A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 25
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S. Mathison and E. Bakker, Anal. Chem., 70,303 (1998).67. A. J. Cunningham, "Introduction to Bioanalytic a l Sensors," Wiley, New York, 1998, Chap. 4.68. H. S. Yim, C. E. Kibbey, S. C. Ma, D. M. Kliza,D. Liu, S. B. Park, C. E. Torre, and M. E. Meyerhoff, Biosens. Bioelectron., 8, 1 (1993).2.6 PROBLEMS2.1 Devise electrochemical cells in which the following reactions could be made to occur. If liquidjunctions are necessary, note them in the cell schematic appropriately, but neglect their effects.(a) H 2 O ^± H + + OH~(b) 2H 2 + O 2 ^± H 2 O(c) 2PbSO4 + 2H 2 O ±± PbO 2 + Pb + 4H + + 2 S O f(d) An T 4- TMPD^ ±± An + TMPD (in acetonitrile, where An and An T are anthracene and itsanion radical, and TMPD and TMPD^ are AW^'^-tetramethyl-p-phenylenediamine and itscation radical. Use anthracene potentials for DMF solutions given in Appendix C.3).(e) 2Ce 3 + + 2H + + BQ ^± 2Ce 4 + + H 2 Q (aqueous, where BQ is p-benzoquinone and H 2 Q is phydroquinone)(f) Ag + + I~ <± Agl (aqueous)(g) F e 3 + + Fe(CN)£~ <=» Fe 2+ + Fe(CN)3," (aqueous)(h) Cu 2+ + Pb «± Pb 2 + + Cu (aqueous)(i) AnT + BQ <=^ BO T .
+ An (in А^Д-dimethylformamide, where BQ, An, and An T are definedabove and BO~ is the anion radical of p-benzoquinone. Use BQ potentials in acetonitrile givenin Appendix C.3).What half-reactions take place at the electrodes in each cell? What is the standard cell potential in eachcase? Which electrode is negative? Would the cell operate electrolytically or galvanically in carryingout a net reaction from left to right? Be sure your decisions accord with chemical intuition.2.2 Several hydrocarbons and carbon monoxide have been studied as possible fuels for use in fuelcells.
From thermodynamic data in references 5-8 and 16, derive E°s for the following reactions at25°C:(a) CO(g) + H2O(/) -> CO2(g) + 2H + + 2e(b) CH 4 (#)+ 2H2O(/) - * CO2(g) + 8H + + 8e(c) C 2 H 6 (g) + 4H2O(/) - * 2CO2(#) + 14H + + Ue(d) C2H2(g) + 4H2O(/) -> 2CO2(#) + 10H + + 10*Even though a reversible emf could not be established (Why not?), which half-cell would ideallyyield the highest cell voltage when coupled with the standard oxygen half-cell in acid solution?Which of the fuels above could yield the highest net work per mole of fuel oxidized? Which wouldgive the most net work per gram?2.6 Problems852.3 Devise a cell in which the following reaction is the overall cell process (T = 298 K):+2Na + 2СГ -> 2Na(Hg) + Cl 2 (aqueous)where Na(Hg) symbolizes the amalgam.
Is the reaction spontaneous or not? What is the standardfree energy change? Take the standard free energy of formation of Na(Hg) as —85 kJ/mol. From athermodynamic standpoint, another reaction should occur more readily at the cathode of your cell.What is it? It is observed that the reaction written above takes place with good current efficiency.Why? Could your cell have a commercial value?2.4 What are the cell reactions and their emfs in the following systems? Are the reactions spontaneous?Assume that all systems are aqueous.+(a) Ag/AgCl/K , СГ (1 M)/Hg2Cl2/Hg3+2+2+(b) Pt/Fe (0.01 M), F e (0.1 M), HC1 (1 M)//Cu (0.1 M), HC1 (1 M)/Cu+(c) Pt/H2 (1 atm)/H\ С Г (0.1 M)//H , С Г (0.1 M)/O2 (0.2 atm)/Pt++(d) Pt/H2 (1 atm)/Na , OH~ (0.1 M)//Na , OH" (0.1 M)/O2 (0.2 atm)/Pt+(e) Ag/AgCl/K , С Г (1 M)//K\ С Г (0.1 M)/AgCl/Ag3+4+2+3+(f) Pt/Ce (0.01 M), C e (0.1 M), H 2 SO 4 (1 M)//Fe (0.01 M), F e (0.1 M), HC1 (1 M)/Pt2.5 Consider the cell in part (f) of Problem 2.4.
