Chemistry driven by electricity
In electrolysis, an electric current forces a reaction that would not happen on its own: metal ions gain electrons at the cathode and are deposited as metal, or water splits into hydrogen and oxygen. It is how aluminum, chlorine and pure copper are produced, and how objects are plated with silver, nickel, zinc or gold.
Faraday’s laws
In 1834, Michael Faraday showed that the amount of substance produced is proportional to the electric charge passed:
- Charge Q = I × t (amperes × seconds = coulombs)
- Moles of electrons = Q ÷ F, with F = 96,485 C/mol
- Mass deposited m = Q × M ÷ (n × F)
- Time for a target mass t = m × n × F ÷ (M × I)
M is the molar mass and n the number of electrons needed per ion (2 for Cu²⁺, 1 for Ag⁺). The current efficiency reduces Q when part of the current drives side reactions.
Worked example: copper plating
A current of 2.0 A passes through a copper sulfate bath for 30 minutes, at 100% efficiency.
- Q = 2.0 × 1,800 = 3,600 C, or exactly 1 ampere-hour.
- Moles of electrons = 3,600 ÷ 96,485 = 0.0373 mol.
- Moles of copper = 0.0373 ÷ 2 = 0.0187 mol.
- Mass = 0.0187 × 63.55 = 1.186 g of copper.
- To deposit 5 g, the same current must flow for 126.5 minutes.
Mass per ampere-hour
| Species | n | Mass per Ah |
|---|---|---|
| Silver Ag⁺ | 1 | 4.02 g |
| Copper Cu²⁺ | 2 | 1.19 g |
| Zinc Zn²⁺ | 2 | 1.22 g |
| Nickel Ni²⁺ | 2 | 1.09 g |
| Gold Au³⁺ | 3 | 2.45 g |
| Aluminum Al³⁺ | 3 | 0.34 g |
| Hydrogen H₂ | 2 | 0.038 g |
Aluminum needs three electrons per atom and has a low molar mass, which is why smelters use enormous currents, several hundred thousand amperes per cell.
Current efficiency
Not every electron does useful work. Hydrogen evolving at the cathode, impurities and leaks reduce the efficiency. Copper refining reaches 95 to 98%, nickel plating about 95%, while hard chrome plating may be only 10 to 25%. Enter the efficiency to get a realistic mass or time.
Plating thickness
To turn a mass into a thickness, divide by the density and the plated area. For example, 1.186 g of copper (8.96 g/cm³) spread over 50 cm² forms a layer about 26 µm thick.
Hydrogen production
Water electrolysis needs 2 electrons per H₂ molecule. One kilogram of hydrogen requires about 26,600 Ah of charge; at the roughly 1.8 to 2 V of a real electrolyzer, that is about 50 kWh of electricity.