AQA GCSE Chemistry

Revision Notes

4.3.4 Electrolysis of Aqueous Solutions

Electrolysis of an Aqueous Solution Using Inert Electrodes

  • 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

Exam Tip

When answering questions on this topic, it helps if you first write down all of the ions present first. Only then you should start comparing their reactivity and deducing the products formed.

Electrode Reactions

Positive Electrode (anode)

  • Negatively charged OH ions and nonmetal ions are attracted to the positive electrode
  • Either OH or nonmetal ions will lose electrons and oxygen gas or the gas of the nonmetal in question is released e.g. chlorine, or bromine
  • The product formed depends on which ion is preferentially discharged, with the other ion remaining in solution
  • An order of the discharge of anions is shown below:
    • Less easily discharged SO42- → NO3→ Cl → Br → I→ OH  More easily discharged
  • Therefore at the anode, oxygen gas will be produced unless the ionic compound contains 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 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, IGCSE & GCSE Chemistry revision notes

The reactivity series of metals including hydrogen and carbon

  • The electrode products are shown below for a series of common electrolytes

Common Electrolytes Table

Common Electrolytes Table, downloadable IGCSE & GCSE Chemistry revision notes

Determining what Gas is Produced

  • The gas produced can be tested to determine its identity
  • If the gas produced at the cathode burns with a ‘pop’ with a lighted splint then the gas is hydrogen
  • If the gas produced at the anode relights a glowing splint dipped into the gas then the gas is oxygen
  • The halogen gases all produce their own colours (bromine is red-brown, chlorine is yellow-green)

Exam Tip

Once you have identified the ions, the next step is to decide towards which electrode will they be drawn and identify the product formed. It helps if you recall the reactivity series.

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.

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