The table below shows a monitoring record from a coastal plant that makes sodium hydroxide solution for cleaning equipment. The plant pumps in concentrated ...
Question 1 Report
The table below shows a monitoring record from a coastal plant that makes sodium hydroxide solution for cleaning equipment. The plant