How Molar Conductivity Changes with Concentration and Electrolyte Type
Are you someone who often gets confused between conductance and conductivity or are you someone who finds the topic of molar conductivity in general difficult? If yes, then you have landed at the right place because here Vedantu will answer all your questions.
The conductance of a unit volume of solution held between two platinum electrodes with a unit area of cross-section and at a distance of unit length is known as specific conductivity or conductivity of an electrolytic solution at any given concentration. As the amount of ions per unit volume that bear the current in a solution decreases with dilution, the conductivity of the solution decreases. It is denoted by kappa(k). Unit of conductivity is given below-
k = siemen x \[m^{-1}\]
Factors Affecting Conductivity Includes-
Conductivity is a very sensitive physical quantity that can be affected by many factors. These factors are listed below:
Nature of an electrolyte
Size of the ion
The concentration of the solution
Temperature
Nature of the solvent
Molar Conductivity
The conductance of volume V of a solution containing one mole of electrolyte held between two electrodes with a region of cross-section A and a distance of unit length is the molar conductivity of a solution at a given concentration.
Molar conductivity = ⋀m = k/c
Where,
⋀m = molar conductivity
k = Specific conductivity
C = concentration in moles per volume
Molar conductivity can be calculated using the equation
⋀m (S cm2 mol-1) = k(S/cm)×1000/molarity(mol/L)
Variation of Molar Conductivity
As the total volume, V, of a solution containing one mole of electrolyte increases, molar conductivity increases with decreasing concentration. The concentration drops as a result of dilution. The molar conductivity of a solution is known as limiting molar conductivity when the concentration reaches zero. Solid and weak electrolytes have different molar conductivity variations with concentration.
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Variations of Molar Conductivity With Concentration
Molar conductivity varies as per the types of electrolytes used in the experiment. These variations are discussed in greater detail below:
1. Strong Electrolyte
For strong electrolytes, the molar conductivity decreases with dilution. This decrease can be represented by the equation given below-
⋀m = ⋀m⁰ - Ac1/2
If the graph is plotted between ⋀m and c1/2, a straight line is obtained with the intercept equal to limiting molar conductivity ⋀m⁰ and the slope equals to -A. This value of A depends on the charges on both cation and anion obtained on the dissociation of an electrolyte in a solution.
So the value of the limiting molar conductivity can be calculated using either the graph or the Kohlrausch law.
Kohlrausch’s law of independent migration of ions states that limiting molar conductivity of an electrolyte is represented as the sum of the lof cation and anion of the electrolyte.
⋀m⁰ = ⋀⁰cation+ ⋀⁰anion
2. Weak Electrolyte
The molar conductivity of weak electrolytes, on the other hand, rises with concentration. Due to a decreased degree of dissociation, such electrolytes have lower molar conductivity at higher concentrations.
When it comes to basic conductivity, it's clear that the conductivity rises as the electrolyte concentration rises. The number of ions in a unit volume of the solution determines the specific conductivity. The dissociation increases with dilution, allowing the number of current-carrying ions in the solution to rise. Dilution, on the other hand, reduces the number of ions present in a unit volume of the solution. The conductivity is reduced as a result of this.
Solved Examples
1. If the Molarity is Given is 0.30M and the Conductivity is 0.023Sm-1. Calculate the Molar Conductivity of the Solution.
⋀m = k×1000/c
⋀m = 0.023×1000/0.30
⋀m = 76.66 cm2 mol-1
2. The Molar Conductivity of a 1.5M Solution of an Electrolyte is Found to be 138.9scm2mol-1. Calculate the Conductivity of this Solution.
⋀m = k×1000/c
k = Λm×c/1000
k = 138.9×1.5/1000
=0.208Scm-1
Did You Know?
The presence of free ions in electrolytes causes them to conduct electricity. It's analogous to how free electrons favour the conduction of electricity in metallic conductors. The Arrhenius equation or principle is used to describe electrolytic conduction.
We're all familiar with electrolytic solutions, which are produced by dissolving some salts. The salts don't have to be ionic all of the time. The only requirement is that the compound is made up of ions of opposite charges.
When a neutral electrolyte is dissolved in water, the electrolyte molecules are divided into two differently charged ions, according to the Arrhenius principle.
The charged particles can freely travel around in the solution. Positive ions, or cations, may travel towards a negative electrode, or cathode, to reduce themselves. At the same time, negative ions or anions will travel towards the positive electrode or anode and oxidise themselves. Electric conduction is generated by the migration of charged particles.
Conclusion
This write-up by Vedantu explains the topic of Variations of Molar Conductivity - Factors, Variations and FAQs in a very holistic manner. It will prepare you for both the Class 12 board exams as well as for various entrance exams such as JEE, JIPMER, NEET, etc.
You can find many other topics of Chemistry by Vedantu on its website. Each and every topic is covered in great detail by subject matter exports of Chemistry at Vedantu.
