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# Version 182 exam 4 holcombe 51395 3 4 1 44

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Version 182 – Exam 4 – holcombe – (51395) 3 4. 1 . 44 Explanation: E = E cathode - E anode = - 1 . 66 - 0 . 22 = - 1 . 88 011 3.0points A copper wire dipped in a solution contain- ing Ag(CN) 2 ions acquires a silver coating because 1. Ag is a stronger reducing agent than Cu. 2. Cu 2+ is a stronger oxidizing agent than Ag + . 3. Ag + oxidizes Cu to Cu 2+ in a spontaneous process until silver metal completely covers the copper. correct 4. H + is reduced by Cu to H 2 , and Ag + ions are reduced by H 2 to metallic silver. Explanation: Ag(CN) 2 provides Ag + ions in solution: Ag(CN) 2 Ag + + 2 CN Ag + + 1 e Ag E 0 = 0 . 7994 V Cu Cu 2+ + 2 e E 0 = - 0 . 337 V Sum: E 0 cell = 0 . 4624 V Since E 0 cell is positive, the process is spon- taneous. Note that Cu is a stronger reducing agent than Ag and Ag + is a stronger oxidizing agent than Cu 2+ . Remember: reducing agents get oxidized and oxidizing agents get reduced . 012 3.0points The rate of formation of oxygen in the follow- ing reaction is 2.28 M/s. 2 N 2 O 5 (g) 4 NO 2 (g) + O 2 (g) What is the rate of formation of NO 2 ? 1. 0.57 M/s 2. 4.56 M/s 3. 2.28 M/s 4. 1.14 M/s 5. 9.12 M/s correct Explanation: The NO 2 forms at 4 × the rate of the O 2 . 013 3.0points A catalyst 1. increases K to favor product formation. 2. changes the reaction mechanism to insure that K is increased. 3. speeds up the reaction and increases K to favor product formation. 4. speeds up the reaction but does not change K . correct Explanation: K is a thermodynamic variable based on Δ G 0 . A catalyst lowers the activation energy of the reaction ( E a ), which speeds it up. It does NOT do anything to the equilibrium position of the system, however. 014 3.0points You like your soft boiled egg to be a 3-minute egg ( i.e. , cooked for 3 min) in Austin. During the summer you vacationed in the mountains in Colorado where water boils at 90 C at 10,000 ft elevation. You found that it took 5 minutes to get the same quality of soft boiled egg. Assuming this is a first order reaction, what is the activation energy for the denatu- ration of the protein ( i.e , the cooking of the egg)? 1. 122.3 kJ 2. 12.3 kJ 3. 57.5 kJ correct

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Version 182 – Exam 4 – holcombe – (51395) 4 4. 86.0 kJ 5. 3.44 kJ Explanation: If the let [egg] t represent the extent of the reaction for the egg at time t and we recognize that we always start and end with the same extent of denaturation of the egg and that this is a first order reaction, then we can write ln parenleftbigg [egg] t [egg] 0 parenrightbigg = - k 1 (3 min) = - k 2 (5 min) k 1 k 2 = 5 3 k has a temperature dependence given by the Arrhenius equation, so for a change in temperatures we can write ln parenleftbigg k 1 k 2 parenrightbigg = E a R parenleftbigg 1 T 2 - 1 T 1 parenrightbigg E a = R ln parenleftbigg k 1 k 2 parenrightbigg 1 T 2 - 1 T 1 = (0 . 008314 kJ K · mol ) ln parenleftbigg 5 3 parenrightbigg 1 363 K - 1 373 K = 57 . 504 kJ / mol 015 3.0points How many moles of of metallic iron (Fe) could be produced from Fe 2+ at a current of 0.004 amperes for 964,853 seconds?
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• Fall '07
• Holcombe

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