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XII- 03 Electrochemistry III
Conductance and Conductors:- Arrhenius theory of electrolvtic dissociation: The main points ofthe theory are: (i) An electrolyte, when dissolved in water, breaks up into two types ofcharged particles one carrying positive charge and the other a negative charge. Positively charged ions are termed cation and negatively charged are called anions AB A+ +B" NaCl Na+ + Cl- K. So, 2K+ + so:- (ii) The process of splitting ofmolecules into ions ofan electrolyte is called ionisation. The fraction ofthe total numberofmolecules present in solution as ions is known as degree of dissociation. Number of molecules dissociated into ions Total number of molecules The electrolytes which completely dissociate into ions are called strong electrol tes. For strong electrolytes o 1 The electrolytes which do not completely dissociate are called weak electrolytes. For such electrolytes o< 1
(iii) At equilib irum AB A+ B A 1B] AB] K ionisation constant >The electrolytes having high value of K are termed as strong electrolytes Those having low value ofK are termed as weak electrolytes. (iv) When an electric current in passedthrough the electrolytic solution, the cations move towards cathode and negative ions move towards anode and get discharged. The ions are discharged always in equivalent amounts, no matter what their relative speeds are current is carried through solutionby the moment ofions example acidic solution will always contain H ions and basic solution contain OH ions (v) The conductivity of the electrolytic solution depends on the nature and number ofions as the (vi) The properties of electrolytes in solution are properties ofions present in the solution for respectively
Factors affecting degree of ionisation: (i) Nature of solute: When a covalent bond is more predominant than an ionic bond in substance less ions are furnished in solution. Such substance are called weak electrolytes for example H2S, HCN, NH4 0H, CH3 COOH etc. NaCl, (Ba(NO3)2, KOH etc are strong electrolytes as they dissociate into the respective ions as soon as they are dissolved They are completey ionised in solution
(ii) Nature of solvent:- The main function of the solvent is to weaken the electrostatic forces of attraction between the two ions and separate them. >Any solvent which has high value of dielectric constant has the capacity of separating ions. Water is considered as the best solvent with a dielectric constant of 81
(iii) Dilution:- The extent of ionisation of an electrolyte is inversely proportional to the concentration of its solution.Thus degree of ionisation increases with increase of dilution (iv) Temperature: The degree of ionisation increases with increase of temperature. This is because at higher temperature molecular speed increases which overcomes the forces of attraction between the ions.
Electrolvtic conductance: The conductance is the property of the conductor (metallic or electrolytic) which facilitiates the flow of electricity through it. it is the reciprocal of resistance. Conductance =- Units -1 or siemens.
Specific conductance or conductivitv: >The resistance of any conductor varies directly as its length () and inversely as its cross-sectional area:- a specific resistance If1 1cm and a -1 cm2 then Specific resistance is therefore the resistance of 1cm3 of a conductor. The reciprocal of specific resistance is called specific conductance
Relationship between specific conductivity and resistance R.K =- Nowconstant ,'. R. K = G* cell constant G In case of electrolytic solution, the specific conductance is defined as the conductance of a solution of definite dilution enclosed in a cell having two electrodes of unit area separated by one centimeter apart.