Electrochemistry Exercise # 4

Exercise # 4 [Subjective Type Questions] 1. Estimate the cell potential of a Daniel cell having 1.0M Zn++ and originally having 1.0M Cu++ after sufficient NH3 has been added to the cathode compartment to make NH3 concentration 2.0M at equilibrium. Given Kf for [Cu(NH3)4]2+ = 1 × 1012, E° for the reaction, Zn + Cu2+  Zn2+ + Cu 1.1V.. 2. Consider the cell Ag|AgBr(s)| Br– || Cl– | AgCl(s)| Ag at 25°C. The solubility product constants of AgBr & AgCl are respectively 5 × 10–13 & 1 × 10–10 . For what ratio of the concentrations of Br– & Cl¯ ions would the e.m.f. of the cell be zero ? 3. Calculate the first dissociation constant of H3PO4 if the e.m.f. of the cell,  Hg | Hg2Cl2(s) | KCl (salt) (conc. = 4N) || H3PO4 (0.1M), NaH2PO4 (0.1M) | H2 (1 atm) | Pt, – 0.3665 V. E red of SCE = 0.2412V. (10–2.12 = 131.82) 4. From the standard potentials shown in the following figure, calculate the potentials Eº 1 and Eº2. Eº1 = ? BrO3– 1.47 V BrO– 1.6 V Eº2 = ? 5. 1 2 Br2 1.07 V Br– 1.335 V Consider the following redox reaction : 2IrCl63– + 3HCOOH  2Ir + 3CO2 + 12Cl– + 6H+ Given : CO2 + 2H3O+ + 2e  HCOOH + 2H2O Eº = – 0.20 V IrCl63– + 3e  Ir + 6Cl– Eº = 0.77 V (a) Determine standard state emf of cell. (b) Is this reaction thermodynamically spontaneous as written ? Briefly explain. 6. The overall formation constant for the reaction of 6 mole of CN¯ with cobalt () is 1 × 1019. The standard reduction potential for the reaction [Co(CN)6]3– + e–  Co(CN)64– is – 0.83V.. Calculate the formation constant of [Co(CN)6]3–. Given Co3+ + e–  Co2+ ; E° = 1.82 V 7. Calculate the potential of an indicator electrode versus the standard hydrogen electrode, which originally contained 0.1M MnO4– and 0.8M H+ and which was treated with Fe2+ to reduce 90% of the MnO4– to Mn2+. MnO4– + 8H+ + 5e  Mn2+ + H2O, Eº = 1.51V 8. Calculate the emf of the cell in mV (atleast first two digits must match with correct answer) + 2 Ag (s), AgO3 (s) Ag (xM), HΙO3 (1M)  Zn (1M) Zn(s) If Ksp = 3 ×10–8 for AgO3 and Ka = (log 3 = 0.48) (Take 1 for HO3 and E 0cell for 2Ag + Zn+2  2Ag+ + Zn is – 1.56 V. 6 RT = 0.059 ) (giving your answer in magnitude only) F ELECTROCHEMISTRY 9. Calculate the voltage, E, of the cell at 25°C Mn(s) | Mn(OH)2(s) |Mn2+ (x M), OH– (1.00 × 10–4 M) || Cu2+ (0.675M)| Cu(s) given that Ksp = 1.9 × 10–13 for Mn(OH)2 (s) Eº (Mn2+ /Mn) = – 1.18 V, Eº (Cu+2 / Cu) = +0.34 V 10. An external current source giving a current of 5.0 A was joined with Daniel cell arrangement opposing the normal current flow and was removed after 10 hrs. Before passing the current the LHE and RHE contained 1L each of 1M Zn2+ and Cu2+ respectively. Find the EMF supplied by the Daniel cell after removal of the external current source. E° of Zn2+/Zn and Cu2+/Cu at 25°C is –0.76 and + 0.34V respectively. 11. A big irregular shaped vessel contained water, the conductivity of which was 2.56 x 10 –5 mho cm–1. 500 g of NaCI was then added to the water and the conductivity after the addition of NaCI, was found to be 3.10 x 10–5 mho cm–1. Find the capacity of the vessel if it is fulfilled with water. (NaCl = 149.9 Scm2 mole–1). Assume negligible conductivity of H2O. 12. 