Class 12 Chemistry – Electrochemistry and Coordination Compounds
Detailed Explanation (Based on Given Questions)
Introduction
In Class 12 Chemistry, topics like Electrochemistry, Standard Electrode Potential, Kohlrausch Law, Oxidising Agents, and Molar Conductivity are very important for board exams as well as competitive exams. These concepts help us understand how chemical reactions produce electricity, how electrolytes conduct current, and how redox reactions occur in electrochemical cells.
The given questions mainly focus on:
- Standard electrode potential (E°)
- Strength of oxidising agents
- Kohlrausch Law
- Molar conductivity and its graphical representation
Let us understand each concept in a clear and structured way.
1. Kohlrausch Law
Statement of Kohlrausch Law
Kohlrausch Law states that:
“At infinite dilution, the molar conductivity of an electrolyte is equal to the sum of the molar conductivities of its individual ions.”
Mathematically:
Λ°m = λ°+ + λ°−
Where:
- Λ°m = Limiting molar conductivity
- λ°+ = Conductivity of cation
- λ°− = Conductivity of anion
This law is mainly applicable to strong electrolytes and helps in calculating limiting molar conductivity of weak electrolytes.
2. Calculation of EMF of the Given Cell
Given Cell:
Al(s) / Al³⁺ (0.15 M) || Cu²⁺ (0.025 M) / Cu(s)
Step 1: Identify Anode and Cathode
Given standard reduction potentials:
E°(Al³⁺/Al) = −1.66 V
E°(Cu²⁺/Cu) = +0.34 V
Higher E° value acts as cathode.
So, Cu²⁺/Cu is cathode and Al/Al³⁺ is anode.
Step 2: Calculate Standard EMF
E°cell = E°cathode − E°anode
E°cell = 0.34 − (−1.66)
E°cell = 2.00 V
Step 3: Apply Nernst Equation
Ecell = E°cell − (0.0591/n) log Q
Balanced reaction:
2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu
Here n = 6
Reaction quotient:
Q = [Al³⁺]² / [Cu²⁺]³
Substitute values:
Q = (0.15)² / (0.025)³
Using given log values, substitute in equation and calculate final EMF.
Thus, actual EMF will be slightly less than 2.00 V due to concentration effect.
---
3. Strongest Oxidising Agent
Given reduction potentials:
Cl₂ + 2e⁻ → 2Cl⁻ E° = +1.36 V
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O E° = +1.51 V
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O E° = +1.33 V
Conclusion
Higher the E° value, stronger the oxidising agent.
Since MnO₄⁻ has highest E° value (+1.51 V),
it is the strongest oxidising agent among the given species.
---
4. Molar Conductivity (Λm)
Definition
Molar conductivity is defined as the conductance of all the ions produced by one mole of an electrolyte when dissolved in a solution.
Unit: S cm² mol⁻¹
Formula:
Λm = κ × 1000 / C
Where:
κ = Conductivity
C = Concentration
5. Nature of Electrolyte from Graph
The graph shows a straight line between Λm and √C.
For strong electrolytes:
Λm = Λ°m − A√C
This is a linear equation.
Since the graph is linear, the electrolyte is a strong electrolyte.
6. Determination of Λ°m
Limiting molar conductivity (Λ°m) is obtained by extrapolating the graph to √C = 0.
The intercept on Y-axis gives Λ°m.
From graph, Λ°m ≈ 150 S cm² mol⁻¹.
7. Determination of Constant A
The equation for strong electrolyte:
Λm = Λ°m − A√C
This is in the form:
y = c − mx
Slope of graph = −A
Thus, value of A can be calculated from slope of straight line:
A = (change in Λm) / (change in √C)
By selecting two points from graph, slope can be calculated.
Conclusion
From the above discussion, we understand that:
- Kohlrausch Law helps calculate limiting molar conductivity.
- EMF of a cell depends on standard electrode potential and concentration of ions.
- Higher standard reduction potential means stronger oxidising agent.
- Linear relation between Λm and √C confirms strong electrolyte behavior.
- Limiting molar conductivity is obtained from graph intercept.
These concepts are extremely important for board examinations and help in understanding electrochemical processes in real life applications like batteries and electroplating.
---
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Statement of Kohlrausch Law
Kohlrausch Law states that:
“At infinite dilution, the molar conductivity of an electrolyte is equal to the sum of the molar conductivities of its individual ions.”
Mathematically:
Λ°m = λ°+ + λ°−
Where:
- Λ°m = Limiting molar conductivity
- λ°+ = Conductivity of cation
- λ°− = Conductivity of anion
This law is mainly applicable to strong electrolytes and helps in calculating limiting molar conductivity of weak electrolytes.
2. Calculation of EMF of the Given Cell
Given Cell:
Al(s) / Al³⁺ (0.15 M) || Cu²⁺ (0.025 M) / Cu(s)
Step 1: Identify Anode and Cathode
Given standard reduction potentials:
E°(Al³⁺/Al) = −1.66 V
E°(Cu²⁺/Cu) = +0.34 V
Higher E° value acts as cathode.
So, Cu²⁺/Cu is cathode and Al/Al³⁺ is anode.
Step 2: Calculate Standard EMF
E°cell = E°cathode − E°anode
E°cell = 0.34 − (−1.66)
E°cell = 2.00 V
Step 3: Apply Nernst Equation
Ecell = E°cell − (0.0591/n) log Q
Balanced reaction:
2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu
Here n = 6
Reaction quotient:
Q = [Al³⁺]² / [Cu²⁺]³
Substitute values:
Q = (0.15)² / (0.025)³
Using given log values, substitute in equation and calculate final EMF.
Thus, actual EMF will be slightly less than 2.00 V due to concentration effect.
---
3. Strongest Oxidising Agent
Given reduction potentials:
Cl₂ + 2e⁻ → 2Cl⁻ E° = +1.36 V
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O E° = +1.51 V
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O E° = +1.33 V
Conclusion
Higher the E° value, stronger the oxidising agent.
Since MnO₄⁻ has highest E° value (+1.51 V),
it is the strongest oxidising agent among the given species.
---
4. Molar Conductivity (Λm)
Definition
Molar conductivity is defined as the conductance of all the ions produced by one mole of an electrolyte when dissolved in a solution.
Unit: S cm² mol⁻¹
Formula:
Λm = κ × 1000 / C
Where:
κ = Conductivity
C = Concentration
5. Nature of Electrolyte from Graph
The graph shows a straight line between Λm and √C.
For strong electrolytes:
Λm = Λ°m − A√C
This is a linear equation.
Since the graph is linear, the electrolyte is a strong electrolyte.
6. Determination of Λ°m
Limiting molar conductivity (Λ°m) is obtained by extrapolating the graph to √C = 0.
The intercept on Y-axis gives Λ°m.
From graph, Λ°m ≈ 150 S cm² mol⁻¹.
7. Determination of Constant A
The equation for strong electrolyte:
Λm = Λ°m − A√C
This is in the form:
y = c − mx
Slope of graph = −A
Thus, value of A can be calculated from slope of straight line:
A = (change in Λm) / (change in √C)
By selecting two points from graph, slope can be calculated.
Conclusion
From the above discussion, we understand that:
- Kohlrausch Law helps calculate limiting molar conductivity.
- EMF of a cell depends on standard electrode potential and concentration of ions.
- Higher standard reduction potential means stronger oxidising agent.
- Linear relation between Λm and √C confirms strong electrolyte behavior.
- Limiting molar conductivity is obtained from graph intercept.
These concepts are extremely important for board examinations and help in understanding electrochemical processes in real life applications like batteries and electroplating.
---
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Nernst equation numericals
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