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

Which of the following are the ionic half-equations for the electrolysis of sodium chloride solution?

1. 2H⁺ + 2e⁻ -> Cl₂ and 2Cl⁻ -> H₂ + 2e⁻

2. 2H⁺ + 2e⁻ -> H₂ and 2Cl⁻ -> Cl₂

3. H₂ + 2e⁻ -> 2H⁺ and Cl₂ -> 2Cl⁻ + 2e⁻

4. 2H⁺ -> H₂ + 2e⁻ and 2Cl⁻ + 2e⁻ -> Cl₂

The correct answer is: 2H⁺ + 2e⁻ -> H₂ and 2Cl⁻ -> Cl₂

The correct answer provides the appropriate ionic half-equations that illustrate the reduction and oxidation processes occurring during the electrolysis of sodium chloride solution, commonly referred to as brine. During electrolysis, at the cathode (where reduction occurs), hydrogen ions (H⁺) from the water are reduced to form hydrogen gas (H₂). The half-equation for this process is represented as 2H⁺ + 2e⁻ -> H₂. This shows that two hydrogen ions gain electrons (2e⁻) to produce one molecule of hydrogen gas. At the anode (where oxidation occurs), chloride ions (Cl⁻) are oxidized to form chlorine gas (Cl₂). The half-equation for this reaction is 2Cl⁻ -> Cl₂ + 2e⁻. In this case, two chloride ions lose their electrons (2e⁻) to generate one molecule of chlorine gas. Combining these two half-reactions correctly illustrates the overall process of electrolysis of sodium chloride solution, capturing both reduction and oxidation reactions in an accurate manner. The other options do not represent the correct ion transformations or the staging of electrons related to the electrolysis process, thus leading to incorrect representations of the chemical changes