Practiceexam42010fall

Practiceexam42010fall - CHEMISTRY C611: ELECTROANALYTICAL...

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Unformatted text preview: CHEMISTRY C611: ELECTROANALYTICAL CHEMISTRY Practice Examination 4 Fall, 2010 I. One of the classic processes in organic electrochemlstl')’ lll\"t}l\’CS the anodic oxidation ofcarhosylates [RCTJU ) at a \-'ariet}-' of electrodes and in a variety ofmedia. This process is known as [a] the Monsanto reaction. (1)} the Kolhe oxidation. [e] the [[ofmann elimination. {d} radical tantlcnrcyclization. (c) the Naleo synthesis. 2. 'fctraalk}-=]alninoriitiin salts (as. lClI‘EtCll1}-’lEtlnll]()l1lUm tetra]lnorohorate. Nlel'l_<']4TBlf.t--). used as electrolytes in organic solvents. can he involved in organic electrochemical processes in a numher o f ways. Which ofthe following processes is not seen with one of these salt‘s‘.J (a) A lL‘ll't'tL'llli}-'l£1l"l]li'l(illile cation can serve as a proton donor toward stroneg hasie earhanions. (hi A tetra'ailkylalnmonium cation can he reduced at a mercury cathode to produce an organoinercury species which is a powerful reducing agent. {c} A letraallift-'lammonium cation can he incorporated into the surface ol'a platinum cathode. allowing electrolyses to he carried out at exlraordinariiy negative potentials. (d '1 A tetraalk}-’iai'tlliitiiiittiii cation can stabilize anionic intermediates Via ion- pairing, tic} A tctraalkylainmrminm cation can react with traces of water to form an alcohol. 3. lill-"hieh of the following sahstances is not ordinarin used as a solvent for organic electrocliemistry'L’ {a} henzene th} diinethylfrn'niamide tc] aeetonilrilc td} telrahydroftn‘an tel methylene chloride {dicltloromcthancl In.) 4. interaction ol‘a functionalizcd alkanethiol with a clean gold surface results in the formation of what is known as L‘- ) an organic conducting polymer. ) a Langmuir—Blodgett film. .J an ion-exchange polymer. 5‘ f“ t a hydroxylatcd surface. a self~assenibted monolayer. fifiAA—efi r'} {'3 5. Using gold electrodes. how did H. A. 0. Hill and co—workers obtain cyclic voltammo— grams for the reversible reduction and oxidation ofthe iron center in cytochrome c! (a) These workers carried out ultrafast cyclic voltanimctry with a bare gold electrode to overcome the effects ofslow electron—transfer kinetics. [13) These workers employed gold ultramicroelcetrodes. the areas of which were so small that adsorption of cytochrome c could not occur. (c) These workers used a promoter molecule, that was adsorhahlc onto gold, to achieve electron—transfer communication between the gold electrode and cytochrome c. (d) These workers used indium~dopcd gold Oxide semiconductor electrodes to achieve reversible reduction and oxidation of the iron center in cytochrome c. (C) These workers employed adsorptive stripping analysis at gold electrodes to observe the reversible reduction and oxidation ofthe iron center in cytochrome C. 6. Suppose that an electrode is coated with a polymer film that contains redox-activc sites and this polymer—coated electrode is used to study the behavior of an electro- active solute in a solution in contact with the polymer-coated electrode. Which of the tbllowing mechanisms for mass or charge transport is least tikelv to be involved in the overall process? ( a) electron diffusion or hopping from one redox—aetive site to another within the polymer film (h) clifl‘ttsiou of the electroactive solute into and through the polymer tilm (C) electron transfer between the electroactive solute and a redox~aetive site within the polymer [ilm (d) direct electron transfer or hopping from the electrode. all the way across or through the polymer film. to the electroactive solute in solution (e) diffusion of the electroactive solute through the polymer film and up to the surface of the electrode at which electron transfer between the electrode and the electroactive solute can occur “.4 7. Which el‘the [bllowing plots 0H3 (the potential] efan ion-selective electrode) versus leg ml {the base—ten logarithm ofl'he activity ol‘a desired monovalent cation Ai') beet represents a situation in which another monex-uleni [and interfering) cation 8' is also present? (a) (b) E E -5 —4 -3 -5 -4 —3 let:T (IA... 10:; as.” (L) (d) E E ~5 —4 —3 —5 —4 H3 10g at.