Batteries: Describe the chemical reactions occurring during the discharge of a Lead storage battery

Chapter: Electrochemistry – Batteries

Topic: Lead Storage Battery – Chemical Reactions During Discharge

A lead storage battery, also called a lead-acid battery, is a rechargeable secondary cell. It is commonly used in automobiles and backup power systems.

It consists of:

• Anode (negative plate): Lead, Pb
• Cathode (positive plate): Lead dioxide, PbO₂
• Electrolyte: Sulphuric acid, H₂SO₄

Electrolyte = H₂SO₄(aq)

During discharge, the battery converts chemical energy into electrical energy.

Negative Plate: Pb

Lead undergoes oxidation and reacts with sulphate ions from the electrolyte.

Pb + SO₄²⁻ → PbSO₄ + 2e⁻

Thus, lead is converted into lead sulphate and electrons are released.

Oxidation occurs at the negative electrode.

Positive Plate: PbO₂

Lead dioxide is reduced in the presence of sulphuric acid and forms lead sulphate and water.

PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Thus, PbO₂ is converted into PbSO₄.

Reduction occurs at the positive electrode.

Combining the Two Half-Reactions

Negative electrode:

Pb + SO₄²⁻ → PbSO₄ + 2e⁻

Positive electrode:

PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Adding the two equations:

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

Overall discharge reaction:

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

Pb → PbSO_4>

⬇️

PbO₂ → PbSO₄

⬇️

H₂SO₄ is consumed

⬇️

Water concentration increases

As discharge continues, both plates gradually become coated with PbSO₄.

At the same time, sulphuric acid is consumed, so the concentration and specific gravity of the electrolyte decrease.

Therefore:
During discharge, Pb and PbO₂ are converted into PbSO₄ and the H₂SO₄ concentration decreases.

At the negative electrode, lead undergoes oxidation and releases electrons:

Pb → PbSO₄ + 2e⁻

These electrons travel through the external circuit from the negative plate to the positive plate.

Pb
Negative → e⁻ → PbO₂
Positive

Discharge	Charging
Chemical energy → Electrical energy	Electrical energy → Chemical energy
Pb and PbO₂ → PbSO₄	PbSO₄ → Pb and PbO₂
H₂SO₄ is consumed	H₂SO₄ is regenerated
Spontaneous reaction	Non-spontaneous reaction driven by external electricity

Q. Write the overall reaction occurring during the discharge of a lead storage battery.

Answer:

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

During discharge, sulphuric acid is consumed and both electrodes form lead sulphate.

Q. Write the electrode reactions occurring during discharge of a lead storage battery.

At negative electrode (oxidation):

Pb + SO₄²⁻ → PbSO₄ + 2e⁻

At positive electrode (reduction):

PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Q. Explain the working of a lead storage battery during discharge.

1. Pb acts as the negative electrode.
2. PbO₂ acts as the positive electrode.
3. H₂SO₄ acts as the electrolyte.
4. At the negative electrode, Pb is oxidised to PbSO₄.
5. At the positive electrode, PbO₂ is reduced to PbSO₄.
6. Electrons flow through the external circuit from Pb to PbO₂.
7. H₂SO₄ is consumed during the overall reaction.

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

Q. Describe the chemical reactions occurring during the discharge of a lead storage battery.

Negative Electrode

Lead undergoes oxidation:

Pb + SO₄²⁻ → PbSO₄ + 2e⁻

Positive Electrode

Lead dioxide undergoes reduction:

PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Overall Reaction

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

Thus, both electrodes are converted into PbSO₄ while sulphuric acid is consumed and water is produced.

Q. Describe in detail the chemical reactions occurring during the discharge of a lead storage battery. Explain the changes taking place at both electrodes and in the electrolyte.

Construction

• Negative electrode: Pb
• Positive electrode: PbO₂
• Electrolyte: H₂SO₄

1. Reaction at Negative Electrode

The Pb electrode undergoes oxidation and releases two electrons.

Pb + SO₄²⁻ → PbSO₄ + 2e⁻

Therefore, Pb is converted into PbSO₄.

2. Reaction at Positive Electrode

PbO₂ undergoes reduction by accepting electrons:

PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Therefore, PbO₂ is also converted into PbSO₄.

