611ProblemSet62010 - Cl-IEMISTRY C611 ELECTROANALYTICAL...

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Unformatted text preview: Cl-IEMISTRY C611: ELECTROANALYTICAL CHEMISTRY Fall, 2010 Problem Set 6 . In the figure below is a cyclic voltammogram obtained at 25°C for a 3.4 x 10"4 M solution of trans—Call5CH=CHCOC(CH3J3 in dimethyl stillbxide containing 0.10 M tetra-n-hutylanimoniuni perchlorate as supporting electrolyte. A circular. planar mercury film electrode (on platinum) with an area 010.035 cm2 was used, and the scan rate was 5.00 V s 'l. Using the solid (experimental) curve. instead ol‘thc (theoretical) points, answer the following questions: (a) Is the electrode process reversible? J ustii‘y your answer. (b) What is the a value for the process? (c) What is the half-wave potential (Eng)? ((1) Using the Randles-chcik equation, calculate the diffusion coefficient for the parent compound. “40‘ 30 20 Current, no _- *15 I —|.6 —I.? -l.8 ‘ E. vs: S.C.E., volts 2. Triphenylamine, (C(,H5)3N. is oxidized in a pattern that is common to a number of aromatic amines :r> o: + (D | “3 || +1.00 V vs. SCE- ll +0.60 V vs. SCE : E l c‘; E” = +0.80 V vs. SCE where A is the amine, B is its cation radical. C‘ is a dimeric product (benzidine), l) is the benzidine cation—radical, and E is the benzidine dication. (a) Sketch a full cycle ol‘a cyclic voltannnogram for triplicnylamine over the potential range from 0 to +1.5 V at a slow scan rate, for which H? (where v is the scan rate) is gel}; la_rgg. (b) Sketch a full cycle of a cyclic voltammogram for triphenylamine over the potential range from 0 to +1.5 V at a E: scan rate, for which k/v (where v is the scan rate) is m small. 3. Suppose that compound A undergoes stepwise reduction as follows: A+e— B l l C‘ If both A and C are very strongly adsOrbcd onto mercury and if the initial concentra- tion ofA is of the level that adsorption waves can be seen. how should a cyclic volt- ammograin obtained with a hanging mercury drop electrode appear? CURRENT POTENTIAL 4. Shown below is a cyclic voltannnogram recorded for a solution containing benzophenone (BP) and tri-p-tolylamine (T PTA"). each species being at a bulk conCentration of 1.00 :nM in acetonitrilc containing 0.10 M tetra-n-butylaniinonEum perchlorate. In this soIvent—snpportingI electrolyte system. benzophenonc can be reduced within the accessible range of working potentials. whereas tri-p—tolylamine can only be oxidized. In addition, henzophcnone cannot be oxidized. and tri-p— tolylamine cannot be reduced. In the figure below, the potential scan begins at {J V vs. a quasi—reference electrode (QRli) and moves first in the positive direction. L5 _ I “2.0 —2.5 ' EW’ vs. QR E1 1 mM TPTA. BF cnzcn 0.1MTBABF‘ -15 (a) Assign each of the voltammetric peaks (waves) between +0.5 and +1.0 V and between —1 .5 and —2.0 V to a specific electron-transler process. Comment on the heterogeneous and homogeneous kinetics pertaining to these electrode reactions. (‘0) Why does the falloff in current appear between +0.7 and +1.0 V"? (c) What proeess or processes cause the anodic and cathodic currents at H] .0 V? ...
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611ProblemSet62010 - Cl-IEMISTRY C611 ELECTROANALYTICAL...

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