MHT-CET Chemistry · Electrochemistry

Batteries, Primary, Secondary and Fuel Cells

A primary cell (dry cell, mercury cell) is used once; a secondary cell (lead accumulator, nickel–cadmium) is recharged by running its reaction backwards as an electrolysis; a fuel cell is fed its reactants continuously.

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Why this matters

9 PYQs, none HARD — one short recall page. The paper asks what happens at the positive electrode of a dry cell (MnO₂ reduced to Mn₂O₃), what is consumed or produced when a lead accumulator discharges or recharges, and which statement about the H₂–O₂ fuel cell is wrong. Know the four half-reactions and the page is done.

Concept 1 of 2

Dry Cell, Lead Accumulator and Nickel–Cadmium Cell

Intuition

Every battery is a galvanic cell with the anode metal named first. Dry cell: zinc case oxidised, MnO₂ reduced. Lead accumulator: lead oxidised and PbO₂ reduced on discharge, both to PbSO₄ while sulphuric acid is consumed — and recharging runs both electrodes backwards.

Definition

Dry cell (Leclanché), primary, 1.5 V. Anode $\text{Zn} \to \text{Zn}^{2+} + 2e^-$ . Cathode (positive, a graphite rod in MnO₂ + NH₄Cl paste): $2\text{MnO}_2 + 2\text{NH}_4^+ + 2e^- \to \text{Mn}_2\text{O}_3 + 2\text{NH}_3 + \text{H}_2\text{O}$ — MnO₂ is reduced to Mn₂O₃. NH₄⁺ reduction also gives some H₂. n = 2, so $E^\circ = -\Delta G^\circ/2F$ .
Mercury (button) cell, primary: Zn–Hg amalgam anode, HgO cathode, KOH paste; steady 1.35 V.
Lead accumulator, secondary, 2 V per cell, 38% H₂SO₄. Discharge — anode $\text{Pb} + \text{SO}_4^{2-} \to \text{PbSO}_4 + 2e^-$ ; cathode $\text{PbO}_2 + 4\text{H}^+ + \text{SO}_4^{2-} + 2e^- \to \text{PbSO}_4 + 2\text{H}_2\text{O}$ ; net $\text{Pb} + \text{PbO}_2 + 2\text{H}_2\text{SO}_4 \to 2\text{PbSO}_4 + 2\text{H}_2\text{O}$ : H₂SO₄ CONSUMED, PbSO₄ PRODUCED, density of the acid falls.
Recharge reverses it: at the positive plate PbSO₄ is OXIDISED to PbO₂; at the negative plate PbSO₄ is reduced to Pb.
Nickel–cadmium, secondary: Cd anode, NiO(OH) cathode, KOH; longer life than lead–acid.

Cell	Type	Anode (−)	Cathode (+)	Note
Dry cell	Primary	Zn → Zn²⁺ + 2e⁻	MnO₂ → Mn₂O₃ (reduced); NH₄⁺ → NH₃ + H₂	1.5 V; n = 2
Mercury cell	Primary	Zn(Hg) → Zn²⁺	HgO → Hg	1.35 V, steady PRIMARY — the 2022 paper's key called only the dry cell primary; the textbook counts the mercury cell too.
Lead accumulator (discharge)	Secondary	Pb → PbSO₄	PbO₂ → PbSO₄	H₂SO₄ consumed, water formed
Lead accumulator (recharge)	Secondary	PbSO₄ → Pb (reduced)	PbSO₄ → PbO₂ (oxidised)	Electrolysis: an external source drives it On recharge the POSITIVE plate is oxidised — the reverse of normal cathode behaviour.
Ni–Cd	Secondary	Cd → Cd(OH)₂	NiO(OH) → Ni(OH)₂	KOH electrolyte

Discharge is galvanic (spontaneous); recharge is electrolytic (driven).