FAQs on Variations of Molar Conductivity: Factors, Effects & Applications
1. What is molar conductivity, and how is it different from specific conductivity (kappa, κ)?
Molar conductivity (Λm) represents the total conducting power of all the ions produced when one mole of an electrolyte is dissolved in a solution. It is distinct from specific conductivity or conductivity (κ), which is the conductance of one unit volume (e.g., 1 cm³) of the solution. Essentially, molar conductivity relates conductivity to the concentration of the electrolyte, providing a measure of conductance per mole.
2. How does the molar conductivity of an electrolyte solution change upon dilution?
For both strong and weak electrolytes, molar conductivity (Λm) increases with dilution (or as concentration decreases). This happens because as the volume of the solution increases, the ions are farther apart, reducing inter-ionic attractions and allowing them to move more freely. For weak electrolytes, dilution also increases the degree of ionisation, further boosting the molar conductivity.
3. What are the key factors that influence the molar conductivity of a solution?
Several factors influence molar conductivity. The most important ones are:
- Nature of the electrolyte: Strong electrolytes, which ionise completely, have higher molar conductivity than weak electrolytes at the same concentration.
- Concentration: Molar conductivity is inversely proportional to concentration; it increases as the solution becomes more dilute.
- Temperature: An increase in temperature raises the kinetic energy of ions, increasing their mobility and thus raising molar conductivity.
- Solvent Viscosity: Lower viscosity of the solvent allows ions to move more easily, resulting in higher molar conductivity.
4. What is the definition of limiting molar conductivity (Λ°m)?
Limiting molar conductivity (Λ°m), also known as molar conductivity at infinite dilution, is the maximum molar conductivity of an electrolyte when its concentration approaches zero. At this theoretical point, the dissociation of the electrolyte is complete, and inter-ionic forces are negligible, allowing each ion to contribute its maximum to the total conductivity.
5. Why does molar conductivity increase sharply for weak electrolytes on dilution but only gradually for strong electrolytes?
This difference is due to their ionisation behaviour. A strong electrolyte is already almost 100% ionised. Dilution only increases the distance between existing ions, slightly reducing their electrostatic drag and causing a gradual increase in conductivity. In contrast, a weak electrolyte has a low degree of ionisation. According to Le Chatelier's principle, dilution shifts the equilibrium to favour more ionisation, causing a sharp increase in the total number of charge-carrying ions. This, combined with increased ionic mobility, results in a steep rise in its molar conductivity.
6. How can a graph of molar conductivity (Λm) vs. √c be used to find the limiting molar conductivity?
This graphical method is primarily used for strong electrolytes. When molar conductivity (Λm) is plotted against the square root of concentration (√c), it yields a nearly straight line as per the Debye-Huckel-Onsager equation (Λm = Λ°m - A√c). By extrapolating this line to zero concentration (where √c = 0), the y-intercept gives the value of limiting molar conductivity (Λ°m). This method fails for weak electrolytes because their graph is a curve that becomes too steep to extrapolate accurately.
7. How is Kohlrausch's law applied to find the limiting molar conductivity of a weak electrolyte like acetic acid (CH₃COOH)?
Since graphical extrapolation is not possible for weak electrolytes, we use Kohlrausch's law of independent migration of ions. This law states that an electrolyte's limiting molar conductivity is the sum of the individual contributions of its anions and cations. To find Λ°m for acetic acid, we can use the known Λ°m values of three strong electrolytes, such as HCl, NaCl, and CH₃COONa. The value is calculated as: Λ°m(CH₃COOH) = [Λ°m(HCl) + Λ°m(CH₃COONa)] - Λ°m(NaCl).
8. Why does specific conductivity (κ) decrease on dilution while molar conductivity (Λm) increases?
This is a common point of confusion. Specific conductivity (κ) measures the conductance of a unit volume of the solution. Upon dilution, the number of ions per unit volume decreases, which leads to a drop in κ. In contrast, molar conductivity (Λm) measures the conductance of one mole of ions, which are now spread over a larger volume. The increased ionic mobility (and increased dissociation for weak electrolytes) overcompensates for the dilution effect, causing Λm to rise.
9. What is the significance of the Debye-Huckel-Onsager equation in understanding molar conductivity?
The Debye-Huckel-Onsager equation, Λm = Λ°m - A√c, is fundamentally important for understanding the behaviour of strong electrolytes. Its significance is that it provides a mathematical model that explains why molar conductivity decreases as concentration increases. The constant 'A' accounts for the retarding forces on ions:
- The asymmetric or relaxation effect (drag from the ionic atmosphere).
- The electrophoretic effect (drag from the solvent moving in the opposite direction of the central ion).
10. What are some important practical applications of molar conductivity measurements?
Molar conductivity measurements are crucial in various chemical applications, including:
- Determining the degree of dissociation (α) of weak electrolytes using the formula α = Λm / Λ°m.
- Calculating the dissociation constant (Ka or Kb) for weak acids and bases, which is a measure of their strength.
- Finding the solubility and solubility product (Ksp) of sparingly soluble salts like AgCl, BaSO₄, and PbSO₄.
- Checking the purity of water, particularly deionised or distilled water, where any increase in conductivity indicates the presence of ionic impurities.