“ log (1A,- (6} E -5 -4 -3 8. A representation ofthe galvanic cell commonly used for pll measurements is Hg l [-IggC‘lfis) I KC‘l(_s) l test solution l glass membrane l-[Cl (0.] M) l AgClts) I Ag (1 b c d c f 1‘ I g") with each interface [or phase boundary) being. denoted by an italicized. boldface letter. Which ofthese interfaces (or phase boundaries) is most responsible for the changes in the overall voltage of the cell (Ecru) that allow us to determine pH? (at) e (b) c (c) d (d} b ('e} g 9. Several electroanalytical methods can be used to study and characterize a chemically modified electrode. Which of the following statements concerning these methods is FALSE? (a) Cyclic voltammetly is useful because the technique provides information about the redox potential of the chemi ‘ally modified surface. (b) Differential pulse voltammctry is useful only ifthe thickness ofthe film is substantial. (c) Chronopotentiometry is useful because the measurement of the transition time at constant current provides a rapid assessment ofthe thickness of a polymer film. (d) Hydrodynamic voltammetry is useful because the technique provides information about the thickness of a polymer film. (6) Cyclic voltammetry and chronolaotentiometry can both provide information about the robustness or pern‘lanence of a film on an electrode surface. 10. From a commercial standpoint. the most well known organic electrochemical reaction is (a) the Kolbe dimerization. (b) the Monsanto process. (c) the l-Iofn'tann elimimition. (d) the Nalco synthesis. (e'} the Kryschenko process. l l. The standard potential (ED) for the one—electron reduction oi‘ cytochrome c in its oxidized (Fe‘i'i form) is -t [1.02 V vs. SCE. However, the Feb form cannot be reduced directly at a bare gold electrode because the rare. ot‘its electrorcduction is prohibitively slow, but it is possible to use an electron-transfer mediator to reduce the Fe'i' form catalytically. Five mediators are available as follows: Mediator A (5° = +0.5 V vs. SCE); Mediator B ( fl = —U.2 V vs. SCE); Mediator C (5" = —U.=i V vs. SCE); Mediator I.) (150 = +0.3 V vs. SCE); and Mediator E (5” = +0.2 V vs. SCE). Which mediatort’s) could be used successfully to reduce the oxidized icon of cytochrome c catalytically‘? ( ) Mediator A ( ) Mediator B (c) Mediators A and E ( ) Mediators B and C ( ) Mediators A. D, and E 12. Organic clcctrochemistry can be carried out on a micro or macro scale. With respect to these two approaches, which of the following statements is FALSE? (a) in comparison with micro methods (such as cyclic voltammetry). macro methods (such as controlled-potential electrolysis) favor the occurrence of hi gher-order processes (such as second-order reactions). (b) In comparison with micro methods (such as cyclic voltammetry), macro methods (such as controlled-potential electrolysis) are apt to suffer from the problem that the pH oi'tlie solution changes during the course ot’the experiment. (c) In comparisou with micro methods (such as cyclic voitammetry), macro methods (such as controlled-potential electrolysis) can be accomplished much more rapidly. (d) in comparison with micro methods (such as cyclic voltammetry), macro methods (such as controlled—potential electrolysis) more readily permit the identification and determination of the electrolysis products. (e) In comparison with micro methods (such as cyclic yoltan'tmetry), macro methods (such as controlled—potential electrolysis) may allow intemiediatcs and products to undergo prolonged contact with the electrode. leading to adsOIption-related processes. l3. 