B.Sc. 2nd Year Chemistry – Detailed and Exam-Oriented Notes
Electrolytic conductance is the conductance of an electrolyte solution due to the movement of ions under the influence of an applied electric field.
Substances that produce ions in aqueous solution or in molten state and conduct electricity are called electrolytes. Electrolytes may be strong or weak depending on the extent to which they ionize or dissociate.
Strong electrolytes are substances that are almost completely ionized or dissociated in solution.
Examples include:
Weak electrolytes are only partially ionized in solution and establish an equilibrium between ionized and unionized species.
Examples include:
| Metallic conduction | Electrolytic conduction |
|---|---|
| Conducted by electrons. | Conducted by ions. |
| No chemical decomposition occurs. | Chemical changes may occur at electrodes. |
| Usually decreases with increase in temperature. | Usually increases with increase in temperature. |
| Occurs in metals. | Occurs in electrolyte solutions or molten electrolytes. |
The opposition offered by a conductor to the flow of electric current is called resistance.
Resistance is represented by R.
Where:
The SI unit of resistance is the ohm (Ω).
Conductance is the reciprocal of resistance.
Therefore:
The SI unit of conductance is the siemens (S), formerly called mho.
The resistance of a conductor depends on its length and cross-sectional area.
Therefore:
Where:
The SI unit of resistivity is Ω m.
Conductivity is the conductance of a solution placed between electrodes of unit length and unit cross-sectional area.
Conductivity is represented by κ.
Since:
Therefore:
If the cell constant is represented by l/A:
The SI unit of conductivity is S m⁻¹.
The ratio of the distance between two electrodes to the area of their cross-section is called the cell constant.
It depends only on the physical dimensions of the conductivity cell.
Its commonly used units are cm−1 or m−1.
Molar conductivity is the conductance of the volume of solution containing one mole of electrolyte placed between two electrodes sufficiently far apart to contain the entire solution.
Molar conductivity is represented by Λm.
When conductivity is expressed in S cm−1 and concentration in mol L−1:
Where C is the molar concentration.
The commonly used unit is:
Equivalent conductivity is the conductance of the volume of solution containing one gram-equivalent of an electrolyte when placed between electrodes sufficiently far apart.
It is represented by Λeq.
Where N is the normality of the solution.
Its commonly used unit is S cm² equivalent−1.
Conductivity and molar conductivity are related but represent different quantities.
Conductivity generally decreases upon dilution because the number of ions present per unit volume decreases.
Molar conductivity, however, generally increases upon dilution because ions experience less interionic interaction and their mobility increases.
When an electrolyte solution is diluted, the concentration of ions per unit volume decreases. Therefore, the conductivity generally decreases.
This behavior occurs for both strong and weak electrolytes.
Strong electrolytes are already highly ionized. Therefore, dilution mainly decreases interionic interactions and increases ionic mobility.
Molar conductivity increases gradually as the solution is diluted and approaches a limiting value at very high dilution.
Weak electrolytes show a much larger increase in molar conductivity on dilution because dilution increases their degree of ionization in addition to increasing ionic mobility.
Kohlrausch's law of independent migration of ions states that at infinite dilution, each ion contributes independently to the total molar conductivity of an electrolyte.
Thus, the limiting molar conductivity of an electrolyte is the sum of the limiting ionic conductivities of its constituent ions.
Here:
The law allows the limiting molar conductivity of an electrolyte to be calculated from ionic conductivity data.
For a weak electrolyte:
Where α is the degree of ionization.
For a weak electrolyte such as a weak acid:
Thus, conductivity measurements can be used to estimate the dissociation constant of weak electrolytes.
Conductivity measurements can be used to determine the solubility of sparingly soluble electrolytes.
Ionic mobility is the speed acquired by an ion under the influence of a unit electric field.
Ionic mobility depends on factors such as:
Hydrogen and hydroxide ions exhibit unusually high ionic mobility in aqueous solution because their charge is transported efficiently through proton-transfer mechanisms.
