Commercial Cells and Batteries
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Direct answer
Shelves of commercial cells hide three families: primary cells (single discharge — the dry Leclanche cell and the mercury button cell), secondary cells (rechargeable — the lead storage battery and nickel-cadmium), and fuel cells (continuous reactant supply — hydrogen-oxygen). The dry cell pairs a zinc anode with a manganese dioxide-carbon cathode in an NH4Cl-ZnCl2 paste and delivers about 1.5 V. The mercury cell, based on zinc amalgam and HgO, holds a remarkably steady 1.35 V throughout its life because its net reaction involves no dissolved ion whose concentration could change. The lead-acid battery (about 2 V per cell) consumes sulphuric acid as it discharges and regenerates it on charge, and the H2-O2 fuel cell converts fuel energy at roughly 70% efficiency — about double a thermal power station.
What you must remember
- Dry cell (Leclanche): anode Zn → Zn^2+ + 2e^-; cathode MnO2 + NH4+ + e^- → MnO(OH) + NH3; the ammonia is trapped as a zinc-ammine complex so the cell does not pressurise.
- Mercury cell constant voltage: net reaction Zn + HgO → ZnO + Hg involves no solute ions, so no concentration term shifts the potential — the textbook explanation for its flat discharge curve.
- Lead-acid discharge: Pb + SO4^2- → PbSO4 + 2e^- (anode); PbO2 + 4H+ + SO4^2- + 2e^- → PbSO4 + 2H2O (cathode); net Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O.
- State-of-charge test: electrolyte density falls as H2SO4 is consumed — the hydrometer reading of the acid tells how charged the battery is.
- Nickel-cadmium: anode Cd + 2OH^- → Cd(OH)2 + 2e^-; cathode NiO2 + 2H2O + 2e^- → Ni(OH)2 + 2OH^-; net Cd + NiO2 + 2H2O → Cd(OH)2 + Ni(OH)2 — the rechargeable dry rival.
- Fuel cell: H2 + 1/2 O2 → H2O with KOH electrolyte and porous Ni electrodes, about 70% efficient; spacecraft use them because the product, water, is drinkable.
Reasoning through the electrode table
Build each cell as an electrochemistry question. In the discharging lead-acid battery, identify the galvanic direction: lead metal oxidises (anode), PbO2 reduces (cathode), both plates grow PbSO4, and the acid thins — so the cell potential slides from about 2.15 V toward 1.8 V as it empties, which is precisely why a battery tester reads voltage to guess remaining charge.
On charging, the battery becomes an electrolytic cell: current is pushed backwards, the positive PbSO4 plate returns to PbO2 and the negative to spongy Pb, and sulphuric acid concentration climbs again. Electrode naming now bites — anode always means oxidation, so during discharge the Pb plate is the anode but during charge it is the cathode, while the signs of the terminals never change. That asymmetry between electrochemical function and terminal sign is the single most examined idea of the chapter.
Where students slip
Match-the-electrode questions catch students who memorise names without roles: the dry cell's cathode is manganese dioxide (the carbon rod is only a conductor), and the mercury cell's steady voltage is about the absence of solution-phase ions, not about "stable mercury". The second trap is the charging direction: writing discharge equations for a charging battery, or labelling the Pb plate the anode in both modes. Third, students quote fuel-cell efficiency as if it were a thermodynamic limit — it is high because chemical energy converts directly to electricity without a heat cycle, about 70% against roughly 40% for thermal plants. JEE Main usually asks one reaction-identification or one advantage-type question; Advanced asks you to write a missing half-reaction from the net equation.
Frequently asked questions
Why does the mercury cell maintain a constant voltage?
Its overall reaction, Zn + HgO → ZnO + Hg, involves only solids and a liquid with no dissolved ions, so no concentration drifts and the Nernst correction stays zero.
What happens to sulphuric acid during lead-acid discharge?
It is consumed in forming PbSO4 on both plates, so electrolyte density falls; recharging regenerates it, which is how a hydrometer estimates charge.
Which electrode is the anode while a lead battery is being charged?
The PbO2-reforming (positive) plate, because anode means oxidation and PbSO4 there is oxidised — electrode roles swap with direction of current.
Why are fuel cells more efficient than thermal power plants?
They convert chemical energy of fuel directly into electricity, bypassing the heat-to-work limit, reaching about 70% against roughly 40% for combustion cycles.
What is the cathode reaction of the dry Leclanche cell?
MnO2 + NH4+ + e^- → MnO(OH) + NH3, with manganese reduced from +4 to +3; the carbon rod merely collects current.