Conductivity or Specific Conductivity of an electrolyte solution is the measurement of its ability to conduct the electricity. Molar conductivity is the conducting power of the ions produced by dissolving one mole of an electrolyte in solution. It is denoted by Λ (lambda).
What is Conductance?
Conductance definition: Conductance the measurement of the easy flow of electricity along a certain path through an electrical element, like electric cable, bulb, etc. Electrical resistance is measured in terms of “Ohm”. It is denoted by R.
Where, R= resistance offered by the conductor, ρ, l, A are specific resistance, length and area of cross-section of the conducting material, respectively.
Unit of conductance: Conductance is considered to be the opposite of electrical resistance, i.e. mho or ohm-1. The SI unit is Siemens per meter (S/m) or ohm-1.
It is measured as ohm-1 = mho = Siemens per meter.
By substituting, R from equation (R= l/A. ρ), we get ( )
G = 1 /R
or G = 1/ (l/A. ρ) = κ. A/l equation (1)
where
κ = 1/ ρ= specific conductance, and ρ is the specific resistance of the conducting material,
From the equation (1),
If area of cross section is A = 1 cm2 and the length is l = 1 cm, then Kappa is equal to specific conductance, κ = G.
Therefore, the specific conductance can be defined as given here:
Specific conductance or conductivity, κ: It is the conductance of a conducting material or a solution between two electrodes of cross-sectional area 1 cm2 and separated by 1 cm distance.
Note that the above solution is occupying 1 cm3 volume.
It is measured in S.I. system as Siemens.m-1 (S/m).
The specific conductance depends on the nature of conducting substance or the electrolyte.
As the concentration of electrolytic solution increases the number of ions per unit area also increases, so does the specific conductance.
Cell constant (G*): The ratio of the distance between the electrodes “l” to the cross sectional area “A” of the electrodes is known as cell constant.
The cell constant can be determined by using following relations given below:
Based on the material’s electrical conductivity, the materials can be divided into two major types as mentioned below:
1. Insulators: The substances resisting the flow of electricity through them are called insulators. They lack free movement of the charged particles or free electrons. For example, glass, wood, organic polymers (like plastics), diamond, quartz, etc.
Conductors: The substances allowing the flow of electricity through them with slight resistance are called conductors.
They are again divided into:
I. Metallic and semi-metallic conductors: These are conductors which conduct the electricity through the electrons. For example, all the metals, graphite, etc.
In case of metallic conductors:
No chemical reaction occurs during the conduction of electricity.
With rise in temperature, due to vibrational disturbances among the molecules of a substance conductivity decreases.
II. Electrolytes: They are the substances or medium that give oppositely charged ions for the conduction of electricity. For example, NaCl, KCl, CH3COOH, HCl etc.
In case of electrolytes:
There is flow of ions towards the oppositely charged electrodes.
During conduction of electricity through electrolytes, oxidation occurs at anode, whereas reduction occurs at cathode, so the chemical reaction occurs.
The conductivity increases with increase in temperature as the extent of ionization also increases.
The electrolytes undergo dissociation to give ions either in molten state or in aqueous solutions. Depending on the extent of ionization (or dissociation) in water, the electrolytes are further divided into:
A. Strong electrolytes undergo complete ionization in water. A few examples are: NaCl, KCl, HCl, H2SO4, NaOH, etc.
B. Weak electrolytes undergo partial ionization in water. For example, HF, CH3COOH, NH4OH, HCOOH, etc.
III. Non-electrolytes: They are the substances that are unable to give ions or electrons for electrical conduction. For example, urea, glucose, sucrose, etc.
Molar Conductivity
It is defined as the conducting power of all the ions produced by 1 mole of an electrolyte in a given solution.
Molar conductivity (Λm or μ): The conductance of that volume of solution containing 1 mole of an electrolyte is known as molar conductivity. It is denoted by Λm or μ.
It is related to specific conductance, κ as:
μ = κ V
Or, μ = κ /M
where, M is the molar concentration.
If M is in the units of molarity i.e., moles per litre the Λ may be expressed as,
Λ = κ × 1000 / M
Consider a solution containing 1 gm mole of electrolyte placed between two parallel electrodes of 1 cm2 area of cross-section and one cm apart,
Conductance = Conductivity (μ) = Molar conductivity (Λ)
Now, if solution contains 1 gm mole of the electrolyte, then the measured conductance will be the molar conductivity. Thus,
Molar conductivity (μ) = 100 × Conductivity
In other words, (μ) = κ × V, where V is the volume of the solution in cm-1 containing one-gram mole of the electrolyte.
If M is the concentration of the solution in mole per litre, then M mole of electrolyte is present in 1000 cm -1
1 mole of electrolyte is present in 1000/M cm-1 of solution.
Thus, (μ) = κ × V in cm-1 containing 1 mole of electrolyte.
or μ = κ × 1000 / M
Units - ,
where M is molarity of the electrolytic solution.
Units of μ is: cm2.ohm-1.mol-1 = cm2.mho.mol-1, or m2.Siemens.mol-1.
The relation between equivalent conductance, Λ and molar conductance, μ can be given by:
μ = Λ × equivalent factor of the electrolyte
The equivalent factor of the electrolyte is usually the total charge on either anions or cations present in one formula unit of it. It may be equal to basicity in case of acids or equal to acidity in case of bases.
Factors Affecting the Conductance of Electrolyte Solution:
Temperature: The conductance of an electrolyte solution increases with increase in the temperature due to increase in the extent of ionization.
Nature of electrolyte: The strong electrolytes get completely ionized in water and therefore, show higher conductivities as they provide more numbers of ions. For example, HCl and HNO3.
Weak electrolytes undergo partial ionization in the presence of water and therefore show low conductivities in their solutions than strong electrolytes. For example, acetic acid (CH3COOH).
Ionic size and mobility: With increase in the size of ion, its mobility decreases, so the conductivity also decreases. For example, in molten state, the conductivity of sodium salts is greater than those of cesium salts as the size of Na+ ion is smaller than the Cs+ ion.
However, in aqueous solutions the extent of hydration affects the mobility of the ion, which in turn affect the conductivity. Heavily hydrated ions show low conductance values due to its larger size. For example, in aqueous solutions Na+ ion with high charge density is heavily hydrated than Cs+ ion with low charge density. Hence, hydrated Na+ is bigger than hydrated Cs+. As a result, sodium salts show lower conductivities compared to those of cesium salts in water.
The nature of solvent and its viscosity: In more viscous solvents, the ionic mobility gets reduced. So, the conductivity gets decreased.
Concentration: The specific conductance (κ) increases with increase in concentration of the solution (as the number of ions per unit volume increases).
Whereas, both the equivalent conductivity and molar conductance increase with decrease in concentration (i.e. upon dilution), as the extent of ionization also increases.
