● Salt bridge—U-shaped tube containing concentrated solu- tion of an inert electrolyte like KCl, KNO3 or K2SO4 or a solidi- fied solution of such electrolytes in agar-agar and gelatin. Salt bridge completes the inner circuit by flow of ions and maintains electrical neutrality in the solution of the half cells. ● EMF of cell—If it has positive value for a cell, the redox
reaction in the cell is spontaneous, otherwise not.
● Standard free energy change (ΔΔΔΔG°°°°)—The relation is – ΔG° = nFEcell° , where ‘n’ is number of electrons involved and F is Faraday.
● Calomel electrode—Electrode of Hg2Cl2 paste is used as
reference electrode in place of hydrogen electrode. Standard reduction potential of this electrode depends on conc. of KCl used.
● Concentration cell—The most common concentration cell is the one in which both the electrodes are of the same type but the concentration of ions is different, e.g.
H2 | H+ (C1) || H+(C2) | H2 or Zn | Zn2+ (C
1) || Zn2+ (C2) | Zn The EMF of concentration cell at 298 K is given by
E = 0·0592n log CC2 1
● Edison storage cell—It is nickel-iron accumulator and cell reaction is as :
Fe + Ni2O3 → FeO + 2 NiO
● Standard cell—One whose EMF does not change with tem- perature. The most common standard cell is Weston stan-
dard cell.
● Factors which enhance corrosion
(i) Presence of impurity in metal (Pure metals do not corrode)
(ii) Presence of moisture
(iii) Presence of electrolytes (saline water)
● Discharge potential—For a particular ion to be deposited on the electrode, it requires minimum voltage that must be
applied across the electrodes. This minimum voltage is known as discharge potential of that ion.
● Conductivity = Cell constantResistance = Cell constant × Conductance ● Equivalent conductivity (ΛΛΛΛeq) = κ × 1000N (N = Normality)
● Molar conductivity (ΛΛΛΛmol) = κ × 1000M (M = Molarity) ● Anode—In a cell it is electrode at which oxidation takes place
and it is negative electrode in electrochemical cell and
positive electrode in electrolytic cell.
● Cathode—In a cell it is an electrode at which reduction takes place. It is positive electrode in electrochemical cell and
negative in electrolytic cell.
● Electrochemical series—Arrangement of all electrodes (ele- ments) in the decreasing order of their standard reduction potentials. It is also known as activity series of elements. ● Nernst equation EMn+/M = E°Mn+/M – 2·303 RTn F log [M] [Mn+] = E°Mn+/M – 0·0592 n log [M] [Mn+]
E°cell = 0·0592n log Kc (Kc = Equilibrium constant) ΔG° = – n F Ecell° (ΔG° = Standard free energy change) ● The ratio of specific conductivity to observed conductance is
called cell constant.
● The weight of a substance deposited by one coulomb of electricity is known as electrochemical equivalent.
● The electrochemical equivalent of an element is equal to Atomic weight of element
Valency of element × 96‚500 ● MnO2 used in a dry cell acts as depolarizer.
● The number of coulombs = No. of amperes × Time in seconds No. of amperes = No. of coulombsTime in seconds
OBJECTIVE QUESTIONS
1. Which is wrong statement regarding a electrochemical (gal- vanic) cell ?
(A) Oxidation takes place at anode
(B) When EMF of cell is posi- tive, cell reaction is sponta- neous
(C) Electrons flow around exter- nal circuit from cathode to anode
(D) Ions carry the current inside the cell
2. The standard electrode poten- tials of Ag, Zn and Cu are + 0·8, – 0·76 and 0·34 volt respec-
tively. Which statement is correct ?
(A) Zn can reduce both Ag+ and Cu
(B) Cu2+ can oxidise Ag and Zn (C) Ag+ can oxidise Zn and Cu (D) Cu2+ can oxidise Ag+ and Zn
3. The unit of specific conductivity is—
(A) mho cm (B) ohm cm–1 (C) mho (D) mho cm–1 4. The minimum equivalent con-
ductance in fused state is shown by—
(A) MgCl2 (B) BeCl2 (C) CaCl2 (D) SrCl2 5. The unit of cell constant is—
(A) ohm–1 cm–1 (B) ohm cm (C) cm (D) cm–1
6. Zn can displace following ion from aqueous solution—
(A) Cu2+ (B) Ag+ (C) Fe2+ (D) All of these 7. Which of the following metals will
effect the concentration of ZnCl2 solution if solution is in contact with metal ?
