Electrochemistry - Part 5 | Class 12th

31. The substances which allow the passage of electricity through them are known as conductors.

32. Every conducting material offers some obstruction to the flow of electricity which is called resistance. It is denoted by R and is measured in ohm.

33. The resistance of any object is directly proportional to its length l and inversely proportional to its area of cross section A. 

                      R = ρ l/ A

ρ is called specific resistance or resistivity. Its SI unit is ohm metre.

34. The inverse of resistance is known as conductance, G

                      G = 1/R

  Unit of conductance is ohm^-1 or mho. It is also expressed in Siemens denoted by S.

35. The inverse of resistivity is known as conductivity. It is represented by the symbol ‘κ’. The SI unit of conductivity is Sm^-1. But it is also expressed in Scm^-1.

        

              

          So, conductivity = Conductance × Cell constant

36.  For measuring the resistance of an ionic solution, there are 2 problems:

  ü a) Firstly, passing direct current changes the composition of the solution

  ü b) Secondly, a solution cannot be connected to the bridge like a metallic wire or a solid conductor.

37.  This can be resolved by using a source of alternating current and the second problem is resolved by using a specially designed vessel called conductivity cell.

38.  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:

              

39. 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.

            

             Where κ = Conductivity

             M is the molarity Unit of Molar conductivity is Scm² mol^-1.

40.  Equivalent conductivity is the conductivity of all the ions produced by dissolving one gram equivalent of an electrolyte in solution.

            

                          Unit of equivalent conductivity is S cm² (g equiv)^-1.

41. 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 λ⁰₊ and that of the anions by λ⁰_- then the limiting molar conductivity of electrolyte is:

                 λ⁰_m  = v₊λ⁰₊ ⁺  v₊λ⁰₊

Where v₊ and v_- are the number of cations and anions per formula of electrolyte.

42. 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.

          

43.

  where Ka is acid dissociation constant ‘c’ is concentration of electrolyte, α is degree of ionization.

44.  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.

45. Dry Cell: 

                 At anode : Zn(s) → Zn²⁺ (aq) + 2 e^-

                 At cathode: MnO₂(s) + NH₄⁺(aq) + e^-  →  MnO(OH) + NH₃

The net reaction: Zn + NH₄⁺ (aq) + MnO₂ (s) →  Zn²⁺ + MnO (OH) + NH₃

46. 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 + 2 e^- → Hg(l) + 2 OH^-

The net reaction: Zn (Hg) + HgO(s) → ZnO(s) + Hg(l)

47. Secondary cells. Those cells which are used for storing electricity, e.g., lead storage battery, nickel – cadmium cell. They can be recharged.

48. Lead storage battery:

                       Anode: Pb(s) + SO₄^2-(aq) → PbSO₄(s) + 2 e^-

                       Cathode: PbO₂(s) + SO₄^2-(aq) + 4 H⁺(aq) + 2 e^- →  PbSO₄(s)+ 2 H₂O (l)

The overall cell reaction consisting of cathode and anode reactions is:

                 Pb(s) + PbO₂(s) + 2 H₂SO₄(aq) → 2 PbSO₄(s) + 2 H₂O(l)

On recharging the battery, the reaction is reversed.

49. 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) + 2 Ni(OH)₂(s) + H₂O(l)

50. Fuel cells:     At Anode: 2 H₂(g) + 4 OH^- (aq) → 4 H₂O(l) + 4 e^-    

                           At Cathode: O₂(g) + 2 H₂O(l) + 4 e^- → 4 OH^-(aq)    

Overall reaction:    2 H₂(g) + O₂(g) → 2 H₂O(l)

51. Corrosion:        Oxidation: Fe(s) → Fe²⁺ (aq) + 2 e^-      

                               Reduction: O₂(g) + 4 H⁺ (aq) + 4 e^- → 2 H₂O(l)      

Atmosphere oxidation:       2 Fe²⁺(aq) + 2 H₂O(l) + ½ O₂(g) → Fe₂O₃(s) + 4 H⁺(aq) 

52. Galvanization. It is a process of coating zinc over iron so as to protect it from rusting.

53. Cathodic protection: Instead of coating more reactive metal on iron, the use of such metal is made as sacrificial anode.