Class 12 Chemistry Chapter 3 Notes: Electrochemistry | Important Questions & PYQs

Class 12 Chemistry – Chapter 3: Electrochemistry (NCERT Notes)

1. Electrochemical Cells

Electrochemical cells are devices that convert chemical energy into electrical energy using redox reactions.

• Oxidation → loss of electrons

• Reduction → gain of electrons

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2. Galvanic (Voltaic) Cells

A galvanic cell consists of:

• Anode → Oxidation (negative electrode)

• Cathode → Reduction (positive electrode)

Key Points

• Electrons flow from anode → cathode

• Salt bridge maintains electrical neutrality

• EMF of cell:

$$E_{\text{cell}}^{\circ }=E_{\text{cathode}}^{\circ }-E_{\text{anode}}^{\circ }$$

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3. Standard Electrode Potential (E°)

• Measured with respect to Standard Hydrogen Electrode (SHE)

• Higher E° → stronger oxidising agent

• More negative E° → stronger reducing agent

Applications

• Predict feasibility of reactions

• Arrange metals by reactivity

• Calculate EMF, ΔG°, equilibrium constant

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4. Nernst Equation

Used to calculate electrode potential at non-standard conditions.

$$E=E^{\circ }-\frac{0.0591}{n}\log Q(at298K)$$

Where:

• n = number of electrons transferred

• Q = reaction quotient

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5. Gibbs Free Energy & Equilibrium Constant

$$\mathrm{\Delta }{G}^{\circ }=-nF{E}^{\circ }$$$$\mathrm{\Delta }{G}^{\circ }=-RT\ln K$$

• E° > 0 → reaction feasible

• Larger E° → larger equilibrium constant

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6. Conductance of Electrolytic Solutions

Conductivity (κ)

• Conductance of solution of 1 cm length & 1 cm² area

• Unit: S cm⁻¹

Molar Conductivity (Λm)

$${\mathrm{\Lambda }}_m=\frac{\kappa \times 1000}{C}$$

Variation with Concentration

• Conductivity (κ) ↓ on dilution

• Molar conductivity (Λm) ↑ on dilution

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7. Electrolytic Cells & Electrolysis

Used to drive non-spontaneous reactions using electricity.

Faraday’s Laws

1. Mass deposited ∝ quantity of electricity

2. For same charge, masses deposited ∝ equivalent weights

$$Q=It\text{and}Q=nF$$

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8. Batteries

Lead Storage Battery

• Anode: Pb

• Cathode: PbO₂

• Electrolyte: H₂SO₄

On discharge

• PbSO₄ formed on both electrodes

On charging

• Reverse reaction occurs

• Battery is rechargeable

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9. Fuel Cells

Convert fuel directly into electrical energy.

Hydrogen–Oxygen Fuel Cell

• Fuel: H₂

• Oxidant: O₂

• Product: H₂O

Advantages

✔ High efficiency
✔ Eco-friendly
✔ Continuous supply of electricity

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10. Corrosion

Corrosion is slow destruction of metals by chemical/electrochemical reactions.

Rusting of Iron

• Acts as an electrochemical cell

• Anode: Fe → Fe²⁺ + 2e⁻

• Cathode: O₂ + H₂O + e⁻ → OH⁻

Final product → hydrated ferric oxide (rust)

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11. Electrochemical Series – Uses

• Predict displacement reactions

• Decide extraction method

• Compare reducing & oxidising power

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12. Important Exam Formulas

• $E_{\text{cell}}^{\circ }=E_{\text{cathode}}^{\circ }-E_{\text{anode}}^{\circ }$

• $E=E^{\circ }-\frac{0.0591}{n}\log Q$

• $\mathrm{\Delta }{G}^{\circ }=-nF{E}^{\circ }$

• ${\mathrm{\Lambda }}_m=\frac{\kappa \times 1000}{C}$

• $Q=It$

• $1F=96500C$

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13. Key Exam Tips

✔ Galvanic cell → spontaneous
✔ Electrolytic cell → non-spontaneous
✔ Anode always oxidation
✔ More negative E° → more reactive metal
✔ Molar conductivity graph important

✍️ Top 10 Short Question–Answers (Board Exam Oriented)

1. What is an electrochemical cell?
   A device that converts chemical energy into electrical energy using redox reactions.

2. Define oxidation and reduction.
   Oxidation is loss of electrons, while reduction is gain of electrons.

3. What is a galvanic cell?
   A cell in which a spontaneous redox reaction produces electrical energy.

4. Which reactions occur at anode and cathode in a galvanic cell?
   Oxidation at anode and reduction at cathode.

5. In which direction do electrons flow in a galvanic cell?
   From anode to cathode through the external circuit.

6. What is the role of a salt bridge?
   It maintains electrical neutrality by allowing ion migration.

7. What is standard electrode potential (E°)?
   Electrode potential measured with respect to standard hydrogen electrode.

