Electrolysis: electrolytes, ions and electroplating

Pass a current through salt water and bubbles of gas appear at the wires. Pass it through copper sulphate and copper coats the negative plate. Electrolysis — chemical change caused by electric current — is used to plate cutlery, refine metals and charge car batteries.

Learning objectives

By the end of this lesson you should be able to:

• define electrolysis, electrolyte, electrodes, anode and cathode
• explain conduction in electrolytes by the movement of ions
• describe the electrolysis of copper(II) sulphate and of acidified water
• distinguish conductors, electrolytes and non-electrolytes

Key terms

Substances that do not conduct when dissolved or molten (sugar solution, pure water, paraffin) are non-electrolytes.

How electrolytes conduct

In an electrolyte the current is carried by ions (charged atoms or groups of atoms), not by electrons:

• positive ions (cations) — such as H⁺, Cu²⁺, Na⁺ — move to the cathode (−)
• negative ions (anions) — such as SO₄²⁻, OH⁻, Cl⁻ — move to the anode (+)

At the electrodes the ions gain or lose electrons and are discharged as atoms or molecules.

Electrolysis of copper(II) sulphate with copper electrodes

Figure 1. Electrolysis of copper sulphate with copper electrodes. WASSCE Tutor diagram.

• At the cathode: Cu²⁺ ions gain electrons and are deposited as copper. $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$. The cathode gains mass.
• At the anode: copper atoms lose electrons and go into solution. $\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$. The anode loses mass.
• The concentration (and blue colour) of the solution stays the same: copper is simply transferred from anode to cathode.

This is the principle of copper refining and electroplating.

Electrolysis of acidified water (Hofmann voltameter)

Pure water barely conducts, so a little dilute sulphuric acid is added. With inert platinum electrodes:

• at the cathode: hydrogen ions are discharged — hydrogen gas is released
• at the anode: oxygen gas is released
• the volume of hydrogen is twice the volume of oxygen (water is H₂O)

Applications of electrolysis

1. Electroplating: coating a cheap metal with a thin layer of a better-looking or corrosion-resistant one (chromium on bicycle handlebars, silver on cutlery, gold on jewellery).
2. Refining of metals such as copper to high purity.
3. Extraction of reactive metals such as aluminium and sodium from their molten ores.
4. Charging of secondary cells (lead–acid car batteries).
5. Electrotyping and anodising aluminium.

Electroplating a spoon with silver

• Cathode: the spoon to be plated (cleaned first)
• Anode: a bar of pure silver
• Electrolyte: a solution of a silver salt (such as silver nitrate)
• Use a small current for a long time, so the coating is even and firmly attached.

Key points

• Electrolysis: chemical decomposition by an electric current; current carried by ions.
• Cations (+) go to the cathode (−); anions (−) go to the anode (+).
• CuSO₄ with copper electrodes: copper moves from anode to cathode; concentration unchanged.
• Acidified water: hydrogen at the cathode, oxygen at the anode, volume ratio 2 : 1.
• Uses: electroplating, refining, extraction, charging batteries.

WASSCE examination focus

Quick check

1. To which electrode do positive ions move?
2. In electroplating a spoon with silver, what is the spoon connected to?
3. What happens to the colour of copper sulphate solution during electrolysis with copper electrodes?

Answers: 1. The cathode (negative electrode). 2. The negative terminal: it is the cathode. 3. It stays the same, because copper dissolves from the anode as fast as it is deposited on the cathode.