Important Topic of Chemistry: Kohlrausch’s Law

 

Friedrich Kohlrausch, a German physicist, researched several electrolyte solutions to establish their conductivity qualities between 1874 and 1879. He was investigating the electrical characteristics of electrolytes in order to determine their behaviour and examine the anomalies involved. The study of several salt solutions revealed an intriguing fact:

‘The limiting molar conductivities of each sort of migrating ion are unique and particular to that kind of ion,’ and an ion’s conductivity is unique and does not depend on the other ions present in the solution or their nature.

The ‘Kohlrausch Law of Independent Migration of Ions’ was developed as a consequence of the investigation into solutions. After concluding that, at infinite dilution, every ion present in an electrolyte makes a ‘definite’ contribution to the complete molar conductivity (total molar conductivity) of the electrolyte, he dubbed the ‘individual contribution’ of a particular ion to the total molar conductivity of the electrolyte ‘Molar ionic conductivity. In 1876, he proposed the generalization known as Kohlrausch law based on his findings.

The limiting molar conductance of an electrolyte to its component ions is described by Kohlrausch Law (also known as the Kohlrausch Law of Independent Migration).

It simply asserts that an electrolyte’s limiting molar conductivity is equal to the sum of the individual limiting molar conductivities of the cations and anions that make up the electrolyte.

In general, if a dissociation electrolyte produces v+ cations and v– anions, its limiting molar conductivity is given by

λ_m= λ₀⁺ v⁺ +v^– λ ^–₀

Here, λ₀⁺ and λ ^–₀ are the limiting molar conductivities of cations and anions respectively.

Limiting molar conductivity is the molar conductivity of a solution at infinite dilution.

When two electrolytes share the same cation, the difference in their limiting molar conductivities is unaffected by the cation and is solely altered by an anion change. The above assertion is likewise valid for electrolytes containing the same anion.

According to the law of independent migration of ions, the limiting molar conductivity of an electrolyte (i.e., the conductivity of an electrolyte at infinite dilution when all of it is fragmented into ions) is equal to the sum of the limiting molar conductivity of each constituent ion.

The limiting conductivity of sulphuric acid is an example of this law (H₂SO₄). Sulphuric acid’s limiting molar conductivity is equal to the sum of the limiting conductivities of hydrogen cation and sulfate anion. The following is the mathematical expression of the preceding statement:

λH₂SO₄=2 λH⁺ +λSO₄^2-

Kohlrausch law and its applications are critical in the study of dilute fluids as well as electrochemical cells. Among other essential uses, this rule is utilized to determine the limiting conductivity of a weak electrolyte.

Kohlrausch Law Applications

• This rule is frequently used to calculate the degrees of dissociation of weak electrolytes.
• This rule is frequently used to compute the solubility constants of different salts.
• It is used to calculate an electrolyte’s dissociation constant.
• It is used to calculate a weak electrolyte’s limiting molar conductivity.
• It is also used to calculate the cell potential in a variety of electrochemical cells.

1. Weak Electrolytes and Molar Conductivities

Kohlrausch’s law is used to calculate the total molar concentration of weak electrolytes. It is because the degree of dissociation in weak electrolytes, such as acetic acid, is relatively low at greater concentrations. As a result, the increase in the degree of dissociation causes a change in the molar conductivities of such electrolytes with dilution. As a result, the number of ions per total volume of the solution containing 1 mole of electrolyte increases. As a result, the 0m value rises dramatically, even with dilution at near-zero concentration. As a result, extrapolating molar conductivities to zero dilution cannot provide the total molar conductivities of such electrolytes (weak electrolytes). As a result, because the molar conductivities of weak electrolytes at infinite dilution cannot be calculated, Kohlrausch’s law is used.

2. Sparingly Soluble Salts’ Solubility

The salts that are sparingly soluble are those that do not dissolve well in water (or have very little dissolution in water). Such salts include AgCl, PbSO₄, BaSO₄, and others. They are at infinite dilution because they are very little dissolved. And they have the same solubility and concentration. Solubility may be calculated using the total molar conductivities (through Kohlrausch’s law) and specific conductivities (K) of these salts.

3. Electrolyte Dissociation Degree

The degree of dissociation of weak electrolytes (α) is calculated using Kohlrausch’s law. The degree of dissociation is determined by the molar conductivity of the electrolyte at any concentration, C, and at infinite dilution.

4. To Determine the Weak Electrolyte Dissociation Constant

The dissociation constant, K_c, for weak electrolytes, may be computed using the electrolyte’s degree of dissociation.

To conclude; when studying the conductivities of electrolytes, Friedrich Kohlrausch, a German scientist, discovered that the limiting molar conductivities of each type of migrating ion are unique and peculiar to that type of ion. As a result, Kohlrausch’s law of independent ion migration was developed. The rule and its mathematical formulations are applicable to all electrolytes, whether strong or weak. As a result, the rule may be used to calculate the degree of dissociation of weak electrolytes, which would normally be done experimentally, as well as the solubility of sparingly soluble salts.

FAQs

How did Kohlrausch's law come to be discovered?

Kohlrausch found this rule while experimenting with dilute liquids. He supported his theory with the finding that when two distinct electrolytes share the same anode (or cathode), the difference in their limiting molar conductivities is independent of the nature/type of the common anode (or cathode).

Describe the applications of Kohlrausch's law of independent ion movement.

It is used to compute weak electrolytes' limiting molar conductivity, degree of dissociation, and dissociation constant. It is also used to calculate the solubility of the salt.

What is the law of ion independent migration?

It is the law that describes the link between an electrolyte's limiting molar conductivity and its component ions. It asserts that an electrolyte's limiting molar conductivity is equal to the sum of the limiting molar conductivities of the ions it divides into after dilution.