Key2 - CHEM. 31 EXAM 2 09.APRIL 2010 NAME: KB This exam...

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Unformatted text preview: CHEM. 31 EXAM 2 09.APRIL 2010 NAME: KB This exam must be completed without the aid of books, notes or colleagues. You may use a non- programmable calculator; if your calculator is programmable, you must present it to a proctor and have it cleared before you begin the exam. All answers must be clearly written in the s ace indicated and mathematical work must be shown for full credit to be awarded for numerical answers and for all answers for which calculations are carried out. Please write your name on the top of each page. A periodic chart and several equations are included as the last page of the exam. GRADING: (Page): Total Total points points awarded u ossible (6) 9 (7) 10 l (8): 18 (9): 20 i TOTAL: 100 ’ Remember: “In the beginner’s mind there are many possibilities; in the expert’s, there are few.” '-— Shunryu Suzuki NAME: L17 'j PART I: (48 points) Circle the letter of the correct answer or write the answer in the space provided. If numerical work is required, show it in the space provided. Points are shown for each problem. [1] (3 points) The dashed line in the following figure shows an uncatalyzed reaction path. Which of the other lines shows the effect of a catalyst? Energy Reaction path (a) (c) (d) [2] (6 points) The reaction of NO (g) with H2(g) is described by the following rate law: Rate = k[Hz] [N012~ A proposed mechanism is Step 1: H2(g)+2 NO(g) —'> N20(g) + H2005) Step 2: NZO(g)+ H2(g) ~> N2(g) + H20(g) What is the molecularity of Steps 1 and 2 a. l: bimolecular 2: bimolecular l: termolecular 2: bimolecular c. lztermolecular 2: unimolecular d. l: unimolecular 2: unimolecular Which step is the rate—detennining step? tep 1 b. step 2 c. both steps are rate determining d. need value of k to determine NAME: (CE %1 [3] (9 points) The following figure shows Arrhenius plots for four different reactions. IIT Rank the reactions in order of activation energy (Ea) — lowest to highest: a. a<b<c<d b.d<c<b<a c. b < c < a < d none of these is the correct order Which reaction has the largest frequency factor (A)? m l b. b c. c d 6. they all have the sameA Rank the reactions in order of their dependence on temperature — greatest to smallest: a. d>(a=c)>b b.b>(a=c)>d @>c>a>b c. d > c > b > a d. all have the same temperature dependence [4] (3 points) The second-order reaction (2A —> B) with a rate constant of 0.350 M‘ls'1 has a half—life of 3.45 5. What was the initial concentration of the reactant [A]o? 0.2;st b.0.201M 0.1.00M d.1.21M 1’3 3 i . O a -| -' .. l / , - 5M 5 "' 7L“ =- 1 kill» 355$ z: X NAME: LE ‘1! [5] (3 points) Is the equilibrium constant Kc for the reaction N204(g) 2 N02(g) in air the same in Los Angeles, CA as in Denver, CO if the atmospheric pressure in Denver is lower but the temperature is the same? / a. yes a no [6] ( 3 points) The equilibrium constant for the formation of calcium carbonate from the ions in solution is 2.2 x 108 : CaCO3(s) K = 2.2 x 108 Ca2+(aq) + C032'(aq) What is the equilibrium constant (Ksp) describing the solubility of calcium carbonate in water? a. the same: 2.2 x 108 b. —2.2 x 108 c. 2.2 x 10'8 .5 x 10'9 [7] ( 3 points) Two students measured an equilibrium constant for the same chemical reaction. Stan obtained a value of 130 for the equilibrium constant, while Eric obtained a value of 11.4. The lab instructor, Mr. Garrison, checked their results and said they were both correct. How can this be? a' The values vary according to the way the measurement is made. Stan must have measured product concentrations, while Eric measured reactant concentrations. b. The values vary according to the starting conditions of the reaction prior to equilibrium. Stan must have started with all reactants, while Eric must have started with all products. The values vary according to the way the reaction is written. The balancing coefficients that Stan used must have been twice those that Eric used. d. The values vary according to the way the reaction is written. The balancing coefficients that Stan used must have been half those that Eric used. [8] (3 points) The equilibrium expression for the reaction of calcium oxide (5) and sulfur dioxide (g) to scrub stack gases from power plants by making calcium sulfite (s) 0. KC = [Ca0] + [$02]. = 1 (1. [08303] Kc [3021- KC=W. NAME: :1 [9] (3 points) Nitric oxide (NO) has an important role as a neurotransmitter. NO used in laboratories for studies of its reactivity is stored in pressurized cylinders. The gas in these cylinders must be purified before use, because the cylinders contain small quantities of other oxides like N20 and N02 in equilibrium with NO. Is the contamination of NO with other oxides of nitrogen dependent on the pressure in the tank? 3N0 N20 + N02 3 more other oxides will be present at higher pressure b. yes fewer other oxides will be present at higher pressure 0. The pressure has no effect on the presence of the other oxides d. Insufficient information — need to know KP to answer this. [10] (4 points) Using the terms