Define Molar Conductivity

In chemistry, many reactions are carried out in liquid form due to the ease of measurement and calculation. Energy plays a big role in carrying out these experiments, and one of the most popular methods to apply external energy is by passing electricity.

The conduction of electricity forms the basis for a lot of phenomena that we observe in today’s world. Though primarily, the conduction of electricity is observed in solids, the same phenomenon in liquids plays a major role in many important technologies like batteries. The capacity of any liquid to conduct electricity gives rise to the concept of Molar Conductivity. It provides a measure of how much electricity a particular electrolyte conducts at a given concentration.

Conductivity

In physics, every material is considered to have a characteristic amount of resistance, irrespective of what shape or size it is in. This characteristic resistance is termed the specific resistance of the material. The Greek letter, (rho) denotes resistivity.

Electricity is passed through a liquid when a special substance called an electrolyte is dissolved in it. Usually, electrolytes are dissolved in water to create an electrolytic solution. Applying voltage to an electrolytic solution causes electricity to pass through it, and it provides a certain resistance to this electricity.

By measuring the amount of resistance that an electrolytic solution provides to the passing current in it, and then by using Ohm’s law, we can calculate the specific conductance or conductivity of the electrolytic solution. Conductivity is typically measured in Siemens per metre. In the industries, a smaller unit called micro Siemens per metre is also used. 

The greek letter (kappa) is used to denote conductivity. The equation for conductivity can be found as 

According to Ohm’s law

R = ρl/A

⇒ρ=AR/l

According to the definition of conductivity

κ=1/ρ

⇒κ=l/AR

⇒κ=l/A×1/R

Here, l/A is a constant value called the cell constant. And 1/R gives the conductance of the electrolytic solution, therefore,

k=Cell constant x Conductance (G)  Siemens/metre

Molar Conductivity

An electrolyte is a special substance that can conduct electricity when dissolved. It contains ions that are formed due to the splitting of the compounds present in the liquid. Every electrolyte differs in its capacity to allow an electric current to pass through it.

Two broad classifications are made in regards to the strength of an electrolyte. Electrolytes can be classified into either strong or weak electrolytes. The strength of an electrolyte refers to the degree to which an electrolyte will undergo dissociation when an electric current is passed through it to facilitate its conduction.

Strong electrolytes split or dissociate into ions easily which means that electricity is easily transferred through a strong electrolyte. But in a weak electrolyte, the passing of an electric current does not cause a large amount of dissociation which means that it is tougher to pass electricity through a weak electrolyte.

To gauge the conduction capacity or conductivity of an electrolyte, a universal measure is required. Hence we consider molar conductivity whenever we talk about the capacity of an electrolyte to conduct electricity. 

Molar conductivity can be defined as the conducting power that is created when one mole of a particular electrolyte is dissolved in a solvent like water. It gives a measure of the conducting capacity of all the ions that are produced as a result of dissolving the electrolyte in the solution. It is denoted by the Greek letter, (lambda).

Every electrolyte has a specific conductance that is dependent on its nature and composition. The ratio of the specific conductance to the molar concentration of the electrolyte, gives its molar conductivity.

λ=κ/c

where

λ is the molar conductivity

is specific conductance

c is molar concentration

From the equation, we can observe that molar conductivity has a direct relation with specific conductance. Hence with an increase in specific conductance, the molar conductivity of an electrolyte will also increase. This is supported by the fact that higher specific conductance indicates less overall resistance offered by the electrolytic solution.

Variations in Molar Conductivity

Molar conductivity is represented by the equation;

λ=/c

where

λ is the molar conductivity

is specific conductance

c is molar concentration

In this equation, we can observe that there is an inverse relationship between molar conductivity and molar concentration of electrolyte c. But molar conductivity variates differently according to the concentration of strong and weak electrolytes.

Strong electrolytes

For strong electrolytes, molar conductivity first increases and then decreases if the concentration decreases after a certain point. That is, for most strong electrolytes, molar conductivity and concentration have a direct relationship. With an increase in dilution, the concentration of ions decreases since no more new ions are produced.

This is due to the fact that strong electrolytes have a high tendency to almost completely dissociate into their constituent ions once they are dissolved in a solvent. When dilution happens, no more new ions are produced as the amount of solvent per ion increases. This means that the conducting capacity of the electrolytic solution decreases since the concentration of ions decreases. This decrease is translated as a decrease in molar conductivity.

Weak Electrolytes

When considering weak electrolytes, molar conductivity and molar concentration share an inverse relationship. With an increase in dilution of the weak electrolytic solution, the molar conductivity of the solution is observed to increase.

This happens because weak electrolytes do not completely dissociate when dissolved in a solvent. Only a fraction of the weak electrolytes get dissociated when it is first dissolved in a solvent. But when more solvent is added, further dissociation takes place.

Due to this extra dissociation, the number of ions that are present in the solution increases. A greater number of ions causes the conduction capacity of the solution to increase which is translated as an increase in the molar conductivity of the solution.

Conclusion

Molar conductivity can be defined as the conducting power that is created when one mole of a particular electrolyte is dissolved in a solvent like water. It gives a measure of the conducting capacity of all the ions that are produced as a result of dissolving the electrolyte in the solution. It is denoted by the Greek letter, (lambda).