GCSE Chemistry- Summary of Electrolysis and Half-EquationsWatch
This is just the process by which aqueous ionic substances (electrolytes) are decomposed into simpler substances when an electric current is passed through them.
In an electrolyte, positive ions go to the cathode. They receive electrons and are reduced.
Conversely, negative ions go to the anode. They lose electrons and are oxidised.
Let's say your electrolyte is a molten salt (e.g. liquid lead(II) bromide). The metal (lead) is formed at the cathode because metal ions are positive. Similarly, the non-metal (bromine) is formed at the anode because non-metal ions are negative.
This is slightly different when your electrolyte is an ionic solution, but it's not too difficult. You'll need to learn the reactivity series of metals, so I've attached an image at the end of this post.
Cathode: The metal is formed only if it is more reactive than hydrogen. If the metal is less reactive than hydrogen, hydrogen will be formed.
Anode: The non-metal is formed only if it is a simple ion (e.g. Cl-, Br-). If the non-metal is a complex ion (e.g. NO3-, SO42-, CO3-), oxygen will be formed instead.
Lastly, if your electrolyte is a dilute solution of a halide compound, oxygen will be formed at the anode instead of the halogen. This is because there are many more hydroxide ions than halide ions in a dilute solution.
Ionic half equations
These are just equations that show you what happens at each electrode during electrolysis.
When positive metal ions arrive at the cathode, they gain electrons to form neutral metal atoms. This is called reduction. If we stick with the previous molten lead(II) bromide, this is the reaction that happens at the cathode:
Pb2+ + 2e- → Pb
The lead(II) ion is positive, so it receives electrons the cathode. As a result, it forms a neutral lead atom.
When negative non-metal ions arrive at the anode, they lose electrons to form neutral atoms or molecules. This is called oxidation:
2Br- → Br2 + 2e-
Bromine ions are negative, so they loses electrons at the anode. As a result, they form diatomic bromine.
The easiest way to remember how to write half equations, is as follows:
1. Write down what ions become what atoms at the electrode: Br- → Br2
2. Balance the number of atoms on both sides to make them equal: 2Br- → Br2
3. Add electrons in order to balance the charge on both sides: 2Br- → Br2 + 2e-
*If the reaction is a reduction/at the cathode, add electrons to the left side. If the reaction is an oxidation/at the anode, add electrons to the right side.
That about sums it up!