Electrochemistry - Kohraush Law

Slide 1: Introduction to Electrochemistry

Electrochemistry is a branch of chemistry that deals with the study of the interchange of chemical energy and electrical energy.
It encompasses the study of redox reactions, electrochemical cells, and various applications in our daily lives.
Electrochemistry plays a significant role in several areas, including batteries, corrosion, and electrolysis.
In this lecture, we will focus on one of the fundamental laws in electrochemistry, Kohlrausch Law.

Slide 2: Redox Reactions

Redox (oxidation-reduction) reactions involve the transfer of electrons between species.
Oxidation is the loss of electrons, while reduction is the gain of electrons.
These reactions are crucial in electrochemistry, as they form the basis of various electrochemical processes.
Example: The reaction between magnesium and oxygen to form magnesium oxide: 2Mg + O2 → 2MgO

Slide 3: Electrochemical Cells

An electrochemical cell is a system that allows for the conversion of chemical energy into electrical energy.
It consists of two electrodes, an anode (where oxidation occurs) and a cathode (where reduction occurs).
The electrodes are immersed in an electrolyte solution, which facilitates the flow of ions.
The flow of electrons from the anode to the cathode creates an electric current.

Slide 4: Types of Electrochemical Cells

There are two main types of electrochemical cells: galvanic cells and electrolytic cells.
Galvanic cells (voltaic cells) generate electrical energy from spontaneous redox reactions.
Electrolytic cells use an external electrical source to drive non-spontaneous redox reactions.
Both types of cells play crucial roles in various applications, such as batteries and electrolysis.

Slide 5: Galvanic Cells

Galvanic cells convert chemical energy into electrical energy.
The redox reactions occur spontaneously, producing an electric current.
The anode is the site of oxidation, while the cathode is the site of reduction.
Electrons flow from the anode to the cathode through an external circuit.

Slide 6: Electrolytic Cells

Electrolytic cells use electricity to drive non-spontaneous redox reactions.
They require an external power source (such as a battery) to operate.
The anode is the positive electrode, attracting negative ions.
The cathode is the negative electrode, attracting positive ions.

Slide 7: Kohlrausch Law - Introduction

Kohlrausch Law, also known as the law of independent migration of ions, was formulated by Friedrich Kohlrausch.
This law describes the limiting molar conductivity of an electrolyte at infinite dilution.
Infinite dilution refers to the scenario where the concentration of the electrolyte approaches zero.
The law states that the molar conductivity of an electrolyte is the sum of the individual molar conductivities of its constituent ions.

Slide 8: Molar Conductivity (λ)

Molar conductivity (λ) is a measure of how well an electrolyte conducts electricity.
It is defined as the conductivity of a 1 Molar solution of the electrolyte divided by the concentration of the electrolyte.
Molar conductivity depends on the nature of the electrolyte and the concentration of ions present.
It is usually reported in units of siemens per meter squared per mole (S·m²·mol⁻¹).

Slide 9: Limiting Molar Conductivity (λ₀)

The limiting molar conductivity (λ₀) is the molar conductivity of an electrolyte at infinite dilution.
λ₀ is a fundamental property of electrolytes that helps in understanding the behavior of ions in solution.
It provides insights into the ionic mobility and conductivity of the electrolyte.
λ₀ can be determined experimentally by measuring the conductance of the electrolyte at different concentrations and extrapolating it to infinite dilution.

Slide 10: Kohlrausch Law Equation

Kohlrausch Law can be mathematically represented as: λ₀ = λ⁺ + λ⁻ where λ₀ is the limiting molar conductivity of the electrolyte, λ⁺ is the molar conductivity of the cation, and λ⁻ is the molar conductivity of the anion.
This equation emphasizes that the molar conductivity of an electrolyte is the sum of the individual molar conductivities of its constituent ions.

Slide 11: Factors Affecting Molar Conductivity

The molar conductivity of an electrolyte depends on several factors, such as temperature, concentration, and size/charge of ions.
Temperature: Molar conductivity generally increases with increasing temperature due to increased ion mobility.
Concentration: Molar conductivity typically decreases with increasing concentration due to ion-ion interactions and decreased mobility.
Size/Charge of ions: Smaller and more highly charged ions tend to have higher molar conductivities due to their greater mobility.

