Redox is electron transfer
A redox reaction is any reaction where electrons move from one species to another. Two things always happen together: one substance loses electrons and the other gains them. The mnemonic settles it — OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
The substance that gives electrons away is oxidised; the one that takes them is reduced. An oxidising agent accepts electrons (so it is itself reduced); a reducing agent donates them. In the older sense, oxidation can also mean gaining oxygen or losing hydrogen — the electron definition is the one that always works.
Electrolysis: forcing a reaction with electricity
Electrolysis uses an electric current to split an ionic compound into its elements. It only works when the ions are free to move — so the compound must be molten or dissolved; a solid ionic compound will not electrolyse because its ions are locked in the lattice.
Two electrodes dip in. The cathode is negative: positive ions (cations) are attracted there and gain electrons — reduction. The anode is positive: negative ions (anions) go there and lose electrons — oxidation. So reduction always happens at the cathode, oxidation at the anode.
Two anchors that never change
Cathode = negative = reduction (cations gain electrons). Anode = positive = oxidation (anions lose electrons). Memorise this pair and every electrolysis question becomes "which ion goes where".
Molten vs aqueous
Electrolysing a molten compound is simple: the metal forms at the cathode, the non-metal at the anode. Molten lead(II) bromide gives lead at the cathode and brown bromine vapour at the anode.
A solution is trickier because water also provides H⁺ and OH⁻. At the cathode you get the metal only if it is less reactive than hydrogen; otherwise hydrogen bubbles off. At the anode you get a halogen if a halide is present; otherwise oxygen. So electrolysing brine (concentrated NaCl solution) gives hydrogen, chlorine and sodium hydroxide — the basis of a whole industry.
Why it matters
Electrolysis extracts the most reactive metals (aluminium from molten aluminium oxide — nothing else can pull the oxygen off), electroplates objects with a thin metal layer (the object is the cathode, the plating metal the anode), and purifies copper for wiring. It is expensive because it needs so much electricity, which is exactly why recycling aluminium is worth it.
A worked example
Molten lead(II) bromide is electrolysed. What forms at each electrode, and which process is which?
- The ions free to move are Pb²⁺ and Br⁻.Molten, so the lattice has broken and ions can migrate.
- Pb²⁺ goes to the cathode and gains electrons → lead metal. This is reduction.Cathode is negative; cations gain electrons (OIL RIG: gain = reduction).
- Br⁻ goes to the anode and loses electrons → bromine. This is oxidation.Anode is positive; anions lose electrons (loss = oxidation).
Lead at the cathode (reduction), bromine at the anode (oxidation).
Oxidation numbers — keeping score
An oxidation number is a signed value that tracks how many electrons an atom has gained or lost. A few rules cover most cases: uncombined elements are 0; a simple ion equals its charge; oxygen is usually −2 and hydrogen +1; and the numbers in a compound add up to zero (or to the ion's charge).
They make redox easy to spot: an atom whose oxidation number rises has been oxidised, and one whose number falls has been reduced — even in reactions where no obvious electron transfer is written.
Useful electrolysis: brine, plating and purifying copper
Electrolysing concentrated sodium chloride solution (brine) is a major industry: it gives hydrogen at the cathode, chlorine at the anode, and sodium hydroxide left in solution — three valuable products used for margarine, bleach and soap.
Electroplating coats a cheap object with a thin layer of an expensive metal by making the object the cathode in a solution of that metal's ions. And impure copper is purified by electrolysis: the impure copper is the anode and dissolves, while pure copper deposits on the cathode, leaving impurities behind as sludge.
Choosing the products in solution
At the cathode, the less reactive of hydrogen and the metal is produced (so hydrogen forms unless the metal is below it, like copper). At the anode, a halide gives the halogen; otherwise oxygen is released from water.