100ml CuSO4(aq) was electrolyzed using inert electrodes by passing 0.965 A till the pH of the resulting solution was 1. The solution after electrolysis was neutralized, treated with excess K and titrated with 0.04M Na2S2O3. Volume of Na2S2O3 required was 35 ml. Assuming no volume change during electrolysis, calculate: (a) duration of electrolysis if current efficiency is 80%. (b) initial concentration (M) of CuSO4. 13. During the discharge of a lead storage battery, the density of sulphuric acid fell from 1.294 to 1.139 g.ml–1. H2SO4 of density 1.294 gmL–1 is 39% and that of density 1.139 gmL–1 is 20% by weight. The battery holds 3.5L of acid and the volume practically remains constant during discharge. Calculate the number of ampere hours for which the battery must have been used. The discharging and charging reactions are : Pb + SO42–  PbSO4 + 2e— (Anodic reaction) PbO2 + 4H+ + SO42– + 2e¯  PbSO4 + 2H2O (Cathode reaction) 14. A silver coulomb meter is in series with a cell electrolyzing water. In a time of 1 minute at a constant current, 1.08 g silver got deposited on the cathode of the coulometer. What total volume (in mL) of the gases would have produced in other cell if in this cell the anodic and cathodic efficiencies were 90% and 80% respectively. Assume STP conditions and the gases collected are dry. (Ag – 108) (Molar volume of any ideal gas at STP = 22.4 L) 15. Kd for dissociation of [Ag(NH3)2]+ into Ag+ and NH3 is 6 × 10–8. Calculate E° for the following half reaction; [Ag(NH3)2]+ + e–  Ag + 2NH3 Given Ag+ + e–  Ag, Eº = 0.799V 16. The resistance of an aqueous solution containing 0.624 g of CuSO4 . 5H2O per 100 cm3 of the solution in a conductance cell of cell constant 153.7 per metre is 520 ohms at 298 K. Calculate the molar conductivity. (CuSO4 . 5H2O = 249.5) 17. Calculate the e.m.f. of the cell Pt H2 (1.0 atm)| CH3COOH (0.1M)|| NH3 (aq, 0.01 M) H2 (1.0 atm)Pt Ka (CH3COOH) = 1.8 × 10–5, Kb, (NH3) = 1.8 × 10–5. 18. When a solution of conductivity 1.342 mho metre–1 was placed in a conductivity cell with parallel electrodes, the resistance was found to be 170. 5 ohm. The area of the electrodes is 1.86 × 10 –4 sq. metre. Calculate the distance between the two electrodes in metre ? 19. The pKSP of Ag is 16.07. If the E° value for Ag+ / Ag is 0.7991V, find the E° for the half cell reaction Ag (s) + e–  Ag + – . 20. Voltage of the cell Pt, H2 (1 atm) | HOCN (1.3 × 10–3 M) || Ag+ (0.8 M) | Ag (s) is 0.982 V. Calculate the Ka for HOCN. Neglect [H+] because of oxidation of H2(g). 21. Calculate the equilibrium concentrations of all ions in an ideal solution prepared by mixing 25 mL of 0.1M T+ with 25mL of 0.2M Co3+ which undergoes chemical change as T+ + Co3+ T+3 + Co+2 Eº (T+ / T3+) = – 1.25 V ; Eº (Co3+ /Co2+) = 1.84 V 22. The experimental setup for a typical Zn-Ni galvanic cell as shown below in figure : e e– – e Salt Bridge M X 2+ M SO4 2– 2+ 2+ + Eº (Zn / Zn) = – 0.76 V 2+ Eº (Ni / Ni) = – 0.25 V MSO4 = 0.01 M XSO4 = 1.00 M 2– X SO4 (A) Identify M and X and determine cell potential at 25ºC. (B) If concentration of M2+ ion changes to 1.0 M during its usage, what would be the new cell voltage ? (C) Describe, what would happen to cell voltage if salt bridge was removed. 23. At 18°C the mobilities of NH4+ and ClO4¯ ions are 6.6 x 10–4 and 5.7 x 10–4 cm2 volt–1 sec–1 at infinite dilution. Calculate equivalent conductance of ammonium chlorate solution. 