medium difficulty10 min

Batteries, Corrosion, and Fuel Cells

Batteries and fuel cells are electrochemical devices that supply electrical energy through redox reactions, while corrosion is an unwanted electrochemical oxidation process of metals.

In one line: Battery stores reactants; fuel cell is fed reactants; corrosion wastes metal through tiny cells.

Practice This Concept

Understand it

Primary batteries are not conveniently rechargeable, while secondary batteries can be recharged by reversing the cell reaction.
The Leclanche cell and mercury cell are common primary examples.
Lead storage and nickel-cadmium cells are secondary examples.
A hydrogen-oxygen fuel cell continuously converts the chemical energy of hydrogen and oxygen into electrical energy as long as reactants are supplied.
Corrosion, such as rusting of iron, occurs through small electrochemical cells formed on the metal surface in the presence of oxygen and moisture.

Example

In a hydrogen-oxygen fuel cell, hydrogen is oxidised and oxygen is reduced, producing water and electrical energy. In rusting, iron is oxidised to iron ions, which ultimately form hydrated iron oxide.

Rule to remember

Fuel cell pattern: fuel is oxidised at anode and oxidant is reduced at cathode.

Corrosion pattern for iron: anodic region Fe → Fe²⁺ + 2e⁻; cathodic oxygen reduction occurs in moist conditions, leading eventually to hydrated iron oxide.

Battery classification rule: primary cells are single-use in normal operation; secondary cells are rechargeable.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Classify lead storage battery and H2-O2 fuel cell.

1A lead storage battery is a secondary battery because it can be recharged by reversing the reaction using external current.
2A hydrogen-oxygen fuel cell is not simply a stored-charge battery; it requires continuous supply of hydrogen and oxygen and forms water as product.

Method you can reuse

1Identify whether the system stores reactants inside the cell or receives reactants continuously.
2Decide whether the reaction is reversible for recharging.
3For corrosion, locate anodic metal dissolution and cathodic oxygen reduction.
4Add prevention logic such as barrier coating, galvanisation, or cathodic protection when asked.

Visual tip

A comparison chart is more useful than a full apparatus diagram: primary cell, secondary cell, fuel cell, and corrosion should be compared by reactants, rechargeability, electrode process, and product.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why is a fuel cell different from an ordinary battery?

Think first, then show answer

A fuel cell keeps producing electricity as long as fuel and oxidant are supplied, while an ordinary battery contains a fixed amount of reactants.

Common wrong answer

A common wrong answer is calling a fuel cell rechargeable in the same sense as a secondary battery, instead of stating that it works with continuous external reactant supply.

Don't mix this up

Students often describe rusting as only a direct reaction of iron with oxygen, ignoring the electrochemical role of moisture, anodic and cathodic regions, and electron transfer.

Exam tip

For application questions, mention the redox role first, then the device condition: rechargeable for secondary cells, continuous reactant supply for fuel cells, and moisture plus oxygen for corrosion.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

CBSE questions often ask differences between primary and secondary batteries, reactions in lead storage battery or fuel cell, advantages of fuel cells, conditions for corrosion, and methods to prevent rusting.

1
Enough for 1 mark
Batteries and fuel cells are electrochemical devices that supply electrical energy through redox reactions, while corrosion is an unwanted electrochemical oxidation process of metals.
2
Add for 2 marks
Example: In a hydrogen-oxygen fuel cell, hydrogen is oxidised and oxygen is reduced, producing water and electrical energy. In rusting, iron is oxidised to iron ions, which ultimately form hydrated iron oxide.
3
Add for 3 marks
Primary batteries are not conveniently rechargeable, while secondary batteries can be recharged by reversing the cell reaction.

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