The Process of Electrolysis
- 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
The basic set-up of an electrolysis cell
Exam Tip
Use the PANIC mnemonic to remember which electrode is the positive and which is the negative: Positive (is) Anode Negative Is Cathode.
- Ionic compounds in the solid state cannot conduct electricity since they have no free ions that can move and carry the charge
- The ions must be able to move and can only do so 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
Particles in ionic compounds are in fixed position in the solid state but can move around when molten or in solution
- During electrolysis the electrons move from the power supply towards the cathode
- Electron flow in electrochemistry thus occurs in alphabetical order as electrons flow from the anode to the cathode
- Positive ions within the electrolyte migrate towards the negatively charged electrode which is the cathode
- Negative ions within the electrolyte migrate towards the positively charged electrode which is the anode
Diagram showing the direction of movement of electrons and ions in the electrolysis of NaCl
Exam Tip
When a metal conducts it is the electrons that are moving through the metal. When a salt solution conducts it is the ions in the solution that move towards the electrodes while carrying the electrons.
Electrolysis of Ionic Compounds
- Lead(II) bromide is a binary ionic compound meaning that it is a compound consisting of just two elements joined together by ionic bonding
- When these compounds are heated beyond their melting point, they become molten and can conduct electricity as their ions can move freely and carry the charge
- These compounds undergo electrolysis and always produce their corresponding elements
- To predict the products of any binary molten compound first identify the ions present
- The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode
- Therefore the cathode product will always be the metal and the product formed at the anode will always be the non-metal
Diagram showing the electrolysis of lead (II) bromide
Exam Tip
Remember electrodes need to be inert such as graphite or platinum so that they don’t participate in a side reaction with the electrolyte.
