Electrolysis of solutions (AQA GCSE Chemistry): Revision Notes
Electrolysis of solutions
What happens during electrolysis of solutions
When you pass electricity through a solution, the water and dissolved substances can break apart. This creates different products at each electrode. The water itself plays an important role because it contains ions that can react.
During electrolysis, both the dissolved substances AND the water molecules participate in the reactions. This means you need to consider ions from both sources when predicting what products will form.
Ions from water
Water is made of molecules held together by covalent bonds. Even though water molecules are mostly stable, a small number of them split apart to form ions:
- Hydrogen ions () - these are positive
- Hydroxide ions () - these are negative
These water ions are always present in solutions and compete with other ions during electrolysis. You cannot ignore them when predicting electrolysis products!
What happens at the cathode (negative electrode)
The cathode attracts positive ions. Two types of positive ions are usually present: metal ions from the dissolved substance and hydrogen ions from water. Which product forms depends on how reactive the metal is.
Scenario 1: Very reactive metals If the metal in solution is more reactive than hydrogen, then hydrogen gas is produced at the cathode. This is because hydrogen ions are easier to turn into atoms than the very reactive metal ions.
Scenario 2: Less reactive metals If the metal in solution is less reactive than hydrogen, then the metal itself is produced at the cathode. The metal ions get turned back into metal atoms.
Scenario 3: What gets left behind The ions that don't react stay dissolved in the solution.
The reactivity series is your key tool for predicting what happens at the cathode. Metals above hydrogen in the reactivity series will produce hydrogen gas, while metals below hydrogen will produce the metal itself.
What happens at the anode (positive electrode)
The anode attracts negative ions. The main negative ions present are usually hydroxide ions from water and sometimes halide ions from dissolved salts. What gets produced depends on which negative ions are present.
Scenario 1: No halide ions present If there are no halide ions (like chloride, bromide, or iodide), then oxygen gas is produced at the anode. This comes from the hydroxide ions.
Scenario 2: Halide ions present If the solution contains halide ions (, , ), then the halogen element is produced instead of oxygen. For example, chloride ions would produce chlorine gas.
Scenario 3: Which ions react Halide ions react more easily than hydroxide ions, so they get chosen first if they're present.
Predicting products - worked examples
Worked Example: Sodium chloride solution
- At the cathode: Sodium is very reactive, so hydrogen gas forms
- At the anode: Chloride ions are present, so chlorine gas forms
- Left in solution: Sodium and hydroxide ions remain
Worked Example: Copper chloride solution
- At the cathode: Copper is less reactive than hydrogen, so copper metal forms
- At the anode: Chloride ions are present, so chlorine gas forms
How to work out what happens
Simple Rules for Predicting Electrolysis Products:
- For the cathode: Check if the metal is more or less reactive than hydrogen using the reactivity series
- For the anode: Check if halide ions are present in the solution
- Remember: Hydrogen and oxygen come from water, whilst metals and halogens come from the dissolved substance
Summary
Key Points to Remember:
- Water splits into hydrogen ions () and hydroxide ions () during electrolysis
- At the cathode: reactive metals give hydrogen gas, less reactive metals give the metal itself
- At the anode: solutions without halides give oxygen, solutions with halides give the halogen
- The reactivity series helps you predict which products form at the cathode
- Halide ions (chloride, bromide, iodide) always produce halogens at the anode instead of oxygen