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1ElectrochemistryMODERATE

Which of the following reactions occurs at cathode during discharging of lead accumulator?

[Q70 · 4th May Shift 1 · 2023]

Swapping discharge and recharge at the positive plate

On DISCHARGE the positive plate reduces PbO₂ → PbSO₄. On RECHARGE the same plate oxidises PbSO₄ → PbO₂. The paper asks for one and offers the other.

Drill 6 more on dry cell, lead accumulator and nickel–cadmium cell

Concept 2 of 2

The Hydrogen–Oxygen Fuel Cell

Intuition

A fuel cell is a galvanic cell that never runs down because hydrogen (the fuel and reducing agent) and oxygen (the oxidising agent) are fed in continuously. The electrodes are porous carbon impregnated with a platinum or palladium catalyst, sitting in hot aqueous KOH; the only product is water.

Definition

Anode: $2\text{H}_2 + 4\text{OH}^- \to 4\text{H}_2\text{O} + 4e^-$ — H₂ is oxidised, so H₂ is the REDUCING agent.
Cathode: $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \to 4\text{OH}^-$ — O₂ is the oxidising agent.
Net: $2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$ ; $E^\circ = 1.23$ V; efficiency ~70%, pollution-free; used in spacecraft.
Electrodes are POROUS CARBON (graphite) with Pt/Pd catalyst — 'platinum wires' is the planted false statement. Electrolyte: hot concentrated KOH (not NaOH as a reagent).

Fuel cell net reaction

2\text{H}_2(g) + \text{O}_2(g) \to 2\text{H}_2\text{O}(l),\qquad E^\circ = 1.23\ \text{V}

Worked example

Identify the oxidising agent, the reducing agent and the electrolyte of the H₂–O₂ fuel cell.

Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2ElectrochemistryMODERATE

Which of the following statements is NOT correct for

\text{H}_2\text{-O}_2

fuel cell?

[Q98 · 3rd May 2nd Shift · 2023]

Calling H₂ the oxidising agent because it 'burns'

Burning IS oxidation — of the hydrogen. The species that is oxidised is the reducing agent. H₂ reduces; O₂ oxidises.

Drill 1 more on the hydrogen–oxygen fuel cell

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The Hydrogen–Oxygen Fuel Cell
Fuel cell net reaction
$2\text{H}_2(g) + \text{O}_2(g) \to 2\text{H}_2\text{O}(l),\qquad E^\circ = 1.23\ \text{V}$

Reference tables (1)

Dry Cell, Lead Accumulator and Nickel–Cadmium Cell5 rows

Cell	Type	Anode (−)	Cathode (+)	Note
Dry cell	Primary	Zn → Zn²⁺ + 2e⁻	MnO₂ → Mn₂O₃ (reduced); NH₄⁺ → NH₃ + H₂	1.5 V; n = 2
Mercury cell	Primary	Zn(Hg) → Zn²⁺	HgO → Hg	1.35 V, steady PRIMARY — the 2022 paper's key called only the dry cell primary; the textbook counts the mercury cell too.
Lead accumulator (discharge)	Secondary	Pb → PbSO₄	PbO₂ → PbSO₄	H₂SO₄ consumed, water formed
Lead accumulator (recharge)	Secondary	PbSO₄ → Pb (reduced)	PbSO₄ → PbO₂ (oxidised)	Electrolysis: an external source drives it On recharge the POSITIVE plate is oxidised — the reverse of normal cathode behaviour.
Ni–Cd	Secondary	Cd → Cd(OH)₂	NiO(OH) → Ni(OH)₂	KOH electrolyte

Discharge is galvanic (spontaneous); recharge is electrolytic (driven).

Watch out for (2)

Swapping discharge and recharge at the positive plate
→ Dry Cell, Lead Accumulator and Nickel–Cadmium Cell
Calling H₂ the oxidising agent because it 'burns'
→ The Hydrogen–Oxygen Fuel Cell

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