Diaphragm Cell

Question 1

What is the most important part of the diaphragm cell?

Answer 1

The most important part of this cell is the diaphragm, which separates the cathode and anode compartment.

Question 2

What is the diaphragm usually made out of? (2)

Answer 2

• The diaphragm is usually made of asbestos which is porous and allows the electrolyte to pass slowly from anode to cathode compartment.
• A pressure gradient is maintained to ensure that the flow of electrolyte remains from anode to cathode compartment and ‘back flow’ is limited.

Question 3

What is pumped in?

Answer 3

Concentrated brine is pumped into the anode compartment, where oxidation of the chloride ions will take place.

Question 4

What is oxidised?

Answer 4

Although oxidation of water is preferential under standard conditions, as discussed previously, the concentration of the NaCl ensures that chloride ions are oxidised in preference. [ ±0.3V] Chlorine gas is collected from the anode compartment and stored separately.

Question 5

What is reduced?

Answer 5

The electrolyte (now containing Na+ ions and water) moves into the cathode compartment, where water will be reduced to form H2 gas. H2 gas is collected from the cathode compartment and stored separately.

Question 6

What happens to the remaining Na+ ions and OH- ions?

Answer 6

The remaining Na+ ions and the OH- ions formed as part of the reduction of water, combine to form NaOH, which is tapped out of the cell.

Question 7

What are the limitations of the cell? (3)

Answer 7

o Asbestos is toxic and very dangerous to use and dispose of
o Cl- ions can pass through the diaphragm into the cathode compartment and contaminate the NaOH with NaCl. The NaOH must then be purified further before use, which is an added cost. (recrystaliser)
o Although the pressure gradient prevents much backflow, it is possible for the OH- ions produced to move back into the anode compartment and be oxidised to form O2. This contaminates the Cl2 collected, requiring further purification and cost.

Question 8

Why is the level of water on the left higher?

Answer 8

So that the liquid only flows in one direction.

Question 9

How can the rate of the reaction be increased?

Answer 9

Increase the surface area of an electrode.