Electrolysis of Aqueous Solutions
- Aqueous solutions will always have water (H2O)
- In the electrolysis of aqueous solutions, the water molecules dissociate producing H+ and OH– ions:
H2O ⇌ H+ + OH–
- These ions are also involved in the process and their chemistry must be considered
- We now have an electrolyte that contains ions from the compound plus ions from the water
- Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved
- Concentrated and dilute solutions of the same compound give different products
- For anions, the more concentrated ion will tend to get discharged over a more dilute ion
Positive Electrode (anode)
- Negatively charged OH– ions and non-metal ions are attracted to the positive electrode
- If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine)
- If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen
- In both cases the other negative ion remains in solution
Negative Electrode (cathode)
- Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons
- Either hydrogen gas or the metal will be produced
- If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode
- This is because the more reactive ions will remain in solution, causing the least reactive ion to be discharged
- Therefore at the cathode, hydrogen gas will be produced unless the positive ions from the ionic compound are less reactive than hydrogen, in which case the metal is produced
The reactivity series of metals including hydrogen and carbon
- The electrode products are shown below for a series of common electrolytes
Common Electrolytes Table
Examiner Tip
When answering questions on this topic, it helps if you first write down all of the ions present. Then compare their reactivity, work out which ones move to which electrode and deduce the products formed.