When an electric current passes through an electrolyte solution, both cations and anions contribute to the transport of electricity.
The fraction of the total current carried by a particular ion is called its transport number or transference number.
Transport number of cation:
Transport number of anion:
Since the total current is carried by cations and anions:
Strong and weak electrolytes differ in ionization and therefore show different conductance behavior.
Conductivity depends strongly on the concentration of ions present per unit volume.
Conductance generally increases with temperature because ion mobility increases and solvent viscosity often decreases.
Solvent viscosity and dielectric properties influence ionization and ionic mobility.
Ionic size, charge and hydration affect ionic mobility.
Conductance of an electrolyte solution is measured using a conductivity cell connected to a suitable conductivity meter.
The cell generally consists of two inert electrodes, commonly platinum electrodes, placed at a known distance from each other.
The cell constant is determined using a standard electrolyte solution with known conductivity.
A conductivity cell consists essentially of two electrodes immersed in the electrolyte solution.
The electrodes are commonly made of platinum and may be coated with platinum black to increase the effective surface area and reduce polarization.
Once the cell constant is known, the conductivity of an unknown solution can be calculated from its measured conductance.
Conductometric titration is a titration method in which the conductance of the solution is measured as the titrant is gradually added.
The endpoint is determined from the change in conductance rather than by using a conventional visual indicator.
Consider titration of HCl with NaOH:
Initially, the conductance is high because H+ ions have very high mobility. As NaOH is added, H+ ions are replaced by Na+ ions and conductance decreases.
After the equivalence point, excess OH− ions appear and the conductance increases.
In the titration of a weak acid such as acetic acid with a strong base, the initial conductance is relatively low because acetic acid is only partially ionized.
As sodium hydroxide is added, sodium acetate is formed and the conductance gradually increases.
Beyond the equivalence point, excess OH− ions cause a more rapid increase in conductance.
Conductometric methods can also be used for precipitation titrations.
For example, chloride ions can be precipitated using silver nitrate:
The change in ionic composition during precipitation changes the conductance of the solution, allowing the endpoint to be determined.
| Strong electrolyte | Weak electrolyte |
|---|---|
| Almost completely ionized. | Partially ionized. |
| High concentration of ions. | Lower concentration of ions at comparable concentration. |
| Small increase in molar conductivity on dilution. | Large increase in molar conductivity on dilution. |
| Examples: HCl, NaCl, KNO₃. | Examples: CH₃COOH, NH₄OH. |
| Conductivity (κ) | Molar conductivity (Λm) |
|---|---|
| Conductance of a unit volume of solution with appropriate cell geometry. | Conductance associated with one mole of electrolyte. |
| Depends on number of ions per unit volume. | Depends on total ionic contribution from one mole. |
| Generally decreases on dilution. | Generally increases on dilution. |
| Unit: S m⁻¹. | Common unit: S cm² mol⁻¹. |
Electrical conductivity is widely used as a rapid indicator of the concentration of dissolved ionic substances in water.
Natural water containing dissolved salts conducts electricity. Higher concentrations of dissolved ions generally result in higher electrical conductivity.
Conductivity measurements are therefore useful in:
Electrolytic conductance is associated with the movement of ions through an electrolyte solution. Electrolytes may be strong or weak depending on their degree of ionization.
Important electrical quantities include resistance, conductance, resistivity, conductivity, cell constant, molar conductivity and equivalent conductivity.
Conductivity generally decreases on dilution, whereas molar conductivity increases. The increase is relatively small for strong electrolytes but much larger for weak electrolytes.
Kohlrausch's law explains the limiting molar conductivity of electrolytes in terms of independent ionic contributions and has several important applications.
Ionic mobility and transport numbers describe the contribution of individual ions to electrical conduction. Conductometric titration provides a useful analytical application of conductance measurements.
Electrolytic conductance is important in chemical analysis, determination of ionization and dissociation constants, water analysis, environmental monitoring and industrial processes.