6 A number of spectroscopic methods can be employed to probe the surface molecular structure ofehemically modilied electrodes. Which ofthe following melhodologies is least attractive and least sensitive for the characterization of surface modifications of monomolecular thickness? (at) ultraviolet—visible transmission spectrophotometry at transparent electrodes (b) X-ray photoelectron spectroscopy (c) Auger electron spectroscopy ((1 scanning, electron microscopy (e) internal reflection infrared spectroscopy 14. Which ofthe following is unlikely to be a chemical functionality at the edge plane of a pyrolytic carbon electrodc‘.’ {a} carboxylic acid group {_—COOII) (b) alcohol group (—011) (c) aldehyde group {—CHOJ (d) acid anhydride group (O=C—O—C=O) | | to) acid chloride group (Cl—C=O) For the glass-membrane electrode for pH measurements, potential differences arise at the interface between the test solution and the outer hytlrated~gel layer and also at the interface between the inner hydrated—gel layer and the internal filling solution ofthe glass—inernln'anc electrode. These interfaeial potential differences are caused by an equilibrium balance of selective charge exchange across an interface; such an equilibrium is known as the )(TI a) potentiolnetric selectivity coefficient, 3'." Hats-.13. ( (b) Donnan equilibrium. (c) liquid-junction potential. (d) membrane potential. ('c) mobility ratio. “mam... 16. A valinomycin liquid iowexchange electrode. designed for the determination ofK+. was found to have the tollowing potentiometric selectivity coefficients for different combinations ol‘alttali-nietal cations: d’lmlfitNal :: l X WK we 2 0.44 = 2 s _ m ftp K-‘.R|‘r- - Which ot‘ the following statements concerning:T the response ofthe electrode is FAI'JSE'? (a) Na+ interferes greatly with the determination of 1C. (b) Cs" and Rb' interfere greatly with the determination ot'K'l'. (c) Rb' interferes more than does Cs' with the detennination ot'K+. (d) This electrode responds better to K'r than to Nah {e} This electrode responds better to Rb+ than to K". 17. What is the key or essential feature of an ISFET (ion—selective field effect transistor) that ultimately provides a solid-state ion sensor“? {a} the reference electrode (b) the n~typc drain (c) the p-typc base (d) the n-typc source (e) the chemically sensitive layer over the gate 18. in the study by Anson and co-workers oftlie use of vanaditun salen species to carry out the Four-electron reduction ofoxygen to water, the reversible one—electron oxidation ot'VWO(salen) to VVO(salen)' was proposed as part of the mechanistic picture. It'you wished to prove that this process is reversible by performing cyclic voltammetry experiments with a planar electrode, which of the following criteria would not be adequate? {a} the shape of the anodic peak should conform to that for a reversible process (b) rm, should be directly proportional to 1'“: (e) tpct’im for the VWO(salen)—\t""0(salen)" couple should be 1 for all 1? (d) the difference in peak potentials should be 59 mV for all v (e) the Randles—Sevcik equation should be strictly obeyed 19. 111 work by Hill and CO-WUI'RCI'S concerning, the electrochemistry ofcytoehrome e at a gold electrode coated with a promoter molecule such as 4.4'-bipyridine, the pll ot‘the solution containing the cytochrome c was found to be critical to the success of the experiment. Why? (a) The pH must be sulliciently low to protonate both nitrogen atoms of4.4‘— bipyridine. so that the promoter molecule can serve as a link between cytochrome c and the gold surface. (b) The pH must be sufficient‘l y high to ensure that neither the promoter molecule nor cytochrome c is protom-tted. (c) The pH must be in a proper range to protonalc the nitrogen atom of=l,4'— bipyridine that points outward into the solution, so that the premoter molecule can dock with cytochrome c. (d) The pH must be in a proper range to protonate the lysine residues at the surface ol‘cytochromc c. so that multiple hydrogen bonds can be formed to link a molecule of cytochrome c to an array of surface-adsorbed promoter molecules. (e) The pH must be sufficiently high to ensure that hydroxide ions are present to be co-adsorbed onto the surface of the gold electrode. 