(A) Au (B) Pb (C) Ag (D) Al 8. In the cell reaction
Cu(s) + 2 Ag+(aq) → Cu2+(aq) + 2 Ag(s) The reduction half reaction is— (A) Ag – e– → Ag+
(B) Cu + 2e– → Cu2–
(C) Ag+ + e– → Ag (D) Cu – 2e– → Cu2+
9. In which of the following cells, the chemical energy is directly converted into electricity ? (A) Dry cell
(B) Concentration cell (C) Fuel cell
(D) Lead storage battery 10. Electrolytic conductance is direct
measure of— (A) Potential (B) Resistance (C) Concentration (D) Dissociation
11. When 0·01 M solution of an elec- trolyte has a resistance of 40 ohm in a cell having a cell cons- tant 0·4 cm–1, the molar con- ductance in ohm–1 cm2 mol–1 is—
(A) 10 (B) 102 (C) 103 (D) 104
12. E° for electrode reactions Zn = Zn2+ + 2e– and Ag = Ag+ + e– are + 0·76 V and – 0·77 V respectively. The EMF of the cell is—
(A) + 1·53 V (B) – 1·53 V (C) – 0·01 V (D) + 0·01 V 13. The resistance of a 0·1 N solu-
tion of acetic acid is 250 ohm, when measured in a cell of cell constant 1·15 cm–1. The equiva- lent conductance in ohm–1 cm2 equiv–1 of acetic acid is— (A) 2·3 (B) 46 (C) 9·2 (D) 18·4 14. The standard reduction poten-
tials at 298 K of Li+/Li, Ba2+/Ba, Na+/Na and Mg2+/Mg are – 3·05, – 2·73, – 2·71 and – 2·37 volt respectively. The strongest oxidi- sing agent is—
(A) Ba2+ (B) Mg2+ (C) Li+ (D) Na+
15. The term conductivity stands for—
(A) Conductance (B) Specific conductance (C) Molar conductance (D) Equivalent conductance 16. The standard reduction poten-
tials at 298 K of half-reactions are as : Zn(aq)2+ + 2e– → Zn (s) ; E° = – 0·762 V Cr3+(aq) + 3e– → Cr (s) ; E° = – 0·740 V 2H(aq)+ + 2e– → H 2(g) ; E° = 0·000 V Fe(aq)3+ + e– → Fe (aq) 2+ ; E° = 0·770 V The strongest reducing agent is—
(A) Zn(s) (B) Cr(s) (C) H2(g) (D) Fe(aq)2+ 17. The normal Al electrode coupled
with normal hydrogen electrode gives an EMF of 1·66 V. The standard electrode potential of Al is—
(A) + 1·66 V (B) – 1·66 V (C) – 0·83 V (D) + 0·83 V 18. The reference calomel electrode
is made from—
(A) ZnCl2 (B) CuSO4 (C) Hg2Cl2 (D) HgCl2
19. From the given half reactions, select the strongest reducing agent : Cl2(g) + 2e– → 2Cl(aq)– ; E° = + 1·36 V Br2(g) + 2e– → 2Br(aq)– ; E° = + 1·09 V Sn2+ + 2e– → Sn(s) ; E° = – 0·14 V (A) Cl– (B) Br– (C) Sn (D) Sn2+ 20. The standard reduction poten-
tials for Fe2+/Fe and Sn2+/Sn electrodes are – 0·44 V and – 0·14 V respectively. For the cell reaction
Fe2+ + Sn → Fe + Sn2+ the standard EMF of the cell is—
(A) + 0·30 V (B) – 0·58 V (C) + 0·58 (D) – 0·30 V 21. In the cell Zn | Zn2+ || Cu2+ | Cu,
the negative terminal of the cell is—
(A) Cu (B) Cu2+ (C) Zn (D) Zn2+ 22. Standard reduction potentials of
four elements A, B, C and D are same – 0·32, 0·00, same respec- tively. The decreasing order of their reactivity is as—
(A) A > D > B > C (B) C > B > D > A (C) B > D > C > A (D) C > A > D > B
23. Which of the following will form a cell with highest voltage ? (A) 1M Ag+, 1M Co2+ (B) 2M Ag+, 2M Co2+ (C) 0·1M Ag+, 2M Co2+ (D) 2M Ag+, 0·1 M Co2+ 24. E° for the cell
Zn | Zn2+ || Cu2+ | Cu is 1·10 V at 298 K. The equili- brium constant for the reaction is—
(A) 10– 37 (B) 1037 (C) 10– 27 (D) 1027 25. The equilibrium constant (Kc) for
the reaction
Cu(s) + 2 Ag+(aq) → Cu2+ + 2 Ag(s) will be— (Given Ecell° = 0·46 V)
(A) Antilog of 12·2 (B) Antilog of 15·6 (C) Antilog of 1·5 (D) Antilog of 2·5
26. Standard EMF of Daniell cell is 1·1 V. The maximum electrical work obtained from the cell is— (A) 21·23 kJ (B) 212·3 kJ (C) 17·54 kJ (D) 175·40 kJ 27. The standard EMF of the cell
Zn | Zn2+ || Cu2+ | Cu is 1·1 V at 298 K. The EMF of the cell reaction, when 0·1 M Cu2+ and 0·1 M Zn2+ solutions are used at 298 K is—
(A) 0·1 V (B) 1·1 V (C) – 1·1 V (D) – 0·1 V 28. What is the potential of half cell
consisting of Zn electrode in 0·01 M ZnSO4 solution at 298 K ?
