🔋 Batteries (Electrochemical Cells)

 

🔋 Batteries (Electrochemical Cells)

A battery is a device that converts chemical energy into electrical energy. It consists of one or more electrochemical cells connected in series or parallel.

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🔹 Types of Batteries

1. Primary Batteries

• These cannot be recharged.

• Once the reactants are used up, the battery stops working.

• Example: Dry cell, Mercury cell.

📌 Dry Cell (Leclanché cell)

• Commonly used in torches, radios, clocks.

• Anode: Zinc container.

• Cathode: Carbon rod surrounded by MnO₂.

• Electrolyte: NH₄Cl and ZnCl₂ paste.

• Reactions:

  • Anode: Zn → Zn²⁺ + 2e⁻

  • Cathode: MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃

📌 Mercury Cell

• Used in hearing aids, watches, calculators.

• Anode: Zinc–mercury amalgam.

• Cathode: HgO + carbon.

• Electrolyte: Paste of KOH and ZnO.

• Reactions:

  • Anode: Zn(Hg) + 2OH⁻ → ZnO + H₂O + 2e⁻

  • Cathode: HgO + H₂O + 2e⁻ → Hg + 2OH⁻

• Voltage: ~1.35 V (constant throughout life).

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2. Secondary Batteries

• Rechargeable batteries.

• Reaction can be reversed by applying external current.

• Example: Lead-acid battery, Nickel-Cadmium battery, Lithium-ion battery.

📌 Lead-Acid Storage Battery

• Used in automobiles.

• Anode: Lead (Pb).

• Cathode: Lead dioxide (PbO₂).

• Electrolyte: Dilute H₂SO₄.

• Discharging Reactions:

  • Anode: Pb + SO₄²⁻ → PbSO₄ + 2e⁻

  • Cathode: PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

• Charging reverses these reactions.

📌 Nickel-Cadmium Battery

• Used in portable devices.

• Anode: Cadmium (Cd).

• Cathode: Nickel(III) oxide-hydroxide (NiO(OH)).

• Electrolyte: KOH.

• Reactions (Discharging):

  • Anode: Cd + 2OH⁻ → Cd(OH)₂ + 2e⁻

  • Cathode: NiO(OH) + H₂O + e⁻ → Ni(OH)₂ + OH⁻

📌 Lithium-Ion Battery

• Used in mobiles, laptops, EVs.

• Anode: Graphite with Li⁺.

• Cathode: Metal oxide like LiCoO₂.

• During discharge: Li⁺ moves from anode to cathode.

• Rechargeable and lightweight.

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🔹 Fuel Cells

• Convert chemical energy of fuel directly into electrical energy.

• Continuous supply of fuel and oxidant needed.

• Example: Hydrogen-Oxygen Fuel Cell.

📌 Hydrogen-Oxygen Fuel Cell

• Anode: H₂ gas → 2H⁺ + 2e⁻

• Cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O

• Electrolyte: Aqueous KOH.

• Efficient and environment-friendly (emits only water).

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📘 Key Differences: Primary vs Secondary Cells

Feature	Primary Cell	Secondary Cell
Rechargeable	No	Yes
Usage	Single time	Multiple cycles
Cost	Usually cheaper	Usually costlier
Example	Dry cell, mercury cell	Lead-acid, Li-ion

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⚡ Important Terms

• EMF (Electromotive Force): The voltage developed by any source of electrical energy.

• Electrode Potential: The tendency of an electrode to lose or gain electrons.

• Standard Electrode Potential (E°): Electrode potential under standard conditions (1M, 1 atm, 25°C).

• Cell Potential (E°cell) = E°cathode – E°anode

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🔋 BATTERIES – NCERT Class 12 Electrochemistry (Detailed Notes)

A battery is a combination of two or more electrochemical cells connected in series or parallel to provide a larger amount of electrical energy.

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🔶 Types of Batteries in NCERT

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1. ✅ Primary Cells (Non-rechargeable)

• Irreversible chemical reactions.

• Cannot be reused once discharged.

• Common in remote controls, toys, flashlights.

📌 Dry Cell (Leclanché Cell)

• Widely used portable primary cell.

• Anode: Zinc container (also acts as the outer body).

• Cathode: Carbon rod surrounded by powdered manganese dioxide (MnO₂) and carbon.

• Electrolyte: Paste of NH₄Cl and ZnCl₂.