20. Which of the following properties or characteristics of a promoter molecule would not be desirable? (a) The promoter should preferably be bifunctional. (b) The promoter must possess a functional group that is adsorbable. (C) The pron'toter should present a cationic functionality to the metalloprotein. (d) The promoter must possess a functionality that can interact with a metalloprotein. (c) The promoter can have a link that is either rigid or flexible. 2]. [n electrochemical studies ol'metalloproteins. there is a tendency for these substances to undergo irreversible adsorption. coupled with conformational changes and loss of redox activity. Electrochemists commonly refer to this phenomenon as __ _ (a) activation ovcrpotential. (b) irreversible electron transfer. to) electrode fouling. (d) Langtnuir-Blodgett film formation. (e) spin coating. 9 22. When Steiger and Anson investigated the triple "‘ruthenation" ot‘compound ('l). they employed the [RutNl-IfingOf'l complex. Which ofthc following statements about this system is TRUE? (a) [RU(Ni'i3)5i'i]O]2T is not electroactive. (b) [Ru(NI-13)5HgO]2' does not interact with Nation, which is part of the matrix used to immobilize “ruthenated” I. (c) [RU(NH3)5]']30]Z+ catalyzes the four-electron reduction ofoxygen to water. (d) [RLI(_NHg)5H30:]2+ forms an adsorbed monomoleeular film on a carbon electrode. (0) I'RutNHmHgOf' is very labile (meaning that the 1-130 ligand is easily removed), making the -CN groups on I easy to “ruthenate.” 23. What electroz-malytical technique did Steiger and Anson employ to determine the catalytic efficiency 01' the reduced form of I for the four—electron reduction of oxygen? (21] hydrodynamic voltammetry with a rotating platinum ring-carbon disk electrode (h) cyclic voltammetry with a carbon disk electrode (e) thin-layer electrochemistry with a planar carbon electrode {(1) electrochemistry with twin glassy carbon band ultramicroelectrodcs (e) double—potential—step chronocoulomctry with a glassy carbon disk electrode l l) 24. Consider the hydrodynamic voitammograin (obtained with a rotating glassy carbon disk) shown by the solid line i3 Panel B ofthc figure below. D (1.5 1.0 E {V vs Agi‘AgCI} For what kind of system would such a voltamlnograrn be obtained? (a) a solution containing a species in a single oxidation state that can reversibly undergo either oxidation or reduction [b] a niore—or—less equimolar solution of two dilTerenl (but int‘ercolwettible) oxidation states of the same species (c) a solution containing a species that undergoes oxidation to form an adsorbable product ((1) an electrode coated with a polymer-bound electrtiactive organmnetallic compound that can undergo either oxidation or reduction (C) an electrode coated with a film ot‘clcmental M that undergoes an n-electron oxidation in the presence oft-t ligand (If) to form the uncharged complex M L". ll 25. An organic electrode process can be Visualized in terms of a set or series of events. Four m-ents—not in the order in which thev normallv occur- --are as follows: (i) follow-up chemical reactions (tag. coupling, proton transfer, isoinerization) (ii) adsorption and geometrical orientation of starting material (iii) diffusion ot’starting material (perhaps with preceding reaction) (iv) electron transfer to produce intermediale What is the most logical sequential order ot‘these events? (a) til—(iiJ—(iiil—(iv) (b) (iiil—(iVJ--(ii)—(i) (C) (iiil—(ii}--(iv)—(i) (d) (ii) — (iV) - (iii) - (i) (‘3) (iil-(iiiJ-(iVJ— (i) ll. 12. 13. 14. 15. [(1. 17. I8. 19. 20. 21. 22. LAM-- UI I} her—1F: 23. a 24. 25. Answers Practice Exmninatiun 4 Fall, 2010 ...
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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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Practiceexam42010fall - CHEMISTRY C611: ELECTROANALYTICAL...

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