Chemistry 1A Fall 2011 Midterm Exam 1 September 20 2011 90 min closed book Name KEY 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 Useful Equations and Constants PV nRT 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 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 bar Name GSI 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 2 4 B H2 C H 2 D H 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 3 H 7 1 g L B 14 3 g L 1 C 28 6 g L 2 D 57 2 g L 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 2 B No because NH3 is a larger molecule and takes up more space C No because the volume pressure and temperature are the same 1 D No because NH3 has a larger molar mass 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 2 Page 2 of 7 Name GSI 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 Answer D 3 points Answer B 2 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 1 C There is a negative formal charge on the C atom 3 D The molecule is not linear 1 SHORT ANSWER Formic acid CH2O2 is a liquid at 25 C It has a strong odor Formamide CH3NO has similar characteristics b Consider the empirical formula CH3NO a The Lewis structure for formic acid CH2O2 is shown below Redraw the One isomer formamide has similar structure to show the shape of the chemical properties to formic acid Draw molecule Specify all the bond angles the Lewis structure of formamide 2 points the geometry around carbon is trigonal planar and the bond angles are 120 2 points the geometry around oxygen is bent with a bond angle of 109 4 points correct Lewis structure for formamide 2 points correct Lewis structure for nitrosomethane Page 3 of 7 Name GSI 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 1 point pyramidal structure for NH3 1 point indication that NaNH2 is ionic with a charge on Na and a charge on NH2 2 points correct Lewis structure for NH2 will be an ionic solid hard and brittle 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 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 Structural formula Spacefilling models Page 4 of 7 Name GSI Vapor pressure 25 C 7 8 0 0022 mm Hg 4 41 x 10 11 mm Hg 0 000143 mm Hg 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 3 D one N atom and C C The hybridization on the N atom in capsaicin is A 9 0 0226 mm Hg sp B sp2 C sp3 3 D none of these D capsaicin The molecule with the weakest intermolecular attractions is A vanillin 3 B eugenol 4 C zingerone 10 The water solubility differences for these molecules is due mostly to A molecular mass 4 B polarity 3 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 4 B soapy water C beer because it has a few percent of alcohol 3 D water Page 5 of 7 Name GSI 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 low Pvap 1 point vapor pressure is low because of strong intermolecular attractions 1 point large London dispersion forces due to large size or two H bonds Strong taste because 1 point binds tightly to receptor site when you eat it 1 point does not dissolve in saliva so it is difficult to remove from the receptor site 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 1 C Water vapor condensation resulted in a sudden decrease of temperature and pressure in the can 2 D Water condensation resulted in a large difference in pressure inside and outside the can 4 Page 6 of 7 Name GSI ELASTIC PROPERTIES OF MATERIALS …