🔋 Reactions:

• Anode (oxidation):

  $$\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$$

• Cathode (reduction):

  $$2MnO_2 + 2NH_4^+ + 2e^- \rightarrow Mn_2O_3 + 2NH_3 + H_2O$$

• EMF: Approximately 1.5 V.

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📌 Mercury Cell (for hearing aids, watches)

• Constant voltage over its lifetime.

• Used in low current devices.

• Anode: Zinc amalgam (Zn-Hg).

• Cathode: Paste of mercuric oxide (HgO) and carbon.

• Electrolyte: Paste of KOH and ZnO.

🔋 Reactions:

• Anode:

  $$Zn(Hg) + 2OH^- \rightarrow ZnO + H_2O + 2e^-$$

• Cathode:

  $$HgO + H_2O + 2e^- \rightarrow Hg + 2OH^-$$

• EMF: Around 1.35 V (constant).

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2. ✅ Secondary Cells (Rechargeable Batteries)

• Reactions are reversible.

• Can be recharged by passing current in the opposite direction.

• Used in automobiles, inverters, laptops, mobile phones.

📌 Lead Storage Battery

• Used in cars and inverters.

• Anode: Spongy lead (Pb).

• Cathode: Lead dioxide (PbO₂).

• Electrolyte: Dilute sulphuric acid (H₂SO₄).

🔋 Discharge Reactions:

• Anode:

  $$Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^-$$

• Cathode:

  $$PbO_2 + SO_4^{2-} + 4H^+ + 2e^- \rightarrow PbSO_4 + 2H_2O$$

• Overall Discharge Reaction:

  $$Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$$

🔄 Recharging:

• External current is applied to reverse the reactions.

• PbSO₄ is converted back to Pb and PbO₂.

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📌 Nickel-Cadmium Battery (Not elaborated in NCERT but briefly mentioned)

• Rechargeable.

• Used in portable electronics.

• Anode: Cadmium (Cd).

• Cathode: Nickel(III) oxide hydroxide (NiO(OH)).

• Electrolyte: KOH.

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📌 Lithium-ion Battery (Briefly mentioned in NCERT)

• Common in mobile phones, laptops, EVs.

• Anode: Graphite.

• Cathode: Lithium metal oxide (e.g., LiCoO₂).

• Electrolyte: Lithium salt in an organic solvent.

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3. ✅ Fuel Cells

• Convert chemical energy directly into electrical energy.

• Fuel and oxidant are supplied continuously.

• High efficiency and eco-friendly.

📌 Hydrogen-Oxygen Fuel Cell

• Used in spacecrafts.

• Anode Reaction (H₂ oxidation):

  $$H_2 + 2OH^- \rightarrow 2H_2O + 2e^-$$

• Cathode Reaction (O₂ reduction):

  $$O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$$

• Overall Reaction:

  $$2H_2 + O_2 \rightarrow 2H_2O$$

• Produces water as the only by-product.

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🔍 Key Points from NCERT

• A battery’s voltage depends on the electrode potential difference.

• Primary cells are irreversible; secondary cells are reversible.

• Fuel cells are sustainable alternatives producing clean energy.

• Lead-acid batteries are an example of commercial secondary cells.

✅ Important PYQs on Battery (Class 12 Chemistry - Electrochemistry)

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1.

Question: What is the difference between a primary cell and a secondary cell? Give one example of each.
📚 (CBSE 2 Marks)

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2.

Question: Write the electrode reactions occurring in a lead storage battery during charging and discharging.
📚 (CBSE Delhi 2019, 3 Marks)

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3.

Question: Draw a labelled diagram of a dry cell. Write the reactions at the anode and cathode.
📚 (CBSE 2018, 3 Marks)

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4.

Question: Why does the mercury cell give a constant voltage throughout its life?
📚 (CBSE 2017, 1 Mark – Conceptual)

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5.

Question: Mention one advantage and one limitation of a lead storage battery.
📚 (CBSE 2016, 2 Marks)

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6.

Question: A device uses a fuel cell in which hydrogen is used as fuel. Write the overall reaction in the cell. Mention any two advantages of using this type of cell.
📚 (CBSE 2020, 3 Marks)

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7.

Question: How is a lithium-ion battery better than a lead storage battery? Give two points.
📚 (CBSE Sample Paper Question, 2 Marks)

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8.

Question: What are the electrode reactions in a mercury cell? Why is its voltage almost constant?
📚 (CBSE 2015, 3 Marks)

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9.

Question: Explain with reactions how a secondary battery is recharged.
📚 (CBSE 2018, 2 Marks)

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10.

