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Conductance of an Electrolyte

 

Conductance is a property of electrolytic solutions which indicates how well an electrolyte can conduct electricity. Its value is numerically equal to the reciprocal of the resistance to the flow of electricity through the solution. I.e. C = 1/R.

The unit of conductance is Ω-1(ohm-1). The conductance of an electrolyte is directly proportional to the surface area, A, of the electrodes, and inversely proportional to the distance between the electrodes, L. l.e. C α A/L ; C = k A/L , where k is called specific conductance (or conductivity).

The conductivity of an electrolyte (k) also indicates how well an electrolyte conducts electricity. The conductivity of an electrolyte (e.g. acids and bases) is the conductance of a volume of solution containing one mole of dissolved electrolyte placed between two parallel electrodes 1 dm apart and large enough to contain between them all the solution - conductivity is affected by temperature.

The molar conductivities of strong electrolytes are high. This is because, by nature, strong electrolytes are highly dissociated when molten or when in solution into large number of ions. These ions are mobile, hence they migrate to the electrodes, resulting in the high conduction of electricity (the higher the number of ions in solution, the higher the conductivity).

On the other hand, weak electrolytes show partial dissociation in solution, producing few ions, which results in low conduction of electricity. E.g. The molar conductivity at room temperature of 0.1 mol dm-3 of HCl is 391 while that of CH3COOH is 5.

Note: * The higher the number of ions in solution, the greater the conductivity of the electrolytic solution.

* Conductivity increases with dilution (i.e. lower concentrations) in both strong and weak electrolytes. However, the extent to which dilution affect their conductivity differs.

The table below shows the trend in conductivity with dilution for a strong and a weak acid.

 

Conc. Moles/dm3

Conductance at 25oC/Mhos
HCl (strong acid)          CH3COOH (weak acid)

1.0
0.1
0.01
0.001
0.0001

 

333
391
413
421
426

 

-
5
16
91
391

 

Explanation of Increase in Conductivity with Dilution:

With increase in dilution (decrease in concentration), ions become farther apart, and inter-ionic forces (i.e. forces of attraction between unlike ions and forces of repulsion between like ions) decrease considerably, so that greater number of ions are able to migrate to the electrodes. In addition, due to change in equilibrium, the electrolyte undergo further ionization from the same mass in solution (in order to balance the effect).

Hence, more ions (conducting species) are introduced into the solution.

The Difference in The Degree of Increase in Conductivity with Dilution between Strong and Weak Electrolytes:

Together with the fact that their ions migrate faster to the electrodes with dilution, weak electrolytes also increase strongly in ionization, thereby increasing (from the same mass of the substance in solution) the number of their ions (conducting species) in solution.

It is assumed that at infinite dilution, weak electrolytes undergo complete ionization. This theoretically makes their         conductivity at infinite dilution higher than those of strong electrolytes. Strong electrolytes undergo complete or almost complete ionization at the start.

Therefore, the number of their ions present (from the same mass of the substance in solution) is constant or almost constant from the beginning, up to the point of infinite dilution. Thus, their conductivity gradually tend to a constant value with dilution.

These explain the high increase in the conductivity of weak electrolytes, e.g. acetic acid, compared with those of strong electrolytes, e.g. HCl with dilution - as given in the table above.

Note: * The greater the number of ions in solution from the same mass of substance dissolved, the greater the conductivity of the solution.

* The greater the dilution of an electrolytic solution (i.e. lower concentrations of solution), the greater the number of its ions in solution, and the greater also is the mobility of its ions to the electrodes– this increases its conductivity.  

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