Slide 12: Application of Kohlrausch Law

Kohlrausch Law finds widespread application in determining the molar conductivity of electrolytes at various concentrations.
It helps in calculating the equivalent conductance (κ), which provides insights into the conductance behavior of electrolytes.
The law is also used in the study of ionic strength and activity coefficients in solutions.

Slide 13: Example Calculation

Let’s consider a hypothetical scenario of an electrolyte dissociating into two ions: A⁺ and B⁻.
The molar conductivity of A⁺ is 70 S·m²·mol⁻¹, and the molar conductivity of B⁻ is 50 S·m²·mol⁻¹.
Applying Kohlrausch Law, the limiting molar conductivity (λ₀) of the electrolyte can be calculated as follows: λ₀ = λ⁺ + λ⁻ = 70 + 50 = 120 S·m²·mol⁻¹

Slide 14: Conductivity and Strong Electrolytes

Strong electrolytes fully dissociate into ions when dissolved in water.
These electrolytes show high electrical conductivity due to the presence of a large number of ions in solution.
Example: Strong acids like hydrochloric acid (HCl) and sulfuric acid (H₂SO₄) are strong electrolytes with high molar conductivity.

Slide 15: Conductivity and Weak Electrolytes

Weak electrolytes only partially dissociate into ions when dissolved in water.
These electrolytes show lower electrical conductivity compared to strong electrolytes.
Example: Weak acids like acetic acid (CH₃COOH) and weak bases like ammonia (NH₃) are weak electrolytes with lower molar conductivity.

Slide 16: Temperature Dependence of Conductivity

Conductivity of solutions is generally affected by temperature.
As temperature increases, ions gain more kinetic energy, leading to increased mobility and higher conductivity.
The temperature coefficient (θ) is a measure of the extent to which the molar conductivity changes with temperature.
Conductivity measurements must be corrected for temperature variations when comparing different electrolytes.

Slide 17: Unit of Conductivity

Conductivity is a measure of a material’s ability to conduct electric current.
The SI unit of conductivity is Siemens per meter (S/m) or mho per meter (℧ /m).
Conductivity is the reciprocal of resistivity (ρ) and is calculated using the formula: σ = 1/ρ.

Slide 18: Conductivity and Electrolyte Concentration

Conductivity increases with increasing electrolyte concentration.
Higher concentration means a greater number of ions available to conduct electricity.
Conductivity is directly proportional to the concentration of ions in solution.
For dilute solutions, conductivity can be related to concentration using the equation: κ = λ₀c, where κ is the equivalent conductance and c is the concentration.

Slide 19: Limitations of Kohlrausch Law

Kohlrausch Law assumes ideal behavior of electrolytes at infinite dilution.
Presence of ion-ion interactions and ionic association can cause deviations from the ideal behavior.
Strong electrolytes may exhibit some deviations due to ion-pair formation or hydration effects.
In such cases, modified equations or empirical corrections may be employed to account for these deviations.

Slide 20: Summary

Kohlrausch Law provides a way to calculate the limiting molar conductivity of an electrolyte at infinite dilution.
It states that the molar conductivity of an electrolyte is the sum of the individual molar conductivities of its constituent ions.
Molar conductivity depends on factors such as temperature, concentration, and size/charge of ions.
Conductivity is affected by temperature and concentration, and conductivity measurements must be corrected for temperature variations.
Kohlrausch Law finds applications in various fields of chemistry and helps understand the behavior of electrolytes in solution.

Slide 21: Application of Kohlrausch Law in Conductivity Measurements

Kohlrausch Law is widely used in conducting conductivity measurements of electrolytes.
Conductivity measurements help determine the electrical conductivity of a solution, which relates to the concentration and mobility of ions.
The law allows us to calculate the molar conductivity of an electrolyte, which is essential for understanding its conducting behavior.
By employing Kohlrausch Law, we can compare the conductivities of different electrolytes and study their variations with concentration.

