Products of Electrolysis
Specification Point 3.25:
Explain the formation of the products in the electrolysis, using inert electrodes, of some electrolytes, including:
a) copper chloride solution
b) sodium chloride solution
c) sodium sulfate solution
d) water acidified with sulfuric acid
e) molten lead bromide (demonstration)
Electrolysis of Aqueous Solutions
- Before describing the products of specific electrolytes, the ions in solution and processes at each electrode must first be considered.
Ions from Water
- Aqueous solutions will always have water molecules.
- The water molecules also break down and form H+ and OH– ions:
H2O → H+ + OH–
Positive Electrode (anode)
- Negatively charged OH– ions and non-metal ions are attracted to the positive electrode.
- Either the OH– or the non-metal ions will lose electrons and oxygen gas or the gas of the non-metal in question is released e.g. Chlorine, Bromine, Nitrogen.
- The product formed depends on which ion loses electrons more readily, with the more reactive ion remaining in solution. A reactivity series of anions is shown below:
More reactive SO42- → NO3– → OH– → Cl– → Br– → I– Less reactive
- Therefore at the anode, oxygen gas will be produced unless the ionic compound contains a halide ions, in which case the halogen will be produced.
Negative Electrode (cathode)
- Positively charged H+ and metal ions are attracted to the negative electrode but only one will gain electrons.
- Either hydrogen or 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, as for the anode reactions, 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
Formation of Products
Edexcel GCSE Chemistry Notes
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