8. Write the formula for EMF of a cell.
   E°cell = E°cathode − E°anode

9. What happens to molar conductivity on dilution?
   It increases with dilution.

10. Write the value of one Faraday.
    1 F = 96500 C

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📝 Long Answer Questions

1. Explain galvanic (voltaic) cell with its working.

A galvanic cell converts chemical energy into electrical energy through a spontaneous redox reaction.

• Anode: Oxidation takes place (negative electrode)

• Cathode: Reduction takes place (positive electrode)

• Electron flow: From anode to cathode

• Salt bridge: Maintains electrical neutrality

• EMF of cell:
  E°cell = E°cathode − E°anode

Thus, electrical energy is produced due to flow of electrons.

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2. Explain standard electrode potential and its applications.

Standard electrode potential (E°):

• Measured relative to standard hydrogen electrode (SHE)

• Indicates tendency of electrode to lose or gain electrons

Applications:

• Predict feasibility of a reaction

• Calculate EMF of electrochemical cell

• Determine ΔG° and equilibrium constant

• Arrange metals according to reactivity

Higher E° → stronger oxidising agent
More negative E° → stronger reducing agent

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3. Explain conductance of electrolytic solutions.

Conductivity (κ):

• Conductance of solution of 1 cm length and 1 cm² area

• Unit: S cm⁻¹

Molar Conductivity (Λm):

• Conductivity of all ions produced by one mole of electrolyte

• Formula:
  Λm = κ × 1000 / C

Effect of dilution:

• Conductivity decreases

• Molar conductivity increases

📝 PYQs (Previous Years’ Questions – CBSE Oriented)

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🔹 1 Mark PYQs

1. What is meant by oxidation?

2. Write the direction of electron flow in a galvanic cell.

3. What is the unit of molar conductivity?

4. Define standard electrode potential.

5. Write the value of one Faraday.

6. Which electrode is negative in a galvanic cell?

7. Write the formula for EMF of a cell.

8. What is the role of a salt bridge?

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🔹 2 Mark PYQs

1. Differentiate between galvanic cell and electrolytic cell (any two points).

2. Why is standard hydrogen electrode used as reference electrode?

3. What happens to conductivity and molar conductivity on dilution?

4. State Faraday’s first law of electrolysis.

5. Why is corrosion considered an electrochemical phenomenon?

6. What is electrochemical series? Write one use.

7. Write the expression for Gibbs free energy in terms of EMF.

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🔹 3 Mark PYQs

1. Write Nernst equation and explain the terms involved.

2. Explain variation of molar conductivity with concentration for strong electrolytes.

3. Describe construction and working of a galvanic cell.

4. State and explain Faraday’s laws of electrolysis.

5. Explain rusting of iron as an electrochemical process.

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🔹 5 Mark PYQs

1. (a) Explain galvanic cell with diagram.
   (b) Write expression for EMF of the cell.

2. (a) What is standard electrode potential?
   (b) How is it used to predict feasibility of a reaction?

3. (a) Derive relation between ΔG° and E°.
   (b) What does positive E° indicate?

4. (a) Explain lead storage battery.
   (b) Write electrode reactions during charging and discharging.

5. (a) What is corrosion?
   (b) Explain rusting of iron with reactions.

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🔢 Numerical PYQs (Very Important)

1. Calculate EMF of a cell using given standard electrode potentials.

2. Calculate electrode potential at non-standard conditions using Nernst equation.

3. Calculate ΔG° for a cell reaction using E° value.

4. Find mass of substance deposited using Faraday’s law.

5. Calculate molar conductivity at given concentration.

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🔑 Most Repeated PYQ Areas (Exam Favourite)

✔ Galvanic vs Electrolytic cell
✔ Nernst equation numericals
✔ ΔG° – E° – K relation
✔ Faraday’s laws (numericals)
✔ Lead storage battery
✔ Corrosion (rusting of iron)
✔ Molar conductivity vs dilution graph

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❓ FAQs (Concept Clarity)

1. Why is anode negative in a galvanic cell?
   Because oxidation occurs and electrons are released.

2. Why is galvanic cell called spontaneous cell?
   Because reaction occurs on its own without external energy.

3. Why does molar conductivity increase on dilution?
   Due to increased ionic mobility.

4. What does E° > 0 indicate?
   The reaction is feasible and spontaneous.

5. Why is corrosion an electrochemical process?
   Because oxidation and reduction occur at different sites on metal surface.

Chapter No.	Chapter Name	Visit
1	The Solid State	Visit
2	Solutions	Visit
3	Electrochemistry	Visit
4	Chemical Kinetics	Visit
5	Surface Chemistry	Visit
6	General Principles and Processes of Isolation of Elements	Visit
7	The p-Block Elements	Visit
8	The d and f Block Elements	Visit
9	Coordination Compounds	Visit
10	Haloalkanes and Haloarenes	Visit
11	Alcohols, Phenols and Ethers	Visit
12	Aldehydes, Ketones and Carboxylic Acids	Visit
13	Amines	Visit
14	Biomolecules	Visit
15	Polymers	Visit
16	Chemistry in Everyday Life	Visit

🧪 NCERT Class 12 Chemistry Notes (Updated)
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