Electrolysis of Aqueous Solutions (Cambridge International IGCSE Chemistry)

Spec point — 4.1.4 · Predict the identity of the products at each electrode for the electrolysis of a halide compound in dilute or concentrated aqueous solution Identify the products formed at the electrodes and describe the observations made during the electrolysis of aqueous copper(II) sulfate using inert carbon/ graphite electrodes and when using copper electrodes

Electrolysis of aqueous solutions

Extended tier only

• Aqueous solutions always have water, H₂O, present
• 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^–) are present, the halogen is produced at the anode

  • The halide ions lose electrons and forms the halogen (chlorine, bromine or iodine)

• If there are no halide ions but OH^– ions are present, oxygen is produced at the anode

  • The hydroxide ions lose electrons and forms oxygen gas (and water)

• In both cases, the other negative ion remains in solution

How concentration affects products at the anode

• The concentration of the solution affects the ion being discharged:

  • If a concentrated halide solution is being electrolysed, the halogen forms at the anode
  • If a dilute halide solution is being electrolysed, oxygen forms at the anode

• For example:

  • For concentrated barium chloride solution :

    • Cl^– ions are discharged more readily than the OH^– ions
    • So, chlorine gas is produced at the anode

  • For dilute barium chloride solution:

    • Only OH^– ions are discharged
    • So, oxygen is produced at the anode

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 metal will be produced

• If the metal is above hydrogen in the reactivity series:

  • The ions of the more reactive metal remain in solution
  • This causes the less reactive hydrogen ions, H⁺, to be discharged
  • So, hydrogen will be produced and bubbling will be seen at the cathode

• If the metal is below hydrogen in the reactivity series:

  • The less reactive metal ions are discharged
  • So, the metal is produced and this will be seen plating onto the cathode

The reactivity series of metals including hydrogen and carbon

Electrolysis of aqueous copper sulfate

• Aqueous copper sulfate contains the following ions:

  • Cu²⁺ and SO₄^2– from the copper sulfate
  • H⁺ and OH^– from the water

Using graphite electrodes:

Apparatus for the electrolysis of copper(II)sulfate using inert / passive graphite electrodes

Observations:

• The cathode is coated with a pink-brown layer of copper metal
• Bubbles of a colourless gas (oxygen) are seen forming at the anode
• The blue colour of the copper(II) sulfate solution fades over time

Product at the cathode:

• Cu²⁺ and H⁺ will both be attracted to the cathode but the less reactive ion will be discharged

  • In this case, copper is less reactive than hydrogen

  • Copper ions are discharged at the cathode

    • They gain electrons and are reduced to form copper metal

  • The half equation for the reaction at the electrode is:

Cu²⁺ + 2e^– → Cu

Product at the anode:

• SO₄^2– and OH^– are both attracted to the anode

  • OH^– ions lose electrons more readily than SO₄^2-
  • OH^– lose electrons and are oxidised to form oxygen gas
  • The half equation for the reaction at the anode is

4OH^– ⟶ O₂ + 2H₂O + 4e^–

Using copper electrodes:

Apparatus for the electrolysis of copper(II)sulfate using active copper electrodes

Observations at the anode and cathode:

• The blue colour of the copper(II) sulfate solution remains unchanged

• The cathode increases in mass as it is coated with a layer of copper

  • This is because copper ions, Cu²⁺, are reduced at the cathode and form copper atoms

• The anode decreases in mass as it dissolves

  • This is because copper atoms are oxidised at the anode and form copper ions, Cu²⁺

• The gain in mass by the negative electrode is the same as the loss in mass by the positive electrode

  • Therefore, the copper deposited on the negative electrode must be the same copper ions that are lost from the positive electrode

• This implies that the concentration of the Cu²⁺ ions in the solution remains constant

Products formed for common aqueous solutions