Electrolysis (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Electrolysis principles

Electrolytic cells

• When an electric current is passed through a molten ionic compound the compound decomposes or breaks down

  • The process also occurs for aqueous solutions of ionic compounds

• Liquids and solutions that are able to conduct electricity are called electrolytes

  • Covalent compounds cannot conduct electricity hence they do not undergo electrolysis

• An electrolytic cell is the name given to the set-up used in electrolysis and which consists of the following:

  • Electrode: a rod of metal or graphite through which an electric current flows into or out of an electrolyte

  • Electrolyte: ionic compound in molten or dissolved solution that conducts the electricity

  • Anode: the positive electrode of an electrolysis cell

  • Anion: negatively charged ion which is attracted to the anode

  • Cathode: the negative electrode of an electrolysis cell

  • Cation: positively charged ion which is attracted to the cathode

Electrical Conductivity of Ionic Compounds

• Ionic compounds in the solid state cannot conduct electricity because they have no free ions that can move and carry the charge

• The ions must be able to move

  • They can only move in the molten state or when dissolved in a solution, usually aqueous

• When the cell is turned on and an electric current is passed through an electrolyte the ions in the solution start to move towards the electrodes

Movement of ions during electrolysis

• During electrolysis, electrons flow from the positive terminal (anode) to the negative terminal (cathode) through the external circuit

• Positive ions in the electrolyte move towards the negatively charged electrode / cathode

• Negative ions in the electrolyte move towards the positively charged electrode / anode

The flow of electrons and ions in electrolysis

Electrolysis of simple ionic compounds

What is produced at the anode and cathode?

• Binary ionic compounds consist of two elements joined together by ionic bonding

  • For example, lead(II) bromide consists of lead(II) ions and bromide ions

• When binary ionic compounds are heated beyond their melting point:

  • They become molten

  • They can conduct electricity because their ions can move freely and carry the charge

• When binary ionic compounds undergo electrolysis, they always produce their corresponding elements

• To predict the products of any binary molten compound first identify the ions present

• Positive ions move towards the cathode 

  • Therefore, the cathode product is the metal

• Negative ions move towards the anode

  • Therefore, the anode product is the non-metal

Electrolysis of molten lead(II) bromide

Method

1. Add lead(II) bromide to a crucible

2. Heat the crucible until the lead(II) bromide becomes molten

   • When molten. the ions are free to move and conduct an electric charge

3. Add two graphite electrodes

4. Connect the electrodes to a power pack / battery

5. Turn on the power pack or battery and allow electrolysis to take place

6. Negative bromide ions move to the positive electrode (anode)

   • The bromide ions lose two electrons to form bromine molecules

   • This is seen as bubbling at the anode as brown bromine gas is given off

7. Positive lead(II) ions move to the negative electrode (cathode)

   1. The lead(II) ions gain electrons to form grey lead metal

   2. This is seen as lead metal depositing on the negative electrode / cathode

What are the products at the anode and cathode?

• Anode: Bromine gas

• Cathode: Lead metal

Half equations for electrolysis of lead(II) bromide

Higher Tier Only

• Half equations show the reactions occurring at each electrode individually

• At the cathode:

  • Lead(II) ions gain electrons

  • The gain of electrons is reduction

Pb²⁺ + 2e^- → Pb

• At the anode:

  • Bromide ions lose electrons

  • The loss of electrons is oxidation

2Br^- → Br₂ + 2e^-