24. Same quantity of electricity is being used to liberate iodine (at anode) and a metal x (at cathode). The mass of x liberated is 0.617g and the iodine liberated is completely reduced by 46.3 cc of 0.124M sodium thiosulphate. Find the equivalent mass of metal x. 25. The resistances of two strong electrolytes X and Y were found to be 45 and 100 respectively. When equal volumes of both the solutions were taken in the same cell in two different experiments. If equal volumes of these solutions are mixed in the same cell, what will be the conductance of the mixture? 26. For 0.0128 N solution of acetic acid at 25°C, equivalent conductance of the solution is 1.4 mho cm3 eq–1 and  = 391 mho cm2 eq–1. Calculate dissociation constant (Ka) of acetic acid. 27. 10 g fairly concentrated solution of CuSO4 is electrolyzed using 0.01F of electricity. Calculate : (a) The mass of resulting solution. (b) Equivalents of acid or alkali in the solution. 28. An electric current is passed through electrolytic cells in series one containing AgNO3 (aq.) and other H2SO4(aq). What volume of O2 measured at 25°C and 750mm Hg pressure would be liberated from H2SO4 if (a) one mole of Ag+ is deposited from AgNO3 solution (b) 8 × 1022 ions of Ag+ are deposited from AgNO3 solution. 29. After electrolysis of NaCI solution with inert electrodes for a certain period of time 600 mL of the solution was left. Which was found to be 1N in NaOH. During the same time, 31.75 g of Cu deposited in the copper voltameter in series with the electrolytic cell. Calculate the percentage yield of NaOH obtained. ELECTROCHEMISTRY 30. A T+ / T couple was prepared by saturating 0.1 M KBr with TBr and allowing the T+ from the relatively insoluble bromide to equilibrate. This couple was observed to have a potential of –0.443 V with respect to Pb 2+ / Pb couple in which Pb2+ was 0.1 M. What is Ksp of TBr. 0 (Report answer in multiplication of 10–8) ( EPb 2  / Pb = – 0.126, E 0  = – 0.336 V) T / T 2.303 RT = 0.059) F (Take antilog (0.5509) = 3.55, 31. The following two cells with initial concentration as given are connected in parallel with each other. (1) Fe(s) | Fe(NO3)2(aq.) (1M) || SnCl2(aq.) (1M) | Sn(s) (2) Zn(s) | ZnSO4(aq.) (1M) || Fe(NO3)2(aq.) (1M) | Fe(s) After sufficient time equilibrium is established in the circuit. What will be the concentrations (in mmoles/L) of Fe 2+ ions in first and second cells respectively. 0 0 0 [Take E Sn 2  / Sn = – 0.14V, E Zn2  / Zn = – 0.76V,, E Fe 2  / Fe = – 0.44 V, 2.3 x RT = 6433, log2 = 0.3] 32. For the cell (at 1 bar H2 pressure) Pt/H2(g) H X (m1), NaX(m2), NaCl(m3) / AgCl / Ag / Pt it is found that the value of  m HX . m Cl –   approaches 0.2814 in the limit of zero concentration. Calculate Ka for the acid HX E – E° + RTF–1 n  mX  at 25°C expressing your answer as 107 Ka. 33. Specific conductance of pure water at 25°C is 0.58 × 10–7 mho cm–1. Calculate ionic product of water (Kw) if ionic conductances of H+ and OH¯ ions at infinite dilution are 350 and 198 mho cm2 respectively at 25°C. Cu2+ 34. + 0.15 volt Cu+ 0.50 Volt The reduction potential diagram for Cu in acid solution is . Eº = x volt Calculate x. Does Cu+ disproportionate in the solution ? 35. For the cells in opposition, Zn(s) | ZnCl2(sol.) | AgCl(s) | Ag | AgCl(s) | ZnCl2(sol.) | Zn(s) C1 = 0.02 M C2 = 0.5 M Find out the emf (in millivolt) of the resultant cell ? (Take log 2 = 0.3, Cu RT at 298 K = 0.060) F