Name Chemistry 1A Fall 2007 KEY Midterm Exam 1 September 18 2007 90 min closed book Name SID GSI Name The test consists of 4 short answer questions and a page of 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 Points Question 1 2 19 Question 2 3 20 Question 3 4 16 Question 4 5 17 T K T C 273 15 Multiple Choice 6 8 1 atm 760 mm Hg 760 torr 1 bar Total Score Useful Equations and Constants Page PV nRT n2 P a V nb nRT V 1 KE mv 2 2 Ptotal PA PB XAPA XBPB N0 6 02214 x 1023 mol 1 R 0 0821 L atm K 1 mol 1 80 Page 1 of 6 Name Question 1 Gases 19 points total Ammonia and trifluorobromide react according to the balanced equation given below NH3 g BF3 g NH3BF3 s a Complete the table below for the reactant molecules NH3 BF3 F F N Lewis Structure B H H H F Name of Electron Pair Geometry tetrahedral trigonal planar Name of Molecular Geometry trigonal pyramidal trigonal planar Formal charge on central atom 0 0 b Draw the Lewis structure of the product NH3BF3 Why would these two molecules tend to react In the forming the product the boron now attains an octet H H F N B H F F Consider an experiment where the two gas phase reactants are held at room temperature in separate flasks The flasks are connected by a narrow tube with valves to control the flow of gas c On average which sample of gas molecules is traveling at a greater velocity NH3 BF3 both are the same d If the valves were opened simultaneously and the gases reacted predict approximately where in the tube would the solid form Circle the position and show your calculation below NH3 BF3 A 1 2 1 mv BF 3 mv 2 NH 3 2 2 v NH 3 v BF 3 B MM BF 3 MM NH 3 C D 67 8 2 17 0 E so the NH3 should travel twice as far as the BF3 The solid should form about 2 3 down the tube from the NH3 e After the reaction is complete what happens to the pressure in the apparatus increases decreases stays the same Page 2 of 6 Name Question 2 Airbags 20 points total In lab you experimented with the reaction between sodium bicarbonate and acetic acid to inflate an airbag Because this reaction is too slow it is not suitable for airbag inflation In an automobile airbag the decomposition of sodium azide NaN3 is used instead NaN3 s 2Na s 3N2 g a A typical passenger side airbag is 120 L Assuming standard temperature and pressure STP how many moles of nitrogen gas would it take to fill the passenger side airbag n PV nRT PV 1 0atm 120 L 5 4moles Latm RT 0 0821 molK 273K b A given car might be driven from sea level where the pressure is 1 0 atm to a mountaintop where the pressure is 0 60 atm The temperature can range from 25 C to 50 C depending on local conditions Under what temperature and pressure conditions will the most moles of nitrogen be required to fill the airbag 1 0 atm 25 C c Will an airbag filled with the 10 moles of water H2O have the same volume as an airbag with 10 moles of N2 Assume both are at STP No d Explain your answer to part c No At STP water will be a liquid and nitrogen will be a gas The liquid will occupy a smaller volume than a gas e Draw a Lewis dot structure for N3 Show all resonance structures and indicate formal charges 2 2 N N N N N N N N N f The bond between two nitrogen atoms in N2 is stronger than the bond between two nitrogen atoms in N3 Explain why In N2 the bond is a triple bond In N3 the bond averages to a double bond The bond in N2 should be stronger because it is a triple bond g Classify the type of bonding in NaN3 ionic covalent polar covalent metallic h Draw a shell model showing Na atoms in elemental sodium Using this model explain how Na in NaN3 obeys the octet rule Na atom Na ion When Na bonds in NaN3 one e is donated from Na to N3 This leaves Na with a complete octet Page 3 of 6 Name Question 3 Liquids 16 points total A diethyl ether B pentane C 1 butanol C H H H C O C H H C H C C H H C H H C H H C H H C H C H H O H C H H C H H C H H C H H C H H C H H H H H O H E 1 propanol H H H H D 2 butanol O H C H H H C H H H C H H H H H Circle all that apply a Which molecules are polar A B C D E b Which molecules have the largest London dispersion forces A B C D E c Which molecules can be attracted to each other by hydrogen bonds A B C D E d Which one of the five molecules has the highest boiling point A B C D E e Explain your reasoning to part d Boiling point depends on the strength of intermolecular interactions The greater the intermolecular forces the higher the boiling point 1 butanol is one of the molecules with the largest London dispersion forces and can also have hydrogen bonding 1butanol will have the greatest forces holding it to other 1 butanols so will have the highest boiling point f Describe one way you can separate a mixture of pentane and1 butanol How do the properties of the molecules allow you to separate them 1 The molecules should have very different boiling points Warming the solution should cause pentane to evaporate first leaving the 1 butanol behind 2 One compound is polar and the other is nonpolar They may for two layers and not mix If so you can just carefully pour off the top layer into another container 3 Add water to the mixture 1 butanol should dissolve in water and two layers are likely to form Drain one layer off Page 4 of 6 Name Question 4 Taste 17 points total In a laboratory experiment you considered the sense of smell Now consider the sense of taste It is believed that the tongue responds to four basic tastes sweet sour salt and bitter a Which of the following do you predict will taste salty Circle all that apply N2 KBr NaNO3 CO2 CH3NH2 b Consider the molecule glycine that is drawn below along with a table of electronegativities H 2 1 H H O C 2 5 N 3 0 H …