Electrolysis: Reactions in Reverse
A battery lets a willing reaction pay you in volts. Electrolysis is you paying volts to force an unwilling reaction backwards.
Builds on: 6.2 Batteries: The Galvanic Cell
Running chemistry uphill
Water will never split into hydrogen and oxygen by itself — that reaction runs steeply uphill (it’s the reverse of hydrogen’s explosive combustion). But push electrons in with a power supply and you can force it: electrolysis, the galvanic cell run in reverse. Below a threshold voltage nothing happens at all; past it, the current drives the reaction and gases bloom at the electrodes:
Hydrogen appears at the cathode (− terminal: reduction, electrons in), oxygen at the anode (+ terminal: oxidation, electrons out) — and twice as much hydrogen as oxygen, because every water molecule carries two H for one O. Electrolysis is a chemical formula you can watch fill two test tubes.
Where the forced reaction earns its keep
- Aluminium — locked in its ore so tightly that only brute electrochemical force frees it. Smelters consume country-scale electricity; recycling a can costs 5% of making a new one.
- Electroplating — a thin forced deposit of chromium, silver or gold onto a cheaper metal. Jewellery, cutlery, corrosion armor.
- Charging a battery — the everyday one: charging is literally electrolysis of the battery’s own discharged chemistry, shoving it back uphill so it can run down again through your phone.
- Green hydrogen — surplus solar/wind electricity stored as H₂, re-payable later in a fuel cell (a galvanic cell burning hydrogen politely).
The volts you invest in splitting water come back — no more, in practice less — when the hydrogen recombines. Electrolysis doesn’t create energy; it stores it in bonds. Thermodynamics keeps flawless books; the profit is in the when and where, not the how much.
⚗️ Lab — The Water Splitter
Two electrodes, two collection tubes, one voltage knob.
- Creep the voltage up — find the threshold below which nothing bubbles.
- Let the tubes fill and check the ratio meter: why 2 : 1?
- Which tube fills at the − electrode, and what redox half is happening there?