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.

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, 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 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, 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, 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, IGCSE & GCSE Chemistry revision notes

Determining what Gas is Produced

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

Electrolysis of an Aqueous Solution Using Inert Electrodes

Objective:
To prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner and dilute acid.

Hypothesis:
A metal will be produced at the negative electrode because metal ions are positive.

Materials:

  • test tubes
  • electrolytic solutions
  • 100 cm3 beaker
  • stand and clamp
  • two carbon rod electrodes
  • two crocodile / 4 mm plug leads
  • low voltage power supply
  • blue litmus paper
  • forceps

image

Electrolysis - Apparatus Inverted Test Tubes, IGCSE & GCSE Chemistry revision notes

Diagram Showing the Electrolysis of Aqueous Solutions

Practical Tip:
Make sure both electrodes are placed to a sufficient and equal depth in the electrolyte whilst being sure that they do not touch each other.

Method:

  1. Set up the apparatus as shown in the diagram.
  2. Add the aqueous solution to the beaker.
  3. Add two graphite rods as the electrodes and connect this to a power pack or battery.
  4. Turn on the power pack or battery and allow electrolysis to take place.
  5. Record the results in a suitable table (see below) and repeat for another solution, checking the electrodes in between runs to see if any metal has been deposited.
  6. The following aqueous solutions are suitable for this investigation: copper chloride, copper sulfate, sodium chloride, sodium bromide, sodium nitrate.
  7. The gases produced can be collected in the test tubes to be tested later.

Results: Record your results in a suitable table:

Investigating the electrolysis of aqueous solutions table, IGCSE & GCSE Chemistry revision notes

Evaluation:
The gases and corresponding tests are:

  • Hydrogen – lighted splint goes out with a squeaky pop
  • Oxygen – a glowing splint relights
  • Chlorine – damp blue litmus paper turns red and is then bleached white

Conclusion:
Describe how the results obtained compare with the expected results based on the hypothesis.

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AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.