Electrolysis of solutions (AQA GCSE Chemistry Combined Science): Revision Notes
Electrolysis of solutions
What happens during electrolysis of solutions
When we use electricity to break down a solution, we call this electrolysis. During this process, hydrogen and oxygen gases are often produced at the electrodes. These gases come from the water molecules that make up the solution.
The production of hydrogen and oxygen gases during electrolysis comes from the breakdown of water molecules in the solution, not just from the dissolved substances. Understanding this concept is key to predicting what products will form during electrolysis.
Ions from water
Water is usually made of molecules held together by covalent bonds. However, a small number of water molecules naturally break apart to form charged particles called ions:
- Hydrogen ions (H) - these are positive
- Hydroxide ions (OH) - these are negative
This breakdown of water molecules is important because these ions move towards the electrodes during electrolysis.
Reactions at the electrodes
Reactions at the cathode (negative electrode)
The cathode attracts positive ions. Here's what happens:
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H ions from water are always attracted to the negative cathode
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If the metal being electrolysed is more reactive than hydrogen, then hydrogen gas is produced at the cathode instead of the metal
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If the metal being electrolysed is less reactive than hydrogen, then the actual metal is produced at the cathode
The reactivity series tells us which metals are more or less reactive than hydrogen.
The reactivity series is crucial here - it determines whether you get hydrogen gas or the actual metal at the cathode. Metals more reactive than hydrogen will leave hydrogen to be produced instead.
Reactions at the anode (positive electrode)
The anode attracts negative ions. Here's what happens:
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OH ions from water are always attracted to the positive anode
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If the solution contains no halide ions, then oxygen gas is produced at the anode
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If the solution contains halide ions (like Cl, Br, I), then the halogen gas is produced at the anode instead of oxygen
Halide ions are negative ions formed from Group 7 elements.
When halide ions are present in solution, they are preferentially discharged over hydroxide ions, which means you get halogen gases instead of oxygen at the anode.
Simple example
Worked Example: Electrolysis of Sodium Chloride Solution
When we electrolyse sodium chloride solution:
- At the anode: Chlorine gas is produced (because chloride ions are present)
- At the cathode: Hydrogen gas is produced (because sodium is more reactive than hydrogen)
- Left behind: Sodium ions and hydroxide ions remain in solution
Key Points to Remember:
- Water molecules break down to form H and OH ions during electrolysis
- At the cathode (negative): Hydrogen gas forms unless the metal is less reactive than hydrogen
- At the anode (positive): Oxygen gas forms unless halide ions are present
- Reactivity series determines what gets produced at the cathode
- Halide ions determine what gets produced at the anode