Galvanic cells and electrode potentials

A galvanic cell (also called a voltaic cell) turns the energy of a spontaneous redox reaction into electricity. Every battery you use is one.

The best-known example is the Daniell cell. Take two beakers. In one, a zinc rod stands in zinc sulphate solution; in the other, a copper rod stands in copper sulphate solution, both at 1 M. Join the rods with a wire through a voltmeter, and join the two solutions with a salt bridge: a tube of a salt such as KCl or KNO₃ in jelly.

Now watch what happens:

• The zinc rod slowly dissolves, so it gets lighter: Zn → Zn²⁺ + 2e⁻. This is oxidation.
• The electrons travel through the wire to the copper rod.
• There, Cu²⁺ ions from the solution take the electrons and settle on the rod as copper metal, so the copper rod gets heavier: Cu²⁺ + 2e⁻ → Cu. This is reduction.
• Added together: Zn + Cu²⁺ → Zn²⁺ + Cu, a redox reaction. The cell is written Zn | Zn²⁺ || Cu²⁺ | Cu and gives 1.1 V.

The salt bridge completes the circuit and keeps both solutions electrically neutral; without it the current stops at once. Electrons flow from zinc to copper in the wire, so the current flows the other way.

The electrode where oxidation happens is the anode; it is negative because electrons are released there. The one where reduction happens is the cathode, and it is positive. A memory aid: LOAN, the Left electrode, Oxidation, Anode, Negative.

Each half of the cell has an electrode potential, the 'push' it gives to electrons. We cannot measure one half alone, so every electrode is measured against the standard hydrogen electrode (SHE): platinum foil in 1 M acid with hydrogen gas bubbling at 1 bar. Its potential is fixed as 0.00 V. Pair any half-cell with the SHE, measure the cell, and the reading is that electrode's standard potential E°.

The cell's standard potential is then:

E°(cell) = E°(right) – E°(left) = E°(cathode) – E°(anode)

If E°(cell) comes out positive, the reaction goes on its own.

The table of E° values tells you more. A very low (negative) E° means the metal gives up electrons easily: it is a strong reducing agent (lithium, potassium). A very high E° means the species grabs electrons: it is a strong oxidising agent (fluorine).

What if you connect an outside voltage against the cell? If it is less than 1.1 V, the cell still works as before. If it is exactly 1.1 V, everything stops: no current flows. If it is more than 1.1 V, everything reverses: zinc is deposited, copper dissolves, and the cell becomes an electrolytic cell.