These are Online Notes on "Electrochemistry" (Chapter=>3 ) Part 3 for practice of CBSE BOARD, CBSE NEET, CSIR NET Chemical Sciences etc.45. A conductivity cell consists of 2 Pt electrodes coated with Pt black.
They have area of cross section A and are separated by a distance l.
Resistance of such a column of solution is given by the equation:
R = ρ ( l / A ) = (1/k ) (l/A)
l/A is called cell constant and is denoted by the symbol G*.
46. Molar conductivity of a solution is defined as the conducting power of
all the ions produced by dissolving 1 mole of an electrolyte in solution.
Molar conductivity ᴧm = ( k x 1000 ) / M
Where κ = Conductivity and M is the molarity
Unit of Molar conductivity is S cm2 mol-1
47. Equivalent conductivity is the conductivity of all the ions produced by
dissolving one gram equivalent of an electrolyte in solution.
ᴧe = ( k x 1000 ) / N
Unit of equivalent conductivity is S cm2 (g equiv) -1
48. Kohlrausch’s Law of independent migration of ions: According to this
law, molar conductivity of an electrolyte, at infinite dilution, can be expressed as the sum of individual contributions from its individual ions.
If the limiting molar conductivity of the cations is denoted by λ and
that of the anions by λ then the limiting molar conductivity of
electrolyte is:
Where v+ and v- are the number of cations and anions per formula of
electrolyte.
49. Degree of dissociation: It is ratio of molar conductivity at a specific concentration ‘c’ to the molar conductivity at infinite dilution. It is denoted by α.
50. Ka = C α2 / 1- α
where Ka is acid dissociation constant , ‘c’ is concentration of electrolyte, α is degree of ionization.
51. Faraday constant: It is equal to charge on 1 mol of electrons. It is equal to 96487 C mol-1 or approximately equal to 96500 C mol-1.
52. Faraday’s first law of electrolysis: The amount of substance deposited during electrolysis is directly proportional to quantity of electricity passed.
53. Faraday’s second law of electrolysis: If same charge is passed through different electrolytes, the mass of substance deposited will be proportional to their equivalent weights.
54. Products of electrolysis: The products of electrolysis depend upon the nature of electrolyte being electrolyzed and the nature of electrodes. If electrode is inert like platinum or gold, they do not take part in chemical reaction i.e. they neither lose nor gain electrons. If the electrodes are reactive then they will take part in chemical reaction and products will be different as compared to inert electrodes.
The products of electrolysis also depend upon the electrode potentials of oxidizing and reducing species. Some of the electrochemical processes although feasible but slow in their rates at lower voltage, these do not take place. They require extra voltage, i.e. over voltage at which these processes will take place. The products of electrolysis also differ in molten state and aqueous solution of electrolyte.
55. Primary cells: A primary cell is a cell in which electrical energy is produced by the reaction occurring in the cell, e.g. Daniell cell, dry cell, mercury cell. It cannot be recharged.
56. Dry Cell:
At anode : Zn(s) → Zn2+ (aq) + 2e-
At cathode : MnO2(s) + (NH4)+ (aq) +e- → MnO(OH) + NH3
The net reaction:
Zn + (NH4)+ (aq) + MnO2 (s) → Zn2+ + MnO (OH) + NH3
56.Mercury Cell: The electrolyte is a paste of KOH and ZnO.
At anode : Zn (Hg) + 2OH- → ZnO(s) + H2O + 2e-
At cathode : HgO(s) +H2O + 2e- → Hg(l) + 2OH-
The net reaction :
Zn (Hg) + HgO(s) → ZnO(s) + Hg(l)
58. Secondary cells. Those cells which are used for storing electricity, e.g.,
lead storage battery, nickel – cadmium cell. They can be recharged.
59. Lead storage battery:
Anode : Pb(s) + SO22-(aq) → PbSO2(s) + 2e-
Cathode: PbO2(s) + SO42-(aq) + 4 H+(aq) + 2e- → PbSO4(s)+ 2H2O (l)
The overall cell reaction consisting of cathode and anode reactions is:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
On recharging the battery, the reaction is reversed.
60. Nickel cadmium cell: It is another type of secondary cell which has
longer life than lead storage cell but more expensive to manufacture.
The overall reaction during discharge is
Cd(s) + 2 Ni(OH)3(s) → CdO(s) + 2Ni(OH)2(s) + H2O(l)
61. Fuel cells:
At anode : 2 H2(g) + 4OH- (aq) → 4H2O(l) + 4e-
At cathode : O2(g) + 2H2O(l) + 4e- → 4 OH-(aq)
Overall reaction:
2 H2(g) + O2(g) → 2H2O(l)
62. Corrosion:
Oxidation : Fe(s) → Fe2+ (aq) + 2e-
Reduction : O2(g) + 4H+ (aq) + 4e- → 2H2O(l)
Atmosphere oxidation:
2Fe2+(aq) + 2H2O(l) + ½ O2(g) → Fe2O3(s) 4H+(aq)
63. Galvanization: It is a process of coating zinc over iron so as to protect
it from rusting.
64. Cathodic protection: Instead of coating more reactive metal on iron,
the use of such metal is made as sacrificial anode.
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