Chemistry 9701/11 — October/November 2010

Background Concept

In the electrolysis of concentrated sodium chloride solution (brine), the ions present are Na+, Cl-, H+ and OH-. At the anode, chloride ions are oxidised to chlorine gas: 2Cl- -> Cl2 + 2e-. At the cathode, hydrogen ions are reduced to hydrogen gas: 2H+ + 2e- -> H2. The remaining hydroxide ions, together with sodium ions, form sodium hydroxide solution.

A diaphragm cell uses a porous barrier between the anode and cathode compartments. This barrier allows ions to pass through to complete the electrical circuit, but it prevents the products formed at the two electrodes from mixing.

Understanding the Question

The question asks for the purpose of the diaphragm specifically. It is not asking about safety, pressure, or electrode design. The diaphragm is a physical separator inside the cell, so its role must be related to keeping the electrode products apart.

Approach

Recall the products of brine electrolysis and think about what would happen if they came into contact. Chlorine gas is produced at the anode and sodium hydroxide is produced at the cathode. These two substances react together, so the cell must be designed to keep them separate.

Step-by-Step Reasoning

1. At the anode, chloride ions are oxidised to chlorine gas.
2. At the cathode, water is reduced to hydrogen gas and hydroxide ions, so sodium hydroxide is formed.
3. If chlorine and sodium hydroxide mix, they react, for example chlorine disproportionates with alkali to form chloride and chlorate(I) ions.
4. The diaphragm prevents this mixing while still allowing ions to move between the electrodes.
5. Therefore, the purpose of the diaphragm is to stop the products of electrolysis from reacting together.

Key Takeaways

The diaphragm or membrane in an industrial electrolysis cell is used to separate the products. This improves the yield and prevents unwanted side reactions between chlorine and sodium hydroxide.

Common Mistakes

• Choosing A: the diaphragm does not stop chlorine escaping into the factory; the cell is closed for that purpose.
• Choosing B: the diaphragm is not a pressure-control device.
• Choosing C: the diaphragm is not an electrode and does not provide a large surface area.

Things to Be Careful About

The diaphragm is permeable to ions, so the electrical circuit is maintained, but it is not a complete barrier to gases or bulk liquid movement. Its function is product separation, not gas containment or pressure regulation.