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Chemistry · Glossary

What is Batteries?

Also known as: primary cell · accumulator · specific energy

Definition 11.3 University Chemistry — Year 2 · Chapter 11 — Batteries, Fuel Cells and Electrolysis

A primary cell is an electrochemical cell used once, until its reagents are spent. An accumulator is a cell that can be recharged by forcing a current through it in the reverse direction, which reverses its reaction. The specific energy of a cell is the electrical energy it delivers per unit mass, usually in W h/kg\mathrm{W}\,\mathrm{h}/\mathrm{kg}.

The operating point of a cell (model curves). At a current I the negative electrode sits on its anodic curve, the positive electrode on its cathodic curve; the difference of the two potentials, less the ohmic drop rI, is the voltage delivered. It falls as I grows.
The operating point of a cell (model curves). At a current II the negative electrode sits on its anodic curve, the positive electrode on its cathodic curve; the difference of the two potentials, less the ohmic drop rIrI, is the voltage delivered. It falls as II grows.
The lead–acid accumulator, schematic. On discharge lead is oxidised and lead dioxide reduced, both to lead sulfate, and the acid is consumed: the density of the solution tells the state of charge.
The lead–acid accumulator, schematic. On discharge lead is oxidised and lead dioxide reduced, both to lead sulfate, and the acid is consumed: the density of the solution tells the state of charge.

Examples

Example 11.6 (The lead–acid accumulator)

At the negative electrode Pb+SOX4X2−→PbSOX4+2 eX−\ce{Pb + SO4^2- -> PbSO4 + 2e-}; at the positive one PbOX2+4 HX++SOX4X2−+2 eX−→PbSOX4+2 HX2O\ce{PbO2 + 4H+ + SO4^2- + 2e- -> PbSO4 + 2H2O}. Overall

Pb+PbOX2+4 HX++2 SOX4X2−→2 PbSOX4+2 HX2O,\ce{Pb + PbO2 + 4H+ + 2SO4^2- -> 2PbSO4 + 2H2O},

ΔrG∘=2(−813.14)+2(−237.13)−(−217.33)−2(−744.53)=−394.1 kJ/mol\Delta_r G^\circ = 2(-813.14) + 2(-237.13) - (-217.33) - 2(-744.53) = -394.1\,\mathrm{kJ}/\mathrm{mol}, E∘=394 100/(2×96 485)=2.04 VE^\circ = 394\,100/(2 \times 96\,485) = 2.04\,\mathrm{V}. Both electrodes are covered by lead sulfate as the cell discharges; charging reverses the reaction. The reduction of HX+\ce{H+} on lead and the oxidation of water on lead dioxide are slow: that is why a cell of 2 V2\,\mathrm{V} can be charged in water, whose thermodynamic window is only 1.23 V1.23\,\mathrm{V} wide.

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