[Eoxi.° = 0·763 V] (A) 8·221 V (B) 0·8221 V (C) 0·5282 V (D) 9·282 V 29. The half cell reactions of the cell
used in hearing aids are as : Zn → Zn2+ + 2e; E° = + 0·76 V Ag2O + H2O + 2e–
→ 2 Ag + 2 OH– ; E° = 0·34 V E° for the cell will be—
(A) 0·42 V (B) 1·1 V (C) 0·84 V (D) 2·2 V 30. The EMF of the cell
Ni | Ni2+ || Cu2+ | Cu
is 0·59 V. The standard reduc- tion potential of Cu electrode is 0·34 V. The standard potential of Ni electrode would be—
(A) – 0·25 V (B) 0·25 V (C) 0·93 V (D) – 0·93 V 31. By passing 0·1 faraday of elec-
tricity through fused NaCl, the amount of chlorine liberated is— (A) 35·45 g (B) 70·9 g (C) 3·545 g (D) 17·77 g 32. The charge required for reduc-
tion of one mole of Cr2O72– ions to Cr3+, will be—
(A) 96500 C (B) 2 × 96500 C (C) 6 × 96500 C (D) 3 × 96500 C
33. What amount of Cl2 evolves, when two amperes of current is passed for 30 minutes in aqueous solution of NaCl ? (A) 33 g (B) 66 g (C) 99 g (D) 1·32 g 34. In a solution of CuSO4, 0·5
ampere of current is passed. How long will it take to deposit 2 gm of copper ?
(A) 102 sec (B) 1020 sec (C) 12157·4 sec (D) 1215·74 sec
35. The charge required to liberate 11·5 g of sodium from fused NaCl is—
(A) 1·0 faraday (B) 1·5 faraday (C) 0·5 faraday (D) 9650 C
36. 3·5 faraday electricity is passed through a solution of AgNO3. The number of gram equivalents of silver deposited on the elec- trode will be—
(A) 1·35 (B) 3·5 (C) 1·25 (D) 2·5
37. The electrochemical equivalent of silver is 0·001118 g. When an electric current of 0·5 ampere is passed through and aqueous solution of AgNO3 for 200 seconds, the amount of Ag deposited is—
(A) 0·1118 g (B) 1·118 g (C) 0·5590 g (D) 5·590 g 38. On passing one faraday of elec-
tricity through the electrolytic cells containing Ag+, Ni2+ and Cr3+ ions solutions, the depo- sited Ag, Ni and Cr are—
Ag Ni Cr (A) 108 g 29·5 g 17·3 g (B) 108 g 108 g 108 g (C) 29·5 g 29·5 g 29·5 g (D) 17·3 g 17·3 g 17·3 g 39. What weight of Cu will be depo-
sited by passing two faradays of electricity through Cu2+ ion solu- tion ?
