Redox reactions and electrode processes

Redox reactions can make electricity. In the Daniell cell, a zinc rod stands in zinc sulphate solution and a copper rod in copper sulphate solution. The rods are joined by a wire, and the solutions by a salt bridge (a tube of salt solution in jelly). Electrons flow through the wire from zinc to copper, and ions move through the salt bridge to keep each solution electrically neutral. The reaction is the same as zinc in copper sulphate, but now the electron transfer happens through the wire, and can do useful work.

Each half of the cell (metal plus its ion) is a redox couple. The tendency of a couple to gain or lose electrons is its electrode potential. Under standard conditions (1 M solutions, 1 atm, 298 K) it is the standard electrode potential, E⊖. Since one half-cell cannot be measured alone, every value is measured against a reference: the standard hydrogen electrode, whose potential is set at 0.00 V.

What E⊖ tells us:

• A more positive E⊖ means a stronger oxidant: the couple's ion is eager to gain electrons. F₂/F⁻ has the highest value, +2.87 V.
• A more negative E⊖ means a stronger reductant: the metal is eager to lose electrons. Li⁺/Li has –3.05 V.
• A redox reaction goes if the oxidant belongs to the couple with the higher E⊖.

Cell notation writes the anode on the left and the cathode on the right: anode | anode solution || cathode solution | cathode. The double line stands for the salt bridge. For example, Zn | Zn²⁺ || Ag⁺ | Ag. At the anode, oxidation happens; in a galvanic cell it is the negative electrode, because electrons leave from it.

In electrolysis, electricity is used to drive a reaction instead. What forms depends on the electrodes and the ions present: ions such as Ag⁺ and Cu²⁺ are reduced in preference to water; a silver anode, being active, dissolves itself; and in dilute sulphuric acid, water is broken down into hydrogen and oxygen.