Primary & Secondary Cells
Chemistry driving a current, and recharging it.
Lesson at a glanceShow the key points
- Two different metals in an electrolyte give a net emf.
- Primary cells only discharge; secondary cells can be recharged.
- Daniell ≈ 1.09 V, ~1 Ω; Leclanché ≈ 1.5 V (slow depolarizer); dry cell ~0.1 Ω.
- Lead accumulator: PbO₂ (+), Pb (−), H₂SO₄; 2.05 V charged, 1.8 V discharged; acid density tracks charge.
- Capacity in ampere-hours.
Electrolysis uses a current to drive chemistry. A cell does the reverse: chemistry drives a current.
How a cell makes an emf
Dip a metal into an electrolyte and ions tend to move between them, setting up an emf between metal and liquid. Two different metals in the electrolyte produce different emfs, leaving a net emf between them: a voltaic cell. The electrode at the higher potential is the positive electrode, the other the negative.
Used only for discharge: current leaves at + and enters at −. Once used up, discarded.
Can be discharged and charged again: current can flow either way through it.
Primary cells
Daniell cell
A zinc electrode in dilute H₂SO₄ and a copper electrode in CuSO₄ solution, separated by a porous pot that lets ions through but keeps the liquids from mixing (the zinc is amalgamated to stop impurities acting locally).
- SO₄²⁻ ions go to the zinc and form ZnSO₄, leaving negative charge on the zinc.
- H⁺ ions pass through the pot and combine with SO₄²⁻ from the CuSO₄ to form H₂SO₄.
- Cu²⁺ ions deposit on the copper electrode, giving it positive charge.
Because hydrogen never reaches the positive electrode, the cell avoids polarization (a layer of hydrogen gas that would choke it). It gives a steady current: emf about 1.09 V, internal resistance about 1 Ω.
Leclanché cell
A zinc rod (negative) and a carbon rod (positive) in ammonium chloride solution, the carbon packed in a porous pot of manganese dioxide. Cl⁻ ions combine with the zinc; NH₄⁺ ions travel to the carbon, splitting into ammonia and hydrogen ions and giving it positive charge. MnO₂ turns the hydrogen into water (forming Mn₂O₃), but this depolarizing action is slow, so the cell suits intermittent use (doorbells). Emf about 1.5 V.
Dry cell
A sealed Leclanché cell with the NH₄Cl and MnO₂ made into a paste inside a zinc can that is itself the negative electrode. Its internal resistance is small, about 0.1 Ω.
The lead accumulator
The most common secondary cell: a lead peroxide (PbO₂) positive electrode and a lead (Pb) negative electrode in dilute sulphuric acid of specific gravity 1.20–1.28.
- Discharging: SO₄²⁻ ions reach the Pb plate and form PbSO₄; H⁺ ions reach the PbO₂ plate and reduce it (PbO₂ + 2H → PbO + H₂O), and the PbO reacts with acid to form PbSO₄ too. Acid is used up and its specific gravity falls. At 1.15 the cell is flat; drawing more current can permanently damage the plates.
- Charging: current is forced in at the positive terminal. PbSO₄ + 2H → Pb + H₂SO₄ at the negative plate and PbSO₄ + SO₄ + 2H₂O → PbO₂ + 2H₂SO₄ at the positive, restoring the plates and the acid.
Emf about 2.05 V fully charged, falling to about 1.8 V when discharged. Real accumulators join several plates of each kind (separated by insulators) to increase capacity, and put cells in series: three for a 6 V battery, six for 12 V.
Capacity is measured in ampere-hours: a 100 A h battery can give 20 A for 5 h, or 10 A for 10 h.
A 60 A h car battery powers a 4.0 A load. How long does it last, and how many cells does a 12 V battery contain?
- 1Two different metals in an electrolyte give a net emf: a voltaic cell.
- 2Primary cells discharge only (Daniell ≈ 1.09 V; Leclanché, dry cell ≈ 1.5 V).
- 3Polarization by hydrogen is prevented by depolarizers (CuSO₄, MnO₂).
- 4Lead accumulator: PbO₂ (+), Pb (−), H₂SO₄; ≈ 2.05 V to 1.8 V; acid density tracks the charge.
- 5Capacity in ampere-hours.
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