sensation of hot peppers, the best thing to drink or eat is A) bread dipped in olive oil B) soapy water C) beer because it has a few percent of alcohol D) water SHORT ANSWER: Explain why capsaicin has a low vapor pressure and therefore very little smell, but a very strong taste that lasts for a long time. No smell because: Strong taste because: DEMONSTRATIONS 12. Recall a lecture demonstration in which a large metal can filled with hot water vapor was closed with a rubber stopper and allowed to cool. After a few minutes, it imploded. This demonstrated that: A) Volume and pressure are directly proportional; hence a decrease of internal pressure resulted in a decrease of the internal volume. B) Pressure inside and outside the can must be always the same. C) Water vapor condensation resulted in a sudden decrease of temperature and pressure in the can. D) Water condensation resulted in a large difference in pressure inside and outside the can.Name_____________________GSI__________ Page 6 of 7 ELASTIC PROPERTIES OF MATERIALS (How much do materials stretch?) Imagine rods of various materials of equal size. Each is placed in a machine that will stretch the rod until it breaks. The force per area in megapascals (MPa) that is required to break different materials is given in the table below. The amount that the material stretches before breaking is also given (% elongation to break). Material Chemical formula Force per area (MPa) % elongation to break Kevlar 3000 3% Nylon 75 20%