International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What is the balanced ionic equation for the formation of oxygen during the electrolysis of water?

1. 4OH⁻ -> 2H₂O + O₂ + 4e⁻

2. 2H₂O -> O₂ + 2H₂ + 4e⁻

3. H₂O -> O₂ + H₂ + e⁻

4. 4OH⁻ + 4e⁻ -> 2H₂O + O₂

The correct answer is: 4OH⁻ -> 2H₂O + O₂ + 4e⁻

The correct answer describes the electrochemical process occurring at the anode during the electrolysis of water. In this process, hydroxide ions (OH⁻) are oxidized, and oxygen gas (O₂) is formed along with water (H₂O). To balance the reaction accurately, one must understand the stoichiometry involved. The equation shows that four hydroxide ions lose four electrons and produce two molecules of water and one molecule of oxygen. This reaction is consistent with the overall reaction for the electrolysis of water, which indicates that water is being split into its elemental components through the application of electrical energy. This balanced ionic equation is significant because it reflects the conservation of mass and charge, ensuring that the number of atoms and charge on each side of the equation is equal. This means that for every four hydroxide ions that react, two water molecules are formed alongside one molecule of oxygen, while also accounting for the electrons that facilitate the process. The other options either do not correctly represent the stoichiometry of the reaction or do not take into account the proper oxidation states and the flow of electrons involved in the electrolysis process. Understanding the specific roles of the ions and molecules in the reaction will help reinforce the concept of redox reactions