Change in Conductivity and Molar Conductivity with Concentration Chemistry Notes

Change in Conductivity and Molar Conductivity with Concentration Chemistry Notes

Effect on Conductivity :

The conductivity of both strong and weak electrolytes decreases with the decrease in concentration because the number of ions conducting per unit volume of electric current decreases on dilution and we know that the conductivity of solution at any concentration is the conductance of unit volume of that solution and the number of ions also decreases in that volume on dilution.

Effect on Molar Conductivity :

→ The molar conductivity of both strong and weak electrolytes increases with decrease in concentration. As we know that molar conductivity is conductance of V volume of that solution in which one mole of electrolyte is dissolved and which is placed between two electrodes apart by unit distance with each other and area (A) of cross-section Molar conductivity increases with decrease in concentration because their total volume also increases in which one mole electrolyte is present. We can understand effect of concentration on conductivity of both strong and weak electrolytes :

→ Effect of Dilution on Strong Electrolytes-The molar conductivity of strong electrolytes also increases on dilution but in less amount because strong electrolytes in aqueous solution or molten state are completely dissociated and pass more amount of electric current. KCl, KOH, NaOH, H₂SO₄, HNO₃, KNO₃, etc., are strong electrolytes. We can also represent effect of molar conductivity by following curve :

→ Debye, Huckel and Onsager expressed the relation between molar conductivity of strong electrolytes and concentration by following equation:

→ This relation has no theoretical concept. On the basis of mathematics, Onsager (1923) gave a long form by modification in above equation:

where, D = Dielectric constant of medium (solvent)
n = Viscosity C = Concentration
T = Absolute temperature Constant terms can be represented by A and B respectively in equation. ie.

→ A graph is plotted between square root of various concentrations of strong electrolyte KCl solution and value of Λ_m which is shown in fig. 3.8. It is clear from graph that the value of molar conductivity of strong electrolyte slightly increases with dilution i.e, decrease in concentration because strong electrolytes dissociated or ionized completely in solution. The movement of ions decreases due to more attraction forces between them at high concentration. So, molar conductivity decreases.

→ On dilution the ions become far from each other and their molar conductivity increases. On dilution, molar conductivity increases till when its highest value is not reached This highest value of molar conductivity with dilution is called molar conductivity at infinite dilution (\(\Lambda_{m}^{\infty}\)) or molar conductivity at zero concentration (\(\Lambda^{\circ} m\)).

→ Effect of Dilution on Weak ElectrolytesIonisation of weak electrolytes is low and according to Ostwald’s law of dilution, the ionisation of weak electrolytes increases with dilution and its ionisation is completed at infinite dilution or zero concentration.

→ So, weak electrolyte acts as strong electrolyte at this condition i.e. at infinite dilution. If we plot a graph between molar conductivity and concentration for a weak electrolyte then it will obtain as fig. 3.9. It is clear from graph that molar conductivity of weak electrolytes increases with dilution and this growth is more than strong electrolytes. This growth is also more at infinite dilution.

• Its reason is increase of its ionization which is expressed previously
• The degree of ionisation of weak electrolytes can be determined from following formula:
• Degree of ionisation (∝) = \(\frac{\Lambda_{m}}{\Lambda_{m}^{\infty}}\)
• Here, Λ_m = Molar conductivity at concentration C
• \(\Lambda_{m}^{\infty}\) = Molar conductivity at infinite dilution

Chemistry Notes