4.2 - Electrolysis - Aqueous Solutions

What forms in an aqueous electrolysis

• A dissolved ionic compound has ions that are free to move, so its solution can be electrolysed without melting it.
• Water breaks up slightly into hydrogen and hydroxide ions:
  • H₂O(l) ⇌ H⁺(aq) + OH^-(aq)
• This means four kinds of ion are present: the positive and negative ions of the dissolved compound, plus H⁺ and OH^- from the water.
• At each electrode, only one of the two competing ions is discharged to form an element, while the other stays in solution.
• Inert electrodes (such as platinum or graphite) are used so they don't react.

Three example solutions

• Dilute sulfuric acid contains H⁺, SO₄^2- and OH^-. Hydrogen forms at the cathode and oxygen at the anode - both colourless gases, with twice the volume of hydrogen collected as oxygen. OH^- is discharged in preference to SO₄^2-, so water is decomposed.
• Copper(II) chloride contains Cu²⁺, Cl^-, H⁺ and OH^-. Copper is deposited at the cathode as a pinkish-brown layer, and chlorine forms at the anode as a pale green gas with a sharp, choking smell.
• Concentrated sodium chloride contains Na⁺, Cl^-, H⁺ and OH^-. Hydrogen forms at the cathode and chlorine at the anode. Na⁺ and OH^- stay in solution, leaving sodium hydroxide.

Industrial use of sodium chloride electrolysis

Electrolysing concentrated sodium chloride produces three useful products:

1. Sodium hydroxide - Used to make soap.
2. Chlorine - Used for bleach and plastics.
3. Hydrogen - Used as a fuel.

Electroplating

• Electroplating is coating an object with a thin layer of a metal using electrolysis.
• It is done for:
  • Appearance - Coating cheap metals with silver or gold for jewellery.
  • Corrosion resistance - Coating cutlery and taps with unreactive metals to protect them from rusting or tarnishing.

Setting up the cell

• Cathode - The object being plated.
• Anode - A piece of the plating metal.
• Electrolyte - A solution containing ions of the plating metal.

Example - Plating a nickel fork with silver

• Cathode - Nickel fork (silver ions gain electrons and deposit as silver).
• Anode - Pure silver bar (dissolves to replace silver ions in solution).
• Electrolyte - Silver nitrate solution.