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

16 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

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).

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 4th May Shift 1 · Q70Moderate

Example 1 · Electrochemistry · Batteries, Primary, Secondary and Fuel Cells

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

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

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.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 3rd May Shift 1 · Q98Moderate

Example 2 · Electrochemistry · Batteries, Primary, Secondary and Fuel Cells

Which of the following statements is NOT correct for

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

fuel cell?

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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Related notes

Electrolysis — what recharging actually is →