Name________________ Page 1 of 6 Chemistry 1A, Fall 2007 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 Page Points Score Question 1 2 19 Question 2 3 20 Question 3 4 16 Question 4 5 17 Multiple Choice 6 8 Total 80 Useful Equations and Constants: PV = nRT nRTnbVVnaP 2 221mvKE  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________________ Page 2 of 6 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 Lewis Structure Name of Electron Pair Geometry Name of Molecular Geometry Formal charge on central atom b. Draw the Lewis structure of the product, NH3BF3. Why would these two molecules tend to react? 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. e. After the reaction is complete what happens to the pressure in the apparatus? increases decreases stays the same A B C D E NH3 BF3Name________________ Page 3 of 6 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? 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? ______________ atm _____________ °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. ____________ d. Explain your answer to part c). e. Draw a Lewis dot structure for N3–. Show all resonance structures and indicate formal charges. f. The bond between two nitrogen atoms in N2 is stronger than the bond between two nitrogen atoms in N3–. Explain why. 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.Name________________ Page 4 of 6 Question 3, Liquids (16 points total) Circle all that apply a. Which molecules are polar? A B C D Eb. Which molecules have the largest London dispersion forces? A B C D Ec. Which molecules can be attracted to each other by hydrogen bonds? A B C D Ed. Which one of the five molecules has the highest boiling point? A B C D Ee. Explain your reasoning to part d). 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? HHCHCOHHCHHCHHHHHCCCHHCHHCHHHHHHHOCCHHCHHCHHHHHHCCHHCHHCHHHOHHOCCHHCHHHHHHA) diethyl ether B) pentane C) 1-butanol D) 2-butanol E) 1-propanolName________________ Page 5 of 6 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 C 2.5 N 3.0 O 3.5 Fill in all the lone pair electrons on the sketch above. What is the formal charge on the oxygen atom with the double bond to carbon? _________ This molecule tastes sour because H+ is released when the molecule is in solution. Circle the H atom in the molecule that you predict will be released. Explain your reasoning. c. In order to have a taste, the substance must be soluble in saliva, which is mostly water. Circle the substances that would definitely NOT have a taste. Circle all that apply. CH4 K2SO4 Si NH4Cl CH3COOH Cu d. The element sulfur, S, is a solid that does not dissolve in water, and hence does not have a taste. Draw a structure that satisfies the octet rule to show how sulfur atoms in the solid bond to one another. e. When you heat sulfur, it forms a gas that does not have a smell. Draw a structure that satisfies the octet rule to show how the sulfur atoms bond to one another to make a gaseous molecule. HNCHCHHOOHName________________ Page 6 of 6 Question 5, Multiple Choice (8 points total) Circle the one best response to the questions below. a. Assume that you have a mixture of He (light dots) and Xe (dark dots) at 300K. Which drawing best represents the mixture? b. What is inside bubbles when water boils? H2O H2 and O2 H2 and O H and O d. What is a stable compound containing H and S? HS H2S H3S HS2 e. Which element X forms a solid of formula XCl2 at 25°C? Li Al Mg C f. Which is the most ionic bond? Li - Cl Cs - F C - F O - H g. Which is the least ideal gas? He H2Se CO2 N2 h. Which