Chemistry 1A Fall 2012 Midterm Exam 1 September 19 2012 90 min closed book Name SID GSI Name Discussion Day Time The test consists of 4 short answer questions and 17 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 17 2 6 51 Ethylene Glycol Short Answer 3 4 Boron Nitride Short Answer 4 4 Allene Short Answer 5 4 Cyanogen Short Answer 5 7 Total 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 70 Name GSI INTERMOLECULAR ATTRACTIONS Substance ethylene glycol propanol Molecular formula HOCH2CH2OH CH3 CH2 2OH propylamine CH3 CH2 2NH2 Structural formula H H H C C H H H H H C C C H H H O H H Molar mass g mol Boiling point O H 62 197 C O H 60 98 C H 59 48 C 59 3 C H H H H C C C N H H H H H trimethylamine CH3 3N H H 1 2 3 C C N H H H H C H Which statement provides the most compelling explanation of the data A O H O is stronger than H N H B O H O is weaker than H N H C Boiling point increases with molar mass D Increasing the number of CH3 groups decreases the boiling point Which explanation s can be used to account for the lower boiling point of trimethylamine compared with the other molecules in the table A Trimethylamine is not polar B Trimethylamine does not form hydrogen bonds C The London dispersion forces are relatively small D Both B and C are correct E A B and C are all correct How would you expect the van der Waals constants to compare for propanol and propylamine A The constant a is larger for propanol but b is about the same for both B The constant a is smaller for propanol but b is about the same for both C Both constants are larger for propanol D Both constants are smaller for propanol Page 2 of 6 Name GSI 4 Which is the best estimate for the boiling point of ethylenediamine NH2CH2CH2NH2 A 35 C B 118 C C 192 C D 247 C SHORT ANSWER The boiling point of ethylene glycol is much higher than the boiling point of propanol Draw a structure showing the interaction between ethylene glycol molecules to explain its high boiling point Each ethylene glycol molecule can form two hydrogen bonds to one or more ethylene glycol molecules BORON COMPOUNDS Substance Formula boric acid B OH 3 Boron oxygen bond lengths 5 Structural formula B O 1 36 Molar mass Boiling point Solubility in water B O bond length 62 300 C 1 0 M 1 36 B O 1 30 B O 1 20 Which model provides the best description of the bonding in B OH 3 based on the information provided A The solubility in water indicates an ionic solid with B and OH B The high boiling point indicates network covalent bonding C The bonding is molecular covalent The boron has an empty p orbital D The bonding is molecular covalent The boron oxygen bonds have a bond order of 1 and the bonding satisfies the octet rule Page 3 of 6 Name GSI 6 7 8 When B OH 3 is added to water a reaction occurs and B OH 4 forms Which statement describes the properties of the solution A The solution remains neutral because the concentration of H is equal to the concentration of OH B The solution is acidic because there is an extra H from H2O C The solution is basic due to the OH on B OH 4 D B OH 3 is not an electrolyte B OH 3 is a nonpolar molecule Boron trioxide B2O3 boils at 1860 C The compound is soluble in water Which statement below is a correct conclusion that you can draw from the data A B2O3 is a relatively hard solid but it fractures readily B The bonding in the solid is not very strong C The atoms are linked together as molecules that satisfy the octet rule D B2O3 is soluble in water because B3 is so small and the B3 and O2 attractions in the solid are relatively weak Which bond is the most polar A C N B B N C B C D B O SHORT ANSWER A sketch of the structure of a layer of boron nitride BN is shown to the right The structure is very similar to that of the graphite form of carbon with a B N bond order of 1 5 However BN is harder than graphite Explain why Possible explanations 1 The formal charge on B is 1 It is 1 on N This causes the next layer to stack such that the B atoms are on top of the N atoms 2 N atoms are more electronegative than B atoms This results in a partial positive charge on B and a partial negative charge on N This causes the next layer to stack such that the B atoms are on top of the N atoms Thus the layers do not slide over one another in BN In contrast the charge is uniform in graphite Note The formal charge and partial charges indicate opposite charges It doesn t matter which is more important just that the charge is not uniform HTTP WWW DOCBROWN INFO PAGE04 4 72BOND BNHEXAGONAL2 GIF Page 4 of 6 Name GSI ALLENE C3H4 AND CYANOGEN C2N2 The structure of the allene molecule is shown to the right 9 H H H C C C H The hybridization around the carbon atom in the middle of the molecule is A sp B sp2 C sp3 D sp4 104 5 C 109 D 120 C3H4 allene C N2 H2 D CF4 10 The H C C bond angle is approximately A 90 B 11 Which molecule is polar A C2H2 B SHORT ANSWER Explain why allene C3H4 is not planar The middle C atom uses two different p orbitals to bond to each of two carbon atoms These p orbitals are perpendicular to each other This causes the CH2 groups to lie in planes perpendicular to one another SHORT ANSWER Cyanogen is a toxic compound with a molecular formula C2N2 Draw two Lewis dot structures for cyanogen that satisfy the octet rule Assign formal charges to all atoms Circle the structure that is more stable Structure 1 N C C N 0 0 0 0 better structure Structure 2 C N N C 1 1 1 1 Page 5 of 6 Name GSI VAPOR PRESSURE Equal volumes of pure water pure ethanol and two aqueous solutions were each injected into four identical sealed evacuated containers The equilibrium vapor pressure data are given in the …