Electrolysis of Aqueous Solutions (WJEC GCSE Chemistry)

Electrolysis of Aqueous Solutions

What is formed at the anode and cathode

• Aqueous solutions will always have water present

• Some water molecules split up into hydrogen and hydroxide ions, H⁺ and OH^–, which participate in the electrolysis reactions

H₂O (l) H⁺ (aq) + OH^– (aq)

Products at the anode

• Negatively charged ions are attracted to the positive electrode, the anode

• When the aqueous electrolyte undergoes electrolysis the OH^– and any other negative ions (e.g. Cl^–, Br^–) in solution are attracted to the anode and they will lose electrons (be oxidised)

  • E.g. 2Cl^– → Cl₂ + 2e^–

Products at the cathode

• Positively charged ions are attracted to the negative electrode, the cathode

• When the aqueous electrolyte undergoes electrolysis the H⁺ and any other negative ions (e.g. Cu²⁺, Ag⁺) in solution are attracted to the anode and they will gain electrons (be reduced)

• Only metals that are below hydrogen in the reactivity series will form at the cathode

  • Cu²⁺ + 2e^– → Cu

Table of products when solutions undergo electrolysis

• Gases formed at the anode and cathode are collected using inverted test tubes

Collecting gases formed in electrolysis

The gases that are collected using the inverted test tubes are filled with water. The gases will displace the water

Competing Ions

Higher Tier

Rules for identifying products formed at electrodes

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 gas

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

Negative Electrode - Cathode

• H⁺ ions and metal ions are attracted to the negative electrode but only one will gain electrons

• Either hydrogen or a metal will be produced

• If the metal is above hydrogen in reactivity series, hydrogen will be produced – bubbling will be seen at the cathode

The reactivity series

The reactivity series of metals enables chemists to predict the products at the cathode in the electrolysis of aqueous solutions

Using Named Electrolytes

• The products at the electrodes from solutions of copper chloride and sodium chloride

The Products of Electrolysing Aqueous Solutions

Explaining the products:

• Copper chloride:

  • Copper is below hydrogen so copper(II) ions are preferentially discharged at the cathode; chlorine is a halogen, so is preferentially discharged at the anode

• Sodium chloride:

  • Sodium is above hydrogen so hydrogen ions are preferentially discharged at the cathode; chlorine is a halogen, so is preferentially discharged at the anode