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Batterieselectricity from chemical reactions

Every battery is a chemical reaction sealed in a can: one material gives up electrons, another takes them, and we put a light bulb or a phone in between and make the electrons go “the long way around.” Build a cell from two metals, drop metals into salt solutions, charge a lithium-ion battery, split water with electricity, and find out what saves a nail from rust.

Oxidation — losing electrons (at the anode). Reduction — gaining electrons (at the cathode). Together: redox.

Galvanic cell — two half-cells

Voltage E
1.100 V
Cell E° (1 M)
1.10 V
Electrons n
2
ΔG = −nFE
−212 kJ
Current
≈ 0
Anode (−)
Zn

The voltage is calculated from standard potentials E° (25 °C) and the Nernst equation. The model ignores internal resistance, overpotentials, and the fact that lithium, potassium, and sodium react with water.

Voltaic pile — cells in series

Number of cells
3
Per cell
≈ 0.8 V
Total
2.4 V

Series connection: the voltages add up, U = N·E. In a voltaic pile, the copper mostly gives off hydrogen, so one pair delivers less than a Daniell cell — about 0.8 V (a rough value).

Lemon battery

Electro­motive force
1.90 V
Internal resistance
3.0 kΩ
LED current
0.03 mA

The current comes from a pair of different metals — the lemon is just the electrolyte. Two copper strips give 0 V. An LED only starts to conduct above its threshold voltage (about 1.8 V for red), and the high resistance of the fruit severely limits the current. Rough values.

Frog legs vs. the pile — Galvani and Volta’s dispute

In the 1780s, Luigi Galvani in Bologna noticed that a frog’s leg twitched when its nerve and muscle were touched with two different metals. In 1791 he announced that this was “animal electricity” stored in the tissues.

Alessandro Volta believed the source of the current was the contact between two different metals, and that the frog was merely a sensitive detector and a damp conductor. To prove it, he built a pile of zinc and copper discs separated by cloth soaked in brine — the first source of steady current. He described it in a letter to the Royal Society dated March 20, 1800.

Both were partly right: the pile’s current comes from a chemical reaction at the metals, and nerves really do work electrically — thanks to differences in ion concentrations (see neuron).

  1. Galvani describes twitching frog legs and “animal electricity.”
  2. Volta’s pile — the first battery. The unit volt is named after Volta.
  3. John Daniell: a Zn|Cu cell with two solutions — a steady ~1.1 V, used to power telegraphs.
  4. Gaston Planté: the lead-acid battery — the first one that could be recharged.
  5. Georges Leclanché: the zinc-carbon cell, ancestor of the AA battery.
  6. Sony brings the lithium-ion battery to market.

How it works

A cell runs a redox reaction, but split between two places. At the anode a metal oxidizes (gives up electrons); at the cathode something is reduced (takes electrons). Electrons can’t flow through the solution, so they go through the wire — and can light a bulb along the way. In the solution, ions carry the charge; the salt bridge closes the circuit.

Cell voltage

E° = E°cathode − E°anodee.g., Zn|Cu: 0.34 V − (−0.76 V) = 1.10 V

Nernst equation (25 °C)

E = E° − 0.0592 Vn· log QQ — reaction quotient, e.g., [Zn²⁺]/[Cu²⁺]; n — number of electrons transferred. For Q = 10 and n = 2, the voltage drops by 0.0296 V.

Energy from the reaction

ΔG = −n · F · EF = 96,485 C/mol — the charge of one mole of electrons (the Faraday constant). Negative ΔG: the reaction runs on its own.

Electrolysis — Faraday’s law

Q = I · t    m = M · Qn · F1 A for 1 hour is 3,600 C, or 0.0373 moles of electrons → about 0.418 L of hydrogen (0 °C) or 0.456 L (25 °C).

Battery capacity and energy

W = U · Q    4,000 mAh · 3.7 V ≈ 14.8 WhmAh tells you the charge, Wh the energy — watt-hours are better for comparisons.

The models on this page are simplified: they ignore internal resistance (except in the lemon battery), electrode overpotentials, and the effect of temperature on E°. Standard potentials are given for 25 °C and 1 mol/L solutions.

CircuitsPower plants and storageLithiumMoleculesAcids and basesNeuronEnergy

Challenges

  1. Build a cell with more than 2 VIn “Cell” mode, pick two metals so that the voltmeter reads more than 2 V.not completed
  2. Light an LED with a lemon batteryConnect enough lemons for current to flow through the LED.not completed
  3. Coat a nail with copper in CuSO₄ solutionIn “Series,” drop an iron nail into copper sulfate and wait until the end.not completed
  4. Charge the battery to 100% and drain the phoneIn “Li-ion,” charge the cell all the way, then discharge it to zero.not completed
  5. Make 1 liter of hydrogen by electrolysisCollect at least 1 L of H₂ in the Hofmann apparatus.not completed
  6. Protect a nail from rust with zincIn “Corrosion,” attach zinc to your nail in water or brine and keep it there for at least 7 days.not completed

Did you know…?

  • A museum in Baghdad held a clay jar, about 1,500–2,000 years old, with a copper cylinder and an iron rod inside. The idea that it was an ancient battery is appealing, but most archaeologists doubt it — it may have been used, for example, to store scrolls.
  • A nerve cell works like a tiny concentration cell: differences in the concentrations of K⁺ and Na⁺ ions on either side of the membrane produce a resting voltage of about −70 mV. The potential for each of these ions is calculated with the same Nernst equation as a battery’s voltage.
  • A phone cell has a voltage of about 3.7 V and an AA battery 1.5 V, even though they are roughly the same size. The voltage is set by the pair of electrode materials, not by the size — the size affects the capacity.

Keys

Q W E R T
switch modes
← →
on the tabs: neighboring mode
Space
start / pause the animation in a mode (electrolysis, corrosion, charging)
1 2
Li-ion: charge / discharge
Esc
stop the animation