AnswersforPracticeExam3

AnswersforPracticeExam3 - CHEMISTRY (3611:...

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Unformatted text preview: CHEMISTRY (3611: ELECTROANALYTICAL CHEMISTRY Practice Examination 3 Fall, 2010 SOLUTIONS KEY 1. Answers For this problem can be obtained 'I'rom pertinent sections in Bard and liaulkner or From the class readings. 2. (a) A key ads-'antage ot‘a divided cell is that products lormed at the auxiliary electrode are prevented from reaching the working electrode. A key disadvantage ot‘a divided cell is that the device (sintered-glass disk or other membrane) which separates the two compartments might limit the current that can be passed. A key advantage of an undivided cell is that high currents can be passed. A key disadvantage ol'an undivided cell is that products generated at the anode and cathode can interminglc. (1)) Remove dissolved oxygen by bubbling argon, helium. or nitrogen through the solution. Alternatively; one can employ a freeze-puntp-thaw procedure. (is) Record a cyclic \-'oltammogram for the species of interest in the same medium and for the same electrode. For a reduction. choose a potential 420 111V more negative than rim; [or an oxidation- choost: a potential -~ I 30 mV more positive than lEpit- {d} Record the background current in a blank electrolysis: then. in an actual electrolysis. continue the experiment until the background current is reached. Alternativ-ely: observe the initial current; then. to gel 90% completion ol’the electrolysis. continue the experiment until the current is l% ol’its initial value. 3. Answers should be obtainable from Bard and Faulkner or From one ot’the. extra reading assignments. 4. (a) +0.4 ('3) +0.4. (c) Whereas curve (a) is a voltannnogram of “normal” appearance, curve (b) is sigmoidal because (with an ultramicroelectrode at a slow scan rate) the product (ferriciniuni ion) essentially escapes the diffusion field and cannot be reoxidized; the essential “retracing” of the forward scan arises because the fcn‘ocene can be replenished (since it is diffusing toward the electrode). 5. (a) i (b) iii (c) iii ((1) iii (c) iv (a iii 6. Answers should be obtainable from Bard and Faulkner or from one of the extra reading assignments. 7'. (3) Among the experimental parameters to be controlled are the following: (i) time ofprc-electrolysis (deposition) (ii) rate of stirring during prevelectrolysis (iii) size ofelectrode (hanging mercuiy drop of thin mercury film) (iv) time for “resting” period (v) volume ot‘solution (Vi) position of working electrode within the cell 1 - - - - —5 —IU {13) 1 he upper and lower concentration limits are approxrinately 10 M and 10 M, respectively. For concentrations higher than 10“” M. it is usually better to choose another analytical methods that is less cumbersome. For concentrations lower than 10—10 M, the level of impurities in the background electrolyte solution may prevent detection of still smaller concentrations. I e 8. (a) B4 C J ' “m b I W._y._?_‘ 1) r i J /i 121 D 'I"’*""~’—i—— __. _.‘I_.__.._‘L.__' _- -1..- _. _ .1“ +9.33 4.3.: +0, r- a aunts ~05 v =25. :CE t1) (1'?) U [19,}: At. ER: ’ +9, ‘./ v“ c H E I ,,i _ 1 . Elm—X _LIIQ-.flc Fr v _F '~ C ‘5’ B C 54A C-atm 01-" (Ha: I, a mil? 3‘ j” ‘ Urey-era 8. (continued) (0) wwwww-..“ _._ “0.25 "0.25 V vs, SCE M (£1 “@4pr “egg-n.3,, I +0.15 +0.5 +0. 2.5 0 ...
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This note was uploaded on 01/18/2012 for the course C 611 taught by Professor Dennisg.peters during the Fall '10 term at Indiana.

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AnswersforPracticeExam3 - CHEMISTRY (3611:...

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