Question: Explain the construction and working of a hydrogen-oxygen fuel cell with the help of reactions.
📚 (CBSE 2020 – HOTS, 3 Marks)

10 important PYQs from Class 12 Chemistry (Battery topic) along with detailed answers, as per the latest NCERT and CBSE board guidelines:

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✅ 1. Difference between primary and secondary cells

Question: What is the difference between a primary cell and a secondary cell? Give one example of each.
Answer:

• Primary Cell: The chemical reaction is irreversible, and the cell cannot be recharged.
  🔹 Example: Dry cell (Leclanché cell)

• Secondary Cell: The chemical reaction is reversible, and the cell can be recharged.
  🔹 Example: Lead storage battery

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✅ 2. Reactions in Lead Storage Battery

Question: Write the electrode reactions occurring in a lead storage battery during charging and discharging.
Answer:

👉 During Discharging:

• Anode (oxidation):

  $$Pb + SO_4^{2-} \rightarrow PbSO_4 + 2e^-$$

• Cathode (reduction):

  $$PbO_2 + SO_4^{2-} + 4H^+ + 2e^- \rightarrow PbSO_4 + 2H_2O$$

• Overall:

  $$Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$$

👉 During Charging (reverse reactions):

• PbSO₄ is converted back to Pb and PbO₂ at respective electrodes.

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✅ 3. Dry Cell Reactions and Diagram

Question: Draw a labelled diagram of a dry cell. Write the reactions at the anode and cathode.
Answer:

• Diagram: (Can be shown in class or as a homework assignment.)

• Anode (Zinc container):

  $$Zn \rightarrow Zn^{2+} + 2e^-$$

• Cathode (MnO₂-carbon mixture):

  $$2MnO_2 + 2NH_4^+ + 2e^- \rightarrow Mn_2O_3 + 2NH_3 + H_2O$$

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✅ 4. Constant Voltage in Mercury Cell

Question: Why does the mercury cell give a constant voltage throughout its life?
Answer:
Because the overall cell reaction involves solids (Zn and HgO), and the concentrations of ions remain nearly constant. Therefore, the EMF does not vary with time.

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✅ 5. Lead Storage Battery: One Advantage & Limitation

Question: Mention one advantage and one limitation of a lead storage battery.
Answer:

• Advantage: Rechargeable and delivers high current.

• Limitation: Heavy and contains toxic lead, which is harmful to the environment.

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✅ 6. Hydrogen-Oxygen Fuel Cell Reaction and Advantages

Question: A device uses a fuel cell in which hydrogen is used as fuel. Write the overall reaction. Mention any two advantages.
Answer:

• Overall Reaction:

  $$2H_2 + O_2 \rightarrow 2H_2O$$

• Advantages:
  🔹 High efficiency
  🔹 Environment-friendly (only water is formed as waste)

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✅ 7. Lithium-ion vs Lead-acid Battery

Question: How is a lithium-ion battery better than a lead storage battery?
Answer:

• Lithium-ion batteries are lighter and more compact.

• They have a longer cycle life and higher energy density compared to lead-acid batteries.

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✅ 8. Mercury Cell Reactions & Constant Voltage

Question: What are the electrode reactions in a mercury cell? Why is its voltage almost constant?
Answer:

• Anode:

  $$Zn(Hg) + 2OH^- \rightarrow ZnO + H_2O + 2e^-$$

• Cathode:

  $$HgO + H_2O + 2e^- \rightarrow Hg + 2OH^-$$

• Reason for Constant Voltage: Since the reaction involves only solids and liquids, the concentration of ions remains nearly constant → constant EMF.

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✅ 9. Recharging Secondary Battery

Question: Explain with reactions how a secondary battery is recharged.
Answer:

• By passing an external current, the discharge reactions are reversed.

• Example for lead storage battery:

  $$2PbSO_4 + 2H_2O \xrightarrow{\text{Electric Current}} Pb + PbO_2 + 2H_2SO_4$$

• This regenerates the original reactants and restores battery capacity.

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✅ 10. Hydrogen-Oxygen Fuel Cell Construction & Reactions

Question: Explain the construction and working of a hydrogen-oxygen fuel cell with reactions.
Answer:

• Electrolyte: Aqueous KOH

• Anode Reaction (H₂):

  $$H_2 + 2OH^- \rightarrow 2H_2O + 2e^-$$

• Cathode Reaction (O₂):

  $$O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$$

• Overall Reaction:

  $$2H_2 + O_2 \rightarrow 2H_2O$$

• Note: Water is the only by-product, making it eco-friendly.