CHEM31EX3key_2011

CHEM31EX3key_2011 - Name 2 1 . A chemist measures the rates...

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Unformatted text preview: Name 2 1 . A chemist measures the rates of change in the concentrations of two chemical species X and Y in a A ' . [Y] < 0 and that MX] = — Ag]— . Circle the overall reaction below that At At At would be consistent with these findings (Z is a third chemical species in the reaction). reaction. She finds that A. X+Z—>Y YTA & ream/W w 0 B. 2Y->X+Z Av @Y+Z.,2X gag : _ 92223 :7“, Q WM at A1: K . . D. 2X—>Y+Z wavflshyokmlto C . o 11"“ UL 2. Which statement is false? A catalyst A. alters the reaction mechanism increases the rate of the forward reaction, but not the reverse rate -‘C. alters :the activation energy D. may be altered during the reaction, but is regenerated at the end 3. As was discussed in lecture, inthe simple collision theory model, the rate for a one—step bimolecular reaction can be written as the product of two factors: Reaction rate = (rate of collisions) x (probability a reaction occurs upon collision) Which of the following will directly affect both of these factors: the temperature B. the activation energy C. the concentrations of reactants D.. the relative orientation of reactants upon collision Name ]< E \t ‘ n—Butyl Chloride and t-butyl chloride have the same molecular formula,-C4H9Cl. They each react with hydroxide ions to form n-butyl alcohol and t—butyl alcohol, resPectively‘, with the same overall stoichiometry: C4H9Cl + OH' —> C4H90H+ Cl" Two possible mechanisms called SNl and 8N2 have been proposed for these reactions: SNl Mechanism: Step 1: C4H9Cl -> C4H9+ + Cl‘ (slow) Step 2: C4H9+ + OH‘ —> C4H90H (fast) 8N2 Mechanism: Single step: C4H9Cl+ OH' —> C4H90H +‘Cl‘ Like the t—butyl bromide reaction discussed in lecture, the t—butyl chloride reaction follows the 3N1 mechanism. Questions 4-6 refer to aspects of the rate law and mechanism for the n—butyl chloride reaction. -4. Given below are initial rate data for the n-butyl chloride reaction: Initial Concentrations (M) Expt. # [C_4'H2Q1_1 [o -] Initial Rate (Ms) 1 1.2 x 10“ 2.2 x 104 1.6 x 10'5 2 3.6 x 104 2.2 x 10“ 4.9 x 10‘5 3 4.87: 104 4.4x104 12.87; 10‘5 Which rate law below best describes this set of data? Cm M Y6; 1— ‘ + Z ‘- A. rate = k[0H‘] B. rate -= k[C4H9Cl] C. rate = k[C4H9Cl]2 Tate = k[C4H9Cl] [OH'] Q 5. Which mechanism is consistent with the rate data? A. SNl neither 8N1 or SNZ Name K E ‘f 6. If the n—butyl chloride reaction on the previous page were flooded with .a high concentration of OH- while keeping the concentration of C4H9C1 low, and the concentration of C4H9C1 was measured as a function of time, which of the plots below would yield .a straight line? A. [C4H9C1] versus time (19 ]n[C4'H9C1] versus time (pg/(ML! D g y 9» VWCQJI,‘ C. [C4H9C1]'1 versus time D. none of the above 7. The appropriate equilibrium constant expression for the reaction below. is 2Ag<s)+2n2+<aq> 2Ag+<aq)+2n<s> K: [245:] [Zn +] [Z11 +] - «1- 2 C. K=2[A‘g;+] _ [Zn ] K _ [Ag+~12[Zn] — 2+ [Zn HA8] K = [Ag+fl2n1 [Zn2*][Ag]2 1‘ Name KE‘( 8. Starting with pure N02(g) ata pressure of 0.500 aim, the following reaction came to equilibrium, and the total pressure was 0.674 atm. Calculate the equilibrium partial pressure of N02(g). 2N02(g) i more + Org) A. 0.326 atm (9H 3% 3 (PM1 :0‘500 “X PNO : ’X B. 0.2mm p ; J-‘X . M -—0 ‘34 c. 0.174atm h“ =O'Soo+:,¥ - ~ 0.152an ‘ ill—Ix 2 O‘Hq ’x 2 (9.3% i E. cannot be calculatedbecause K P is not given 9.. For the reaction: H2(g)+12(g) # 2HI(g) 19:50.2 at 445 °c If [H2] = [12] -= '[HI] = 1.75 x 10"3 M at 445 °C,'whichvof the following statements is true? 1'32 _ :QRS‘LD) _I<K C|.'+S'110’3)CI.7'$¥IO‘3) ‘ A. The system is at equilibrium :and no concentration changes will occur. B. The concentrations of H1 and 12 will increase as the system approaches equilibrium. C. The concentrations -:of H2 and HI will fall as the system moves toward equilibrium. D. The concentrations of H2 and 12 will increase as the system approaches equilibrium. CE) The concentration of 1-11 will increase as the system approaches equilibrium. Name \géj . 