3. Overall Reaction

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

4. Changes During Discharge

• Both Pb and PbO₂ electrodes are converted into PbSO₄.
• H₂SO₄ is consumed.
• Water is formed.
• The concentration of H₂SO₄ decreases.
• The specific gravity of the electrolyte decreases.
• Electrons flow through the external circuit from Pb to PbO₂.

Final Answer:

During discharge, the lead storage battery converts chemical energy into electrical energy. Pb at the negative electrode is oxidised to PbSO₄, while PbO₂ at the positive electrode is reduced to PbSO₄. Sulphuric acid is consumed and water is formed.

1. The negative electrode of a lead storage battery is made of:

A) Cu

B) Pb

C) Zn

D) Ag

✅ Answer

B) Pb

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2. The positive electrode consists of:

A) Pb

B) Zn

C) PbO₂

D) CuO

✅ Answer

C) PbO₂

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3. The electrolyte used is:

A) HCl

B) NaOH

C) H₂SO₄

D) KOH

✅ Answer

C) H₂SO₄

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4. A lead storage battery is a:

A) Primary cell

B) Secondary cell

C) Fuel cell only

D) Dry cell only

✅ Answer

B) Secondary cell

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5. During discharge, Pb undergoes:

A) Reduction

B) Oxidation

C) Neutralisation

D) Hydrolysis

✅ Answer

B) Oxidation

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6. During discharge, PbO₂ undergoes:

A) Oxidation

B) Reduction

C) Sublimation

D) Evaporation

✅ Answer

B) Reduction

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7. The product formed at both electrodes during discharge is:

A) PbO

B) PbSO₄

C) PbCl₂

D) Pb(NO₃)₂

✅ Answer

B) PbSO₄

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8. During discharge, sulphuric acid is:

A) Consumed

B) Produced

C) Unchanged

D) Converted to HCl

✅ Answer

A) Consumed

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9. The overall reaction is:

A) Pb + PbO₂ → PbO

B) Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

C) PbSO₄ → PbO₂

D) Pb + HCl → PbCl₂

✅ Answer

B

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10. Electrons flow during discharge from:

A) PbO₂ to Pb

B) Pb to PbO₂ through the external circuit

C) Electrolyte to Pb

D) Cathode to electrolyte

✅ Answer

B

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11. Oxidation takes place at the:

A) Negative electrode during discharge

B) Positive electrode only

C) Electrolyte

D) Salt bridge

✅ Answer

A

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12. Reduction occurs at the:

A) Negative electrode

B) Positive electrode during discharge

C) Electrolyte

D) Separator only

✅ Answer

B

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13. The concentration of H₂SO₄ during discharge:

A) Increases

B) Decreases

C) Becomes zero immediately

D) Remains exactly constant

✅ Answer

B

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14. The specific gravity of the electrolyte during discharge:

A) Increases

B) Decreases

C) Is unchanged

D) Becomes infinite

✅ Answer

B

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15. A lead storage battery is commonly used in:

A) Automobile starting systems

B) Thermometers

C) Glass manufacture only

D) Water purification only

✅ Answer

A

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16. During discharge, chemical energy is converted into:

A) Nuclear energy

B) Electrical energy

C) Sound energy only

D) Light energy only

✅ Answer

B

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17. The negative electrode reaction releases:

A) 1 electron

B) 2 electrons

C) 3 electrons

D) 4 electrons

✅ Answer

B) 2 electrons

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18. Water is formed at the:

A) Negative electrode

B) Positive electrode

C) Salt bridge

D) External wire

✅ Answer

B) Positive electrode

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19. During charging, the discharge reaction is:

A) Reversed

B) Stopped permanently

C) Converted into combustion

D) Unrelated

✅ Answer

A) Reversed

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20. Which statement is correct?

A) Both plates form PbSO₄ during discharge

B) Only Pb forms PbSO₄

C) Only PbO₂ forms PbSO₄

D) No sulphate participates

✅ Answer

A

🎯 Quick Revision

Negative electrode: Pb + SO₄²⁻ → PbSO₄ + 2e⁻
Positive electrode: PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O
Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
During discharge: H₂SO₄ ↓   |   PbSO₄ ↑   |   Chemical energy → Electrical energy