What would the composition of the system be at the endof a galvanic discharge to an equilibrium condition? What would the cell potential be? What wouldthe potential of each electrode be vs. NHE? Vs. SCE? Take equal volumes on both sides.2.6 Devise a cell for evaluating the solubility product of PbSO4. Calculate the solubility product fromthe appropriate E° values (T = 298 K).2.7 Obtain the dissociation constant of water from the parameters of the cell constructed for reaction (a)in Problem 2.1 (T = 298 K).2.8 Consider the cell:Cu/M/Fe2+, Fe 3 + , H+//Cr/AgCl/Ag/Cu'Would the cell potential be independent of the identity of M (e.g., graphite, gold, platinum) as longas M is chemically inert? Use electrochemical potentials to prove your point.2.9 Given the half-cell of the standard hydrogen electrode,Pt/H2 (a = 1)/H+ (a = 1) (soln)H 2 <=± 2H+(soln) + 2e(Pt)Show that although the emf of the cell half-reaction is taken as zero, the potential difference between the platinum and the solution, that is, ф Р 1 — 0 s , is not zero.2.10 Devise a thermodynamically sound basis for obtaining the standard potentials for new half-reactions by taking linear combinations of other half-reactions (T = 298 K).
As two examples, calculate£"° values for(a) Cul + e ^ Cu + I"(b) O 2 + 2H + + 2e ±± H 2 O 2given the following half-reactions and values for £ ° (V vs. NHE):Cu 2+ + 2e *± CuCu2+0.340+ I" + e <=> CulO 2 + 4H++ 4e ?± 2H2OH 2 O 2 + 2H + + 2e ?± 2H2O0.861.2291.7632.11 Transference numbers are often measured by the Hittorf method as illustrated in this problem.
Consider the three-compartment cell:LСR©Ag/AgNO3(0.100 M)//AgNO3(0.100 M)//AgNO3(0.100 M)/Ag ©86 • Chapter 2. Potentials and Thermodynamics of Cellswhere the double slashes (//) signify sintered glass disks that divide the compartments and preventmixing, but not ionic movement. The volume of AgNO3 solution in each compartment (L, C, R) is25.00 mL. An external power supply is connected to the cell with the polarity shown, and current isapplied until 96.5 С have passed, causing Ag to deposit on the left Ag electrode and Ag to dissolvefrom the right Ag electrode.(a) How many grams of Ag have deposited on the left electrode? How many mmol of Ag have deposited?+(b) If the transference number for Ag were 1.00 (i.e., tAg+ = 1.00, J N O - = 0.00), what would the+concentrations of Ag be in the three compartments after electrolysis?+=(c) Suppose the transference number for Ag were 0.00 (i.e., tAg+ = 0.00, ^NO 3 1-00), what+would the concentrations of Ag be in the three compartments after electrolysis?(d) In an actual experiment like this, it is found experimentally that the concentration of Ag + in theanode compartment R has increased to 0.121 M.
Calculate tAg+ and t^oj>2.12 Suppose one wants to determine the contribution of electronic (as opposed to ionic) conductionthrough doped AgBr, a solid electrolyte. A cell is prepared with a film of AgBr between two Agelectrodes, each of mass 1.00 g, that is, ©Ag/AgBr/Ag©. After passage of 200 mA through the cellfor 10.0 min, the cell was disassembled and the cathode was found to have a mass of 1.12 g. If Agdeposition is the only faradaic process that occurs at the cathode, what fraction of the currentthrough the cell represents electronic conduction in AgBr?2.13 Calculate the individual junction potentials at T = 298 К on either side of the salt bridge in (2.3.44)for the first two concentrations in Table 2.3.3.
What is the sum of the two potentials in each case?How does it compare with the corresponding entry in the table?2.14 Estimate the junction potentials for the following situations (T = 298 K):(a) HCl(0.1M)/NaCl(0.1M)(b) HC1 (0.1 M)/NaCl (0.01 M)(c) KNO 3 (0.01 M)/NaOH (0.1 M)(d) NaNO3 (0.1 M)/NaOH (0.1 M)2.15 One often finds pH meters with direct readout to 0.001 pH unit.
Comment on the accuracy of thesereadings in making comparisons of pH from test solution to test solution. Comment on their meaning in measurements of small changes in pH in a single solution (e.g., during a titration).2.16 The following values of Щ[+^ are typical for interferents / at a sodium-selective glass electrode:K + , 0.001; NH4", 10~5; Ag + , 300; H + , 100.
Calculate the activities of each interferent that wouldcause a 10% error when the activity of Na + is estimated to be 10~3 M from a potentiometric measurement.2.17 Would Na 2 H 2 EDTA be a good ion-exchanger for a liquid membrane electrode? How aboutNa2H2EDTA-R, where R designates a C 2 Q alkyl substituent? Why or why not?2.18 Comment on the feasibility of developing selective electrodes for the direct potentiometric determination of uncharged substances.2.19 Consider the exhaust gas analyzer based on the oxygen concentration cell, (2.4.21). The electrodereaction that occurs at high temperature at both of the Pt/ZrO 2 + Y 2 O 3 interfaces isO 2 + 4e «± 2O 2 ~Write the equation that governs the potential of this cell as a function of the pressures, p e g and p a .What would the cell voltage be when the partial pressure of oxygen in the exhaust gas is 0.01 atm(1,013 Pa)?3KINETICS OF ELECTRODEREACTIONSIn Chapter 1, we established a proportionality between the current and the net rate of anelectrode reaction, v.
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