Ionic theory
Some substances conduct electricity when they are molten or dissolved in water. They are called electrolytes. Examples are salts, acids and alkalis. Non-electrolytes, such as sugar, alcohol and pure water (almost), do not conduct. The ionic theory explains this: an electrolyte contains ions, and the ions are free to move when the substance is molten or in solution. Metals conduct because of moving electrons; electrolytes conduct because of moving ions.
| Substance | Conducts? | Why |
|---|---|---|
| Copper wire | Yes | Free electrons |
| Solid sodium chloride | No | Ions are fixed |
| Molten sodium chloride | Yes | Ions are free to move |
| Sugar solution | No | Molecules, no ions |
| Dilute hydrochloric acid | Yes | H⁺ and Cl⁻ ions |
Mechanism of electrolysis
Electrolysis is the breaking down of an electrolyte by passing a direct electric current through it. The electrodes dip into the electrolyte. The anode is the positive electrode and the cathode is the negative electrode. Anions (negative ions) go to the anode, where they lose electrons (oxidation). Cations (positive ions) go to the cathode, where they gain electrons (reduction).
| Electrolyte | At the cathode (−) | At the anode (+) |
|---|---|---|
| Molten sodium chloride | Sodium metal: Na⁺ + e⁻ → Na | Chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻ |
| Concentrated sodium chloride solution (brine) | Hydrogen gas | Chlorine gas |
| Dilute sulphuric acid (acidified water) | Hydrogen: 2H⁺ + 2e⁻ → H₂ | Oxygen: 4OH⁻ → 2H₂O + O₂ + 4e⁻ |
| Copper(II) sulphate with inert (carbon) electrodes | Copper is deposited | Oxygen gas |
| Copper(II) sulphate with copper electrodes | Copper is deposited | The anode dissolves |
Laws of electrolysis
Faraday found how the amount of substance produced depends on the electricity used. First law: the mass of a substance produced at an electrode is proportional to the quantity of electricity (charge) passed. Second law: the same quantity of electricity produces masses of different substances in the ratio of their equivalent masses. One mole of electrons has a charge of 96 500 coulombs, called one faraday.
- electrons needed to discharge one ion
- Faraday constant, 96 500 C/mol
A current of 2.0 A flows for 30 minutes through copper(II) sulphate solution. What mass of copper is deposited? (Cu = 64, F = 96 500)
- Charge: Q = It = 2.0 × 1800 = 3600 C.
- Cu²⁺ + 2e⁻ → Cu, so z = 2.
Answer: About 1.2 g of copper is deposited.
Choose the current, the time and the metal. Find the mass of metal deposited at the cathode.
Corrosion inhibition and purification of metals
| Use | How electrolysis is used |
|---|---|
| Electroplating | The object to be plated is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal. Used for chromium on taps and silver on spoons. |
| Purification of copper | Impure copper is the anode and pure copper the cathode in copper(II) sulphate solution. Pure copper builds up on the cathode. |
| Corrosion inhibition | Iron objects are protected by plating, by galvanising with zinc, or by joining them to a more reactive metal that corrodes instead (sacrificial protection) |
| Extraction of metals | Aluminium, sodium and magnesium are obtained by electrolysis of their molten compounds |
| Manufacture | Chlorine and sodium hydroxide from brine |
Key terms
- Electrolyte
- A substance that conducts electricity when molten or dissolved because it contains free ions.
- Anode
- The positive electrode, where oxidation happens.
- Cathode
- The negative electrode, where reduction happens.
- Faraday
- The charge of one mole of electrons: 96 500 C.
Summary
- Electrolytes contain ions that are free to move when molten or dissolved.
- Cations go to the cathode and are reduced; anions go to the anode and are oxidised.
- Faraday's laws: mass ∝ charge; m = I t M ÷ (z F), with F = 96 500 C/mol.
- Uses: electroplating, purifying copper, extracting reactive metals, protecting iron from corrosion.