10 min readConcept sheet2 or 3-mark questions on the electrode reactions of the dry cell, mercury cell, lead storage battery or nickel-cadmium cell; 1-mark MCQ; 4-mark case-based question

Batteries: primary and secondary

A primary battery cannot be recharged: dry cell, mercury cell. A secondary battery can, because its reaction reverses: lead storage battery, nickel-cadmium cell.

Dry cell (Leclanché cell)Zinc container anode; carbon rod cathode surrounded by MnO₂ and carbon; moist NH₄Cl and ZnCl₂ paste; about 1.5 V

Mercury cellZinc-mercury amalgam anode; HgO and carbon paste cathode; KOH and ZnO paste electrolyte; steady 1.35 V

Lead storage batteryLead anode; lead grid packed with PbO₂ as cathode; 38% sulphuric acid as electrolyte; rechargeable

Nickel-cadmium cellRechargeable; longer life than the lead storage cell, but costlier to make

Primary vs secondary batteries

Primary batteryThe reaction happens only once. When it is used up, the battery is dead and cannot be recharged. Examples: dry cell (torches, clocks, remotes) and mercury cell (hearing aids, watches).

Secondary batteryAfter use it can be recharged by passing current through it in the opposite direction, which reverses the cell reaction. Examples: lead storage battery (cars, inverters) and the nickel-cadmium cell.

Dry cell (Leclanché cell)

A zinc container is the anode. A carbon (graphite) rod, surrounded by powdered MnO₂ and carbon, is the cathode. The space between them is filled with a moist paste of NH₄Cl and ZnCl₂.

1
Anode: Zn(s) → Zn²⁺(aq) + 2e⁻
2
Cathode: MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃
Manganese is reduced from +4 to +3.
3
The NH₃ formed joins with Zn²⁺ to give the complex ion [Zn(NH₃)₄]²⁺.
4
The cell gives about 1.5 V.

Mercury cell

Used in devices that need a small, steady current, such as hearing aids and watches.

1
Anode: Zn(Hg) + 2OH⁻ → ZnO(s) + H₂O + 2e⁻
2
Cathode: HgO + H₂O + 2e⁻ → Hg(l) + 2OH⁻
3
Overall: Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)
4★
No ion in solution changes its concentration in the overall reaction, so the potential stays constant at about 1.35 V for the whole life of the cell.

★ key step

Lead storage battery

Anode: lead. Cathode: a grid of lead packed with lead dioxide (PbO₂). Electrolyte: a 38% solution of sulphuric acid.

1
Discharge, anode: Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻
2
Discharge, cathode: PbO₂(s) + SO₄²⁻(aq) + 4H⁺(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)
3★
Overall discharge: Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)
4
Charging (current reversed): 2PbSO₄(s) + 2H₂O(l) → Pb(s) + PbO₂(s) + 2H₂SO₄(aq)
5
Sulphuric acid is used up on discharge, so the density of the acid falls. Its density shows how charged the battery is.

★ key step

Nickel-cadmium cell

1
Anode: cadmium. Cathode: nickel hydroxide, written Ni(OH)₃ in your textbook. The plates are separated by a layer soaked in moist NaOH or KOH, an alkaline electrolyte.
2
Overall discharge: Cd(s) + 2Ni(OH)₃(s) → CdO(s) + 2Ni(OH)₂(s) + H₂O(l)
3
It is rechargeable and lasts longer than the lead storage cell, but costs more to make.

How to draw the dry cell

Draw a tall rectangle as the zinc container, labelled anode (−).
Just inside its walls, shade a layer labelled moist paste of NH₄Cl and ZnCl₂.
Inside that, shade a region labelled MnO₂ + carbon powder.
Down the centre, draw a graphite (carbon) rod with a metal cap on top, labelled cathode (+).
Draw a seal across the top and label every part.

In real life

The inverter battery that keeps fans running in a power cut, and the battery of a car or e-rickshaw, are often lead storage batteries. Old ones must go to authorised recyclers, because lead harms the nervous system.

Exam angle

2 marks: “Distinguish between a primary and a secondary battery, with one example of each.” Primary: the reaction happens once and the battery cannot be recharged (dry cell). Secondary: it can be recharged by passing current the opposite way, reversing the reaction (lead storage battery).

3 marks: “Write the reactions at the anode and the cathode of a lead storage battery during discharge, and the overall reaction during charging.” Anode: Pb + SO₄²⁻ → PbSO₄ + 2e⁻. Cathode: PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O. Charging: 2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄.

2 marks: “Why does a mercury cell give a constant voltage throughout its life?” Its overall reaction, Zn(Hg) + HgO → ZnO + Hg, has no ion in solution whose concentration changes, so the cell potential stays at about 1.35 V.

Remember it

Primary: used once, then thrown away. Secondary: a second life by recharging. Lead battery: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O.