chem1a-fall00-mt3b-Pines-exam - Chemistry 1A Fall 2000

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Unformatted text preview: Chemistry 1A Fall 2000 __________________________________________________________________________________ Midterm Exam III, version B November 14, 2000 (Closed book, 90 minutes, 155 points) Name: ______________________ Section Number: ____________________ SID: _______________________ T.A. Name: ________________________ Exam information, extra directions, and useful hints to maximize your score: • • • • • Write your name on all 8 pages. There are two parts to the exam: 1) multiple choice and 2) short answer problems. For the multiple choice problems, fill in the ScantronTM form AND circle the answer on your exam. Answer the questions you know how to do first, then work on the questions you skipped. Show all work on the short answer problems for which you want credit and do not forget to include units! ________________Unit Prefixes________________ milli, m (x 10-3) micro, µ (x 10-6) nano, n (x 10-9) kilo, k (x 103) mega, M (x106) giga, G (x 109) Some possibly useful information: ∆H o = ∑ ∆H o (products) − ∑ ∆H o (reactants) f f ∆S = ∑ S (products) − ∑ S (reactants) o o o ∆G = ∆H - T∆S Cp°(H2O, l)= 1 cal/K·g S = kBlnW CO (gas) ∆Hf° (kJ/mol) at 25oC -110.5 S° (J/mol-K) at 25oC 197.7 CO2 (gas) -393.5 213.6 H2 (gas) 0 130.6 Compound 1 Cal = 1000 cal Average Bond Enthalpy (kJ/mol) 413 C-C 348 C-O 360 O=O 497 O-H 463 H-H 436 -285.8 70.0 CH3OH -238.7 126.8 C2H5OH Bond C-H H2O (liquid) -277.7 160.7 CH3COOH ∆E = q + w -484.5 159.8 (Do not write in the box below.) Page 2-4 Points / 48 5-6 / 60 7-8 / 47 Total / 155 Page 2 of 2 Name:_________________________________________ Part 1: Multiple Choice. (4 pts each, 48 pts total) Instructions: Bubble in the correct answer on your Scantron sheet AND circle the answer on your exam. Each question has one correct answer. 1.) The answer to question 1 is B. Bubble in B on your Scantron™ form. 2.) Which has K ≠ PCO2 at equilibrium? A.) Na2CO3 (solid) Na2O(solid) + CO2 (gas) B.) CaCO3(solid) CaO(solid) + CO2 (gas) C.) C(solid) + O2 (gas) D.) CO2 (solid) CO2 (gas) CO2 (gas) E.) 2 MgO(solid) + C(solid) 3.) 2 Mg(solid) + CO2 (gas) For the exothermic formation of ammonia, N2(gas) + 3 H2 (gas) which will act to favor the formation of ammonia? A.) high pressure B) high volume D.) catalyst 4.) C.) high temperature E.) all of the above Which of the following is true for the isothermal expansion of an ideal gas? A.) w > 0 5.) 2 NH3(gas) B.) ∆T > 0 C.) ∆V < 0 D.) ∆SSys > 0 E.) ∆SSurr > 0 Which of the following has the greatest entropy? A.) 1 mole of water at 25 oC and 1 atm B.) 1 mole of water at 100 oC and 1 atm C.) 0.5 moles of water plus 0.5 moles of steam at 100 oC and 1 atm D.) 1 mole of steam at 100 oC and 1 atm E.) 1 mole of steam at 100 oC and 0.5 atm Page 3 of 3 Name:_________________________________________ 6.) Which is true for the following exothermic reaction at 1 atm? Fe(solid) + CO(gas) FeO(solid) + C(solid) A.) B.) C.) D.) E.) 7.) Proceeds spontaneously at all temperatures Proceeds spontaneously at no temperatures Proceeds spontaneously at high temperatures Proceeds spontaneously only at 0 K Proceeds spontaneously at low temperatures How many glasses of water (200 mL) can be heated from 10 oC to 35 oC by the combustion of a mini-Snickers bar (40 Cal)? A.) 1 8.) C.) 40 D.) 500 E.) 1000 To the formation of which compound from the elements in standard states does the following process of bond-breaking ! bond formation correspond? 2 C-C +2 H-H + ½ O=O ! 