Chemistry · JEE Advanced only

Batteries (Primary, Secondary), Fuel Cells and Corrosion (JEE Advanced)

Not in the JEE Main syllabus, but it is in JEE Advanced. Study time: about 3-4 hours: 1 hour for battery types and reactions, 1 hour for fuel cells and efficiency concepts, 1-2 hours for corrosion mechanism and prevention methods with practice problems.

Last updated: 26 October 2026

Direct answer

Batteries are electrochemical cells that convert chemical energy into electrical energy.

Batteries are electrochemical cells that convert chemical energy into electrical energy. Primary batteries are non-rechargeable (like dry cells), while secondary batteries can be recharged (like lead-acid batteries). Fuel cells continuously convert fuel and oxidant into electricity, and corrosion is the electrochemical degradation of metals, especially iron rusting.

Official syllabus

What the JEE Advanced syllabus says

“Batteries: Primary and Secondary, fuel cells; Corrosion”

Source: JEE Advanced 2026 Information Bulletin, Annexure-I. Closest JEE Main topic: dry cell and lead accumulator, fuel cells.

JEE Main covers basic dry cells and lead accumulators, but Advanced expects you to understand the detailed electrode reactions, efficiency calculations, and the electrochemistry behind corrosion prevention methods like cathodic protection and sacrificial anodes.

Key ideas

What you must understand

Primary batteries: Dry cell (Leclanché cell) uses Zn anode and MnO₂ cathode with NH₄Cl paste; mercury cell uses Zn-HgO system with constant voltage output
Secondary batteries: Lead-acid battery has Pb anode and PbO₂ cathode in H₂SO₄; during discharge both electrodes convert to PbSO₄
Nickel-cadmium and lithium-ion batteries are secondary cells; Li-ion has high energy density due to light Li atoms
Fuel cells: H₂-O₂ fuel cell produces water with ~70% efficiency; no Carnot limitation since it is not a heat engine
Corrosion is an electrochemical process where Fe acts as anode (oxidizes) and impure regions or water droplets act as cathode (O₂ reduces)
Corrosion prevention: galvanization (Zn coating), cathodic protection (connecting to more active metal), and electroplating

Formulas

Formulas and key facts

Dry cell anode: Zn → Zn²⁺ + 2e⁻
Dry cell cathode: MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃
Lead battery discharge: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
Lead battery charging: reverse of discharge reaction
H₂-O₂ fuel cell: 2H₂ + O₂ → 2H₂O, E°cell ≈ 1.23 V
Corrosion anode: Fe → Fe²⁺ + 2e⁻
Corrosion cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O (acidic) or O₂ + 2H₂O + 4e⁻ → 4OH⁻ (neutral/basic)
Rust formation: 4Fe²⁺ + O₂ + (4+2x)H₂O → 2Fe₂O₃·xH₂O + 8H⁺

Avoid

Common mistakes

Confusing primary and secondary batteries: primary cannot be recharged, secondary can be recharged by reversing the cell reaction
Forgetting that in lead-acid battery, H₂SO₄ concentration decreases during discharge (both electrodes consume SO₄²⁻)
Thinking fuel cells violate thermodynamics: they are not heat engines, so Carnot efficiency limit does not apply
In corrosion, the rusting occurs at the anode (where Fe oxidizes), not at the cathode where O₂ is reduced

Worked example

A JEE Advanced-level example

Question. A lead-acid battery has 6 cells connected in series, each with E°cell = 2.0 V. During discharge, 0.5 mol of Pb is oxidized. Calculate (a) the total EMF of the battery, and (b) the charge (in Faradays) transferred during this discharge.

Show solution

Total EMF = 6 × 2.0 V = 12.0 V
At anode: Pb → Pb²⁺ + 2e⁻, so 1 mol Pb releases 2 mol electrons
For 0.5 mol Pb oxidized: electrons released = 0.5 × 2 = 1.0 mol
1 mol electrons = 1 Faraday of charge
Therefore, charge transferred = 1.0 Faraday

Answer: (a) Total EMF = 12.0 V; (b) Charge transferred = 1.0 Faraday (or 96500 C)

Solved twice independently; both solutions agree.

FAQs

Frequently asked questions

Is Batteries (Primary, Secondary), Fuel Cells and Corrosion in the JEE Main syllabus?

No. It is in the JEE Advanced 2026 syllabus only. The closest JEE Main topic is dry cell and lead accumulator, fuel cells.

How long does Batteries (Primary, Secondary), Fuel Cells and Corrosion take to prepare for JEE Advanced?

About 3-4 hours: 1 hour for battery types and reactions, 1 hour for fuel cells and efficiency concepts, 1-2 hours for corrosion mechanism and prevention methods with practice problems, if you already know the related JEE Main topic.