Electrochemistry Exercise 2

ELECTROCHEMISTRY EXERCISE – II BOARD PROBLEMS Q.1 Conductivity of 0.00247 M acetic acid is 7.896 × 10–5 S cm–1. Calculate its molar conducitvity. If º for acetic aicd is 390.5 S cm2 mol–1, what is its dissociation constant ? (Ans. 1.85 × 10–5) Q.2 How does molar conductivty vary with concentration for (i) weak electrolyte (ii) strong electrolyte ? Give reason for these variations (Scm2mol1) Str ong ele ctro lyte Weak electrolyte Q.3 C (mol / litre)1/ 2 (a) In the button cell, widely used in watches and other devices the following reaction takes place: Zn(s) + Ag2O(s) + H2O(l)  Zn2+(aq) + 2Ag(s) + 2OH–(aq) Determine Eº and rGº for the reaction 0 0 Given E Ag / Ag = + 0.80 V, EZn2 / Zn = –0.76 V.. (b) Explain with examples the terms weak and strong electrolytes. How can these be distinguished? Q.4 Predict the products of electrolysis obtained at the electrodes in each case when the electrodes used are of platinum: (i) An aqueous solution of AgNO3 (ii) An aqeous solution of H2SO4 Q.5 (a) State two advantages of H2 – O2 fuel cell over ordinary cell. (b) Silver is electrodeposited on a metallic vessel of total surface area 900 cm2 by passing a current of 0.5 amp for two hours. Calculate the thickness of silver deposited. (Density of Ag = 10.5 g cm–3 , At. mass of Ag = 108 amu) Q.6 (a) Give reasons for the following : (i) Rusing of iron is quicker in saline water than is ordinary water. (ii) Aluminium metal cannot be produced by the electrolysis of aqueous solution of aluminium salt. Q.7 (a) Calculate the e.m.f. of the cell Mg (s) |Mg2+(0.1M||Cu2+(1 × 10–3 M)| Cu(s) Given : Eº Cu2+/Cu = + 0.34 V, Eº Mg+2/Mg = – 2.37 V (b) Explain with examples the terms weak and strong electrolytes. Q.8 (a) The resistance of a conductivity cell containing 0.001 M KCl solution at 298 K is 1500  . What is the cell constant, if the conductivity of 0.001 M KCl solution at 298 K is 0.146 × 10–3 S cm–1? (b) Predict the products of electrolysis in the following : A solution of H2SO4 with platinum electrodes. Q.9 Conductivity of 0.00241 M acetic acid solution 7896 × 10–5 S cm–1. Calculate its molar conductivity in this solution. If 0m for acetic acid in 390.5 S cm2 mol–1, what would be its dissociation constant? Q.10 The conductivity of a 0.20 M solution of KCl at 298 K is 0.0248 S cm–1. Calculate its molar conductivity. Q.11 Express the relation between conductivity and molar conductivity of a solution. Q.12 (a) State Kohlrausch's Laq of independent migration of ions. How can the degree of dissociation of acetic acid in a solution be calculated from its molar conductivity data? ELECTROCHEMISTRY (b) (i) Formulate the electrochemical cell representing the reaction 2Cr(s) + 3Fe+2(aq)  2Cr+3(aq) + 3Fe(s) (ii) Calculate Eºcell (iii) Calculate Ecell at 25ºC if [Cr3+] = 0.1 M and [Fe+2] = 0.01 M Given : E0 Cr 3 / Cr 0 = – 0º74 V, EFe  2 / Fe = – 0.44 V Q.13 What is meant by 'limiting molar conductivity' ? Q.14 Express the relation among the cell constant, the resistance of the solution in the cell and the conductivity of the solution. How is the conductivity of a solution related to its molar conductivity? Q.15 Given that the standard electrode potentials(Eº) of metals are: K+/K = – 2.93 V, Ag+/Ag = 0.80 V, Cu2+/Cu = 0.34 V 2+ Mg /Mg = – 2.37 V, Cr3+ /Cr = – 0.74 V, Fe2+/Fe = – 0.44 V, Arrange these metals in an increasing order of their reducing power. Q.16 Two half-reactions of an electrochemical cell are given below: MnO4– (aq) + 8H+(aq) + 5 e–  Mn2+ (aq) + 4H2O(l), Eº = + 1.51 V Sn2+(aq)  Sn4+ (aq) + 2 e– , Eº = + 0.15 V. Construct the redox reaction equation from the two half -reactions and calculate the cell potential from the standard potentials and predict if the reaction is reactant or product favoured. Q.17 Define the following : (i) Order of a reaction (ii) Activation energy of a reaction What type of cell is a lead storage battery? Write the anode and the cathode reactions and the overall cell reaction occurring in the use of a lead storage battery. Q.18 Q.19 Two half cell reactions of an electrochemical cell are given below: MnO4–(aq) + 8H+(aq) + 5e–  Mn2+(aq) + 4H2O(l), Eº = + 1.51 V Sn2(aq) Sn4+ (aq) + 2e– , Eº = + 0.15 V construct the redox equation from the two half cell reactions and predict if this reaction favours formation of reactants or product shown in the equation. Q.20 (a) What type of a battery is lead storage battery? Write the anode and cathode reactions and the overall cell reaction occurring in the operation of a lead storage battery. (b) Calculate the potential for half-cell containing 0.10 M K2Cr2O7(aq, 0.20 M Cr3+ (aq) and 1.0 × 10–4 M H+(aq) The half -cell reaction is Cr2O72–(aq) + 14H+(aq) + 6e–  2Cr3+ (aq) + 7H2O(l), and the standard electrode potential is given as Eº = 1.33 V Q.21 (a) How many moles of mercury will be produced by electrolysing 1.0 M Hg(NO3)2 solution with a current of 2.00 A for 3 hours ? [Hg(NO3)2 = 200.6 g mol–1] (b) A voltaic cell is set up at 25ºC with the following half-cells Al3+(0.001) and Ni2+(0.50M). Write an equation for the reaction that occurs when the cell generates an electric current and determine the o Q.22 Q.23 Q.24 Q.25 o cell potential. (Given : ENi2  / Ni  0.25 V, EAl3 / Al  1.66 V ) Express the relation among cell constant, resistance of the solution in the cell and conductivity of the solution. How is molar conductivity of a solution related to its conductivity ? or The molar conductivity of a 1.5 M solution of an electrolyte is found to be 138.9 S cm2 mol–1. Calculate the conductivity of this solution. The electrical resistance of a column of 0.05 M NaOH solution of diameter 1 cm and length 50 cm is 5.55 × 103 oh. Calculate its resistivity, conductivity and molar conductivity. The standard electrode potential (E°) for Daniell cell is 1.1 V. Calculate the G° for the reaction Zn(s) + Cu2+ (aq) Zn2+ (aq) + Cu (s) (1 F = 96500 C mol–1). Calculate the emf of the following cell at 25ºC : Ag(s) |Ag+(10–3 M) || Cu2+ (10–1M) | Cu (s) Given Eºcell = + 0.46 V and log 10n = n.