(A) 2·0 g (B) 31·75 g (C) 63·0 g (D) 127·0 g
40. The charge required to deposit 9·0 g of Al from Al3+ ion solution is—
(A) 96,500 C (B) 9650 C (C) 32163 C (D) 3216·3 C 41. The number of coulombs requ-
ired to produce 20·0 g of calcium metal from molten CaCl2 is— (A) 1·93 × 105 (B) 1·35 × 105 (C) 4·8 × 104 (D) 9·65 × 104 42. A copper cup is deposited with
silver by passing an electricity of 965 coulomb. The amount of Ag deposited is—
(A) 1·002 g (B) 107·87 g (C) 1·0787 g (D) 9·65 g 43. When a lead storage battery is
discharged— (A) SO2 gas evolves (B) PbSO4 is consumed (C) H2SO4 is consumed (D) Lead (Pb) is formed
44. On electrolysing a dilute solution of H2SO4 between Pt electrodes, the gas evolved at anode is— (A) SO2 (B) SO3 (C) O2 (D) H2
45. The electrolysis of a solution results in formation of H2 at cathode and Cl2 at anode. The solution is—
(A) Dil. H2SO4 (B) Aqueous NaCl (C) Aqueous CuCl2 (D) Liquid of fused NaCl 46. When a strip of copper is placed
in an aqueous solution of ferrous sulphate—
(A) Copper will be precipitated (B) Iron will be precipitated (C) Both Cu and Fe will dissolve (D) No reaction will take place 47. In the process of electrolysis, the
current through the electrolyte is carried by—
(A) Electrons (B) Cations (C) Anions
(D) Anions and cations
48. The standard reduction potential (Ered.° ) for the half reactions are as :
Zn → Zn2+ + 2e– ; E° = 0·76 V Fe → Fe2+ + 2e–; E° = 0·41 V
The EMF of cell reaction Fe2+ + Zn → Zn2+ + Fe is—
(A) – 0·35 V (B) 0·35 V (C) 1·17 V (D) – 1·17 V 49. Given standard electrode poten-
tials (E°)—
Fe2+ + 2e– → Fe; E° = – 0·44 V Fe3+ + 3e → Fe; E° = – 0·036
The standard electrode potential (E°) for
Fe3+ + e–
→
Fe2+ will be—(A) + 0·772 V (B) + 0·404 V (C) – 0·404 V (D) – 0·476 V 50. Thenumber offaradaysrequired
to deposit one mole atoms of a metal, when a solution of its ions M2+ is electrolysed, is—
(A) 1 (B) 2 (C) 1
2 (D) 96500 51. In order to produce 160 g of oxy-
gen, the number of moles of water required to be electrolysed is—
(A) 2·5 (B) 5 (C) 10 (D) 20
52. If a salt bridge is removed from a electrochemical cell, the vol- tage—
(A) Increases rapidly (B) Drops to zero (C) Increases slowly (D) Does not change
53. One faraday of electricity may liberate one gram atom of metal from the solution of—
(A) AlCl3 (B) CuSO4 (C) BaCl2 (D) NaCl 54. When electric current is passed
through acidulated water, 112 ml of H2 at STP is collected at cathode in 965 sec. The current passed in ampere is—
(A) 0·1 (B) 0·5 (C) 1·0 (D) 2·0
55. The decreasing order of stan- dard electrode potentials of K, Ca and Li is as—
(A) Ca > K > Li (B) Li < K < Ca (C) Li < Ca < K (D) Ca < Li < K
56. In the Faraday law of electrolysis
w = z × i × t
‘z ’ stands for—
(A) Equilibrium constant (B) Chemical equivalent (C) Electrochemical equivalent (D) Cell constant
57. The quantity of electricity expres- sed in coulombs involved in the passage of electric current of 1 ampere for one minute is equal to—
(A) 1 (B) 1 60 (C) 60 (D) 96500 58. If the cell reaction, M + e–→ M–
has a large negative reduction potential, it follows that— (A) M is readily reduced (B) M is readily oxidised (C) M– is readily reduced (D) M– is readily oxidised 59. During the charging of lead
storage battery, the reaction occurring at cathode is as— (A) Pb → Pb2+ + 2e– (B) Pb2+ + 2e– → Pb (C) Pb2+ + SO42– → PbSO4 (D) PbSO4 + H2O → PbO2 + 4H+ + SO 42– + 2e– 60. The specific conductance of 0·1 M HNO3 is 6·3 × 10–2 ohm–1 cm–1. The molar conductance of this solution is—
(A) 6300 ohm–1 cm2 (B) 630 ohm–1 cm2 (C) 315 ohm–1 cm2 (D) 100 ohm–1 cm2
61. The overall chemical reaction that occurs in a Daniell cell is— (A) Oxidation (B) Reduction (C) Redox (D) Precipitation 62. The electrochemical equivalent of a material (element) is equal to—
(A) Atomic wt. × Valency 96‚500 (B) Atomic wt. × 96‚500 Valency (C) Atomic weight Valency × 96‚500 (D) Valency × 96‚500 Atomic weight
63. The volume of oxygen at STP, obtained by electrolysis of 18 g of H2O will be—
(A) 22·4 litre (B) 11·2 litre (C) 5·6 litre (D) 4 litre 64. The number of moles of Al3+
reduced to Al by passage of 1 faraday of electricity is—
(A) 1 (B) 2 (C) 3 (D) 0·33 65. If 2·303 RT/F = 0·0592 and acti-
vity of solids is constant, the EMF of cell Zn | Zn(C1)2+ || Cu(C2)2+ | Cu will be— (A) E = E° – 0·0592 log C2 C1 (B) E = E° – 0·0592 log C1 C2 (C) E = E° – 0·0592 2 log C1 C2 (D) E = E° – 0·0592 2 log C2 C1