practiceexam22010

practiceexam22010 - is CHEMISTRY 0611 ELECTROANALYTICAL...

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Unformatted text preview: is.) CHEMISTRY 0611: ELECTROANALYTICAL CHEMISTRY Practice Examination 2 Fall, 2010 (18 points) Four so-called “pulse voltammetrie" techniques have been devised over the years in an effort to minimize the effect of double-layer charging and to lower the concentration of electroactive species which can be determined analytically. These techniques are (i) tast polarography, (ii) normal pulse polarography, (iii) differential pulse polarography, and (iv) square-wave polarography. Choose any three of these techniques; for each of these three techniques, (a) describe the potential program and current-sampling scheme, (b) sketch the form oi‘a representative polarogram or polarographic response, and {c} indicate what a reasonable lower concentration limit is for chemical analysis. ( 20 points) (a) (4 points) Name two advantages that mercury has over platinum as an electrode material. (b) (2 points) Name one advantage that platinum has over mercury as an electrode material. (c) (2 points) Name one advantage that glassy carbon has over platinum as an electrode material. (d) (6 points) List three important properties or characteristics ot‘a solvent for use in electrochemical experiments. (e) (4 points) List two important properties or characteristics ot‘a supporting electrolyte for use in electrochemical experiments. (I) (2 points) It is often essential to place the working electrode and auxiliary electrode in separate compartments ol‘ an electrolytic cell, with these two compartments being in electrolytic contact with each other via a permeable membrane (such as a sintered glass disk). Why is this separation necessary? Lad t l 6 points) In an acetic acid—soditun acetate buffer. p-nitrosophenol undergoes a reversible two-electron reduction at mercury to yield a l'iydroxylaniine species. This hydroxylamine compound is dehydrated to form p-benzoquinoneimine. which can be reversibly reduced to p-aminophenol: OH OH 211* + 2 e‘ E4- ; a“: = ——0.as v vs. SC‘l—i NO til-DH OH 0 Hr '—"‘ + H30 NHOlI NH O OH + 2H+ + 2c" fl ; 51...; = +0.35 v vs. sets NH NH}.- Suppose that a hanging mercury drop electrode is used to study the cyclic voltammetric behavior of‘p-nitrosophenol in an acetic acid—sodium acetate buffer and that the potential of the mercury electrode is scanned from +0.60 to —U.30 V vs. SCE. (a) On a carefully labeled plot, draw a complete cyclic voltammogram to Show the results ofan experiment carried out at a relatively fast scan rate (Le. sufficiently fast to outrun completely the dehydration reaction]. indicate clearly on the plot what electron-transfer process is associated with each eyelie voitammetric peak. (1)) On a carefully labeled plot, draw a complete cyclic voltatnmogram to show the results ofan experiment carried out at a relatively slow scan rate (5.6.. sufficiently slow that the dehydration reaction is not outraced). indicate clearly on the plot what electron-transfer process is associated with each cyclic voltammetric peak. 4. {l8 points} {a} Distinguish clearly between single-potentiall—step rind dotthle—potCIttifli step ehronoeoulemctry. What are the advantages ol‘one over the other technique“.J (b) In 21 certain donhleupot'entiz-tt-step ehrnnoeoulonietric experiment the following graph was obtained when the data were plotted according to the procedure re— ported by Anson [Anal Chem” 38, S4 {1966)}. Q, microcwtombs O l 2. 3 V2. 8, set: For siteh ti graplh fist ercrjr pa.s:rih.-’e piece tgf'ni/t'w'imirion— end I mean every— thing#-that can he obtained. Be sure to indicate what additional data are needed or required in order to obtain each of these pieces of ini'm'matie-n. 5. (16 points) According to the Cottrcll equation for a chronoainperomctric experimenL the current. i'. is given by the relation i H F fl D“: Cult/Til"? Eli-2 where n is the number of electrons involved in the redox reaction, F is the li‘arattay constant. A is the electrode area D is the diffusion coefficient ol‘the elcctroactive species, (7* is the bulk concentration of the electroactive species. and r is time. (a) [4 points} Describe how one performs or carries out a elironoamperometric experiment. (bl (4 points) Describe or show how the data obtained in a ehronoarnperometrie experiment are plotted to prove that the Cottrell equation is obeyed. (e) (4 points) Using the result of part (b). describe or show how convection [or stirring, or movement) of the solution affects chronoamperometric data. (d) (at points) Using the result of part (b). describe or show how charging ot'the double—layer al't‘ects chronoamperometric data. (i. [12 points) In polarography with a dropping mercury electrode. or in cyclic voltammetry with a hanging mercury drop electrode, the reduction ol‘a solution—plume reactant" species (0) to a solution-phase product species (R). according to the half- reaction 0 + H e a“: R can show a pest-wave it'O is adsorbed. Prove, using appropriate thermodynamic and eteetrochemical arguments, that the reduction ot’ adsorbed 0 will indeed give rise to a posts fave. ...
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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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practiceexam22010 - is CHEMISTRY 0611 ELECTROANALYTICAL...

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