acid, base, conjugate acid, and conjugate base, identify the conjugate acid—base pairs in this reaction by writing the correct term beneath the formula: CH3COOH(aq) + NH3(aq) CH3COO~(aq) + NH4+(aq) \ %'..W 9%.. awc acid. law [11] (3 points) On the pH scale, [H+] changes: a. from 1 M to 14 M y a factor of 10 for each pH unit 0. from —1 M to ~14 M d. by a factor of “e” for each pH unit NAME: ' [12] ( 5 points) You left the small hardware store that was featured on the sample exam and are now working in the lab of a store that sells ‘boutique’ fertilizers —— special plant food for special plants. This store had a similar problem — sprinklers going off in their storage area caused all the labels to fall off containers of their stock of acids, bases and salts that they blend to produce their product. You are given the list of compounds and have the job of identifying them and relabeling the bottles. Your new boss (who aced Chem 31) gives you 30 minutes to do the job of relabeling the bottles. You make aqueous solutions of each unknown and determine its identity by taking the pH. Here is the data set you collect: match the formula of the salt with the pH of the solution Solution 1: pH 1.96 9 A. NH4N03 /’ Solution 2: pH 11.2 L: p B. mixture ofNaHCO3 and Na2CO3 Solution 3: pH 4.32 A C. mixture of NaCl, KCl, and LiClO4 Solution 4: pH 9.01 3 D. mixture of HCl and HN03 Solution 5: pH 7.0 C, E. K28 PART II: (52 total points) [13] (4 points) Nitrous oxide (N20) is used as an anesthetic; it is called ‘laughing gas’ because it induces spontaneous laughter in someone who inhales it. Because it is also soluble in cream, it is used as a propellant in pressurized cans to produce whipped cream. It iS also a potent greenhouse gas. In the atmosphere it decomposes in accordance with the following reaction: 2 N20(g) 2 N2(g) + 02(g) The plot of 1n(N20) as a function of time is linear. What is the rate law for the reaction? W‘LngzO] [14] ( 10 points total) The metabolism of table sugar (sucrose: C12H22011) begins with the hydrolysis of the disaccharide to glucose and fructose (both CGHIZO ): C12H22011(aq) + —‘> 2C5H120(aq) The kinetics of the reaction were studied at 24°C in a reaction system with a large excess of water, so the reaction was pseudo—first order in sucrose. ' Time (s) [sucrose] (M) 0 A 0.562 612 0.541 1600 0.509 / 2420 0.484 3160 0.462 I 1 4800 0.4417 (a) (4 points) Determine the rate law for the reaction: W : ‘Q l CCIZ H2201: ) : 195- CCIZHZZOH>CHZO) (b) (6 points) Determine the pseudo-first order rate constant for the reaction: Work space: 4:3} 1: U" Mez— fine-5729) _ (41722) “(16453 ” A76 (3/60 —— mock 0'60 5 “NIKE/W _ arr-MW “0,6? ‘? _ " W 36 g -’ 1" IO 5 / ' / 5'6 o NAME: (CL: [15] ( 18 points) In Chapter 15 we studied the chemical process that give rise to oxides of nitrogen and sulfur in the atmosphere. These oxides can also react with each other to further complicate the mixture of gases in Earth’s atmosphere. You get an internship at Daimler AG in ' Germany to study the chemistry of the reaction of nitrogen dioxide with sulfur dioxide in automobile exhaust: N02(g) + 802(g) N0(g) + s03(g) On your first day in the laboratory, you analyze an equilibrium mixture of the gases at 500°C that contains the following concentrations of substances: [NOz] = 0.100 M [$02] = 0.300 M [N0] = 2.00 M [503] = 0.600 M While holding the temperature constant, you add more $02 to the reaction mixture to increase its concentration to 0.800 M. Calculate the molar concentration of each substance when the system re—establishes equilibrium. n/l K: ZNOJZSOEJ (immaémg , .3 0,0 END; (0.100103%?) if ’ ’- NoL $0; N0 $03 I 0.100 0.5500 2.00 0.600 C x75 g4 + 7c +2; 6’ 0.(00 ")C 0.?00‘34 $001"; aéoorv); eta..0 : (2.00f7")(0'600fx> (#20 +Z.601¢+7<L) (Moo—ac) (0.900%) (d. 0800~aqdox+3§> 340%.. 38,g7< 4200 :o 76”- 0.0947 8”... NAME: £5 % [16] (20 points) Many acids are physiologically important compounds. (a) (10 points) Muscles become fatigued during exercise because lactic acid builds up'in the tissues. The percent ionization of a 1.00 M solution of lactic acid at room temperature (25°C) is determined to be 2.94%. CH3CHOHCOOH(aq) CH3CHOHCOO—(aq) + H+(aq) Lactic acid (HLac) Use this value of percent ionization to calculate Ka for lactic acid. fflfl M/Jdac [H *J : (/.00)(o.027;9 .: a . 0 3? VM H Lam H “L La c g i I . o b 0 0 g a E 0.?2og, 0.02%, 0.0295! (09?“0’6) (b) (10 points) Uric acid is a highly insoluble compound that can crystallize in joints, causing the severe pain and swelling associated with the condition known as gout. The pKa of uric acid is 3.89. What is the pH of a 0.0150 M solution of uric acid? Uric acid(aq) H+(aq) + urateXaq) _ . a as 7 ,f PH: flit—3.9? ~- Q: m = Aqum 028% I H04 u “‘ W’ 19- 0 6’0 0 a 7’ C O /.Z?)</0 ’: 7‘ /7[s 4/? r 75 r ')¢ + 7c , (a 0/ 5‘0 ~14) 7r ? 7 ’5 0 . 0150 r 1d 34 7c ‘ 0 #29 K (04(0me ~7L) =75” pH: -/05,/,gax/0"Z 23% 7‘Z+/‘27 Hwa »/.%rx/o’5":0 ’ 74”; /~%3>‘/0”5 ...
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Key2 - CHEM. 31 EXAM 2 09.APRIL 2010 NAME: KB This exam...

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