10. The Haber process for the production of ammonia invOlves the reaction between nitrogen and hydrogen gases: N2(g) + 3 H2(g) 33-? .2 NH3(g) AH“: -9l.8 kJ/mol Le Chatelier’s Principle is employed in order to increase the production of ammonia. Of the following possible methods, identify the one that will NOT favor the increased production of ammonia. A. Remove the ammonia by liquefying it and then draining it off B. Increase the concentrations of nitrogen and hydrogen by adding more to the reaction vessel @ Addition of a catalyst D. Raise the pressure by reducing the volume of the reaction vessel E. Run reaction at lower temperatures Name L E FREE-RESPONSE PROBLEMS: (SHOW YOUR WORK TO RECEIVE PARTIAL OR FULL CREDIT) I. (10 points) The molecule cyclohexane (C6H12) can interconvert'between a “boat” form and a “chair” form by twisting about CC bonds: k —‘—> Boat form 5 Chair form k —1 The interconversion is first—order in both directions. The activation energy for chair —> boat is ‘41 kJ/mol, with an Arrhenius pre-exponential “frequency factor” of 1.0 x 1012 s"1 . a. Given that the equilibrium constant Kc .for the reaction as written .above is 9.83 x 103 at 298 K, determine the value of the rate constant k1 at 298K. —- ‘t “A? y. l 0607.1k T ‘5‘ (RT 1 .3 T, «3k. L!“ = k : (‘OY'ID‘ 5-,? Q 3 IL! fishy-rt?— ,_>_)l< i: L :(Ctj’qutfjaugxwvs'l) ' : Q,_§ mwgs" b. The activation energy in the boat —> chair direction is considerably less than the 41 kJ/mol quoted above for the opposite direction. Is the reaction as written above exothermic or endothermic? (Circle your choice below) E exothermic c. Given youranswer to part b, would you predict that Kc would increase, decrease, or stay the same with increasing temperature? (Circle your response below). @ increase endothermic“ decrease stay the same Name Km! 11. (10 points) Chloroform reacts with chlorine in the gas phase to form hydrogen chloride and carbon tetrachloride: A CH613 (g) + 012 (g) —>ch(g> + CCI4<g> The reaction is thought to proceed via the three-step mechanism below: kl fl C12 5 2C1 fast equilibrium k—l Cl+ CHCl3—kz——>HCZ+ (3013 slow Cl + ccz3-—ka—>CCI4 fast a. What rate law does this mechanism predict? {ad-X2 2 kZCQ1CCHC’QS) '1). Given your rate law in part a., what units would you predict for the observed rate constant kobs‘? M l __ l _ "/‘z 4 @ r; m3)» " MHz-s 'M S c. Suppose the reaction were carried out at very high concentrations of CHC13. Assuming that the third step remains fast compared to the first two, how might the mechanism and rate law change? Justify your answer by specifying a new slow step and deriving your new predicted rate law. A4 )1; cams} \Ns‘L cmde TM animailm mm R4112; kzmlfwwslv Mafia, Caz} valifi 23:63 WMOQ £16m rmlth-VLQ m flaw k, mil. Name g i 111. (10 points) At 100 °C, the equilibrium constanth = 6.7 x 10'9 for the decomposition of gaseous phosgene to gaseous carbon monoxide and chlorine: COClz<g>Zco<g) + 012 (g) .a. If an evacuated chamber at 100 °C were filled with COC12 to an initial pressure of 0.250 atm, what would be the partial pressure of .CO after the COC12 decomposes and equilibrium is achieved? CDCQIEZ7 CD’FC/QL 095mm “k 3“ a ' —- _. “X’- ~ at Mr = éq’Y‘ID &,7J§D"'7° A ‘ 69 f” “W ‘ _ am: (o.zSo)Cé.$>uO”°l) "C ‘UX’D 01"“ “ P00 (@0ka gm Warmer) b. Suppose the volume of the chamber was now'reduced by a factor of two with the temperature held constant at 100 °C. According to LeChatlier’ s principle, which way would the equilibrium shift (circle your answer below)? ‘ W ow-l S R91 \M 01: toward reactants toward products W “92% 0. Calculate the new equilibrium partial pressure of CO after the volume reduction described in part b. VAR“ WW 677est by Z . C6 C21 t...) 60‘ ’1“ (/9 2. S I (@130 x2) QMHU 9);; LHchHD— Na), C + we -’\c - o! 6 krzé} x15“ 2- (8.25116 4)" 1WW “X, (35004-06 ' z ,s 6 , _ —r :7 JD :‘BJXCO ~17: : o,soo(g.1xzo '5‘) ~ €7.3wb «M: Co (f ...
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CHEM31EX3key_2011 - Name 2 1 . A chemist measures the rates...

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