3 C-H + C-O + O-H A.) ethanol 9.) B.) 8 B.) formaldehyde C.) methanol D.) acetaldehyde E.) acetic acid Using the information on page 1, estimate ∆Ho for the process in question 8. A.) 246 kJ/mol B.) 45 kJ/mol C.) 3 kJ/mol D.) -45 kJ/mol E.) -246 kJ/mol 10.) When you burn wood, energy is released. Which statement best describes where that energy comes from? A.) The reaction breaks the chemical bonds in the wood, releasing their energy. B.) Energy is released because wood and all organic materials have high energy bonds. C.) When wood burns, its weak bonds break to combine with the stronger oxygen-oxygen double bonds, releasing energy. D.) When you burn wood, the products are more stable than the reactants so energy is released. E.) When wood is a reactant, its high energy bonds release energy when they break. Page 4 of 4 Name:_________________________________________ 11.) Consider the dissociation reaction A2 (g) 2 A (g). The following pictures represent one possible initial state and the equilibrium state for the system. Initial State Equilibrium State Which of the following statements is true for this dissociation reaction? A.) This reaction favors products at all temperatures because the number of A atoms increases and the number of A2 molecules decreases. B.) This reaction favors reactants at all temperatures because the A2 molecules have a bond which makes them more stable. C.) This reaction favors reactants at lower temperatures because heat is absorbed. D.) This reaction favors reactants at higher temperatures because heat is absorbed. E.) This reaction doesn’t favor products or reactants since the equilibrium constant equals 1. 12.) Which process is accompanied by the largest increase in entropy? I II III A.) I!V IV B.) I!II D.) III!IV V C.) II!III E.) IV!V Page 5 of 5 Name:_________________________________________ Part 2: Short Answer Problems (107 pts total) Instructions: Enter answers for all questions in the boxes provided (or as otherwise instructed). Show your work. Where requested write explanations in fifteen words or less. 1.) (60 pts) Consider the following reaction: CO2(gas) + H2(gas) CO(gas) + H2O(liquid) o At equilibrium at 25 C, PCO2 = PH2 = 1.0 atm ; PCO = 3.2 x 10-4 atm . a.) The volume is suddenly reduced by a factor of ½. What are Q and K? How will the reaction proceed toward equilibrium? Q: K: Short explanation of re-establishment of equilibrium: . b.) What are the partial pressures of all the species in the reaction at the new equilibrium? PCO2: PH2: PCO: Page 6 of 6 Name:_________________________________________ c.) Using the information on page 1, determine ∆H° for the reaction from ∆Hf°’s and determine ∆S° for the reaction from S°’s. Explain the sign of ∆S°. ∆H°: ∆S° Explanation: d.) Determine ∆G° for the reaction at 25 oC. ∆G° e.) Below what temperature does the reaction become spontaneous? T: Page 7 of 7 Name:_________________________________________ (47 pts) 2.) For questions a – d consider the following: Different hydrocarbon structural isomers burn differently in oxygen. This is the basis for the ‘octane’ rating of gasoline. Consider three structural isomers, X,Y and Z, of a certain hydrocarbon. The hydrocarbon isomers can react as follows at 298 K. 1) X K1 = 20.0 ∆H° = -40.0 kJ/mol ∆S° = -120 J/K mol 2) Y a.) Y Z K2 = 0.4 ∆H° = 20.0 kJ/mol ∆S° = 60 J/K mol What is the equilibrium constant for X Z? Keq: b.) Arrange the compounds X, Y, and Z on the relative enthalpy scale below? _______ ∆Hf° _______ _______ Page 8 of 8 Name:_________________________________________ c.) Upon combustion, which isomer, X or Z, burns hotter? Circle one and explain. X Z Same Explanation: d.) Sketch a plot of ∆G° vs. T for the equilibrium X Z. ∆Gº 0 T ...
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