Slide 22: Importance of Conductivity Measurements

Conductivity measurements provide valuable information about the strength of electrolytes and their ability to conduct electricity.
These measurements are crucial for various applications, such as designing efficient batteries or understanding the behavior of ionic solutions.
Conductivity data allows for the determination of key parameters like ionic mobility, ionic strength, and dissociation constants.
Using Kohlrausch Law, we can assess the electrical conductivity of electrolytes at different concentrations and make informed decisions.

Slide 23: Conductivity Cell and Its Components

Conductivity cells are used for measuring the conductivity of solutions.
The basic components of a conductivity cell include two electrodes, usually made of platinum, gold, or graphite.
The electrodes are immersed in the solution and connected to an electrical circuit.
The distance between the electrodes and the cell geometry are standardized to ensure accurate and reproducible measurements.

Slide 24: Conductivity Cell and Solution Conductivity

The conductivity cell measures the electrical conductivity of the solution within it.
When a voltage is applied across the electrodes, the solution acts as a conductor.
The conductivity of the solution depends on the concentration and mobility of ions present.
The conductivity cell provides a quantitative measure of the solution’s ability to conduct electricity.

Slide 25: Conductivity Cells: Potentiometric and Amperometric

There are two main types of conductivity cells: potentiometric cells and amperometric cells.
Potentiometric cells measure the potential difference (voltage) across the electrodes, which is related to the conductivity of the solution.
Amperometric cells measure the current flowing through the solution when a constant voltage is applied.
Both types of cells have their advantages and applications, depending on the desired measurements.

Slide 26: Conductivity Cell Calibration

Calibration is an essential step in conducting accurate conductivity measurements.
Calibration involves the use of standard solutions with known conductivities to establish a calibration curve.
The calibration curve relates the measured electrical conductance to the concentration of the standard solutions.
By using the calibration curve, the conductivity of an unknown solution can be determined.

Slide 27: Conductivity Measurements and Electrolyte Concentration

Conductivity measurements are dependent on the concentration of the electrolyte.
For strong electrolytes, conductivity generally increases with increasing concentration due to a greater number of ions available for conduction.
Weak electrolytes may exhibit different behavior, with conductivity increasing initially and then reaching a maximum due to ion-pair formation or other factors.
Conductivity measurements of different concentrations allow for the determination of equivalent conductance (κ) and molar conductivity (λ) using appropriate equations.

Slide 28: Example Calculation using Kohlrausch Law

Let’s consider an example of a solution containing 0.1 M sodium chloride (NaCl).
We want to determine the molar conductivity of NaCl using Kohlrausch Law.
Conductivity measurements of the solution yield a specific conductance (κ) of 7.5 × 10⁻³ S·cm⁻¹.
Applying the equation κ = λ₀c, we can calculate the molar conductivity (λ₀): λ₀ = κ / c = (7.5 × 10⁻³ S·cm⁻¹) / (0.1 mol·cm⁻³) = 75 S·m²·mol⁻¹

Slide 29: Limitations of Conductivity Measurements

Conductivity measurements have certain limitations that need to be considered.
Temperature variations can affect conductivity, necessitating temperature corrections for accurate results.
Conductivity measurements are dependent on the specific ions present in the solution and their individual mobilities.
The presence of impurities or other substances that may interfere with the measurement can affect the accuracy of conductivity data.
Careful calibration and quality control are essential to minimize errors and obtain reliable conductivity measurements.

Slide 30: Summary and Key Points

Kohlrausch Law is a fundamental concept in electrochemistry, facilitating the determination of the molar conductivity of electrolytes.
Conductivity measurements provide quantitative information about the ability of a solution to conduct electricity.
The conductivity cell, calibration, and appropriate equations are essential for conducting accurate measurements.
Conductivity measurements are dependent on the concentration and nature of the electrolyte, and temperature corrections may be necessary.
Conductivity data aids in understanding the behavior of electrolytes, assessing their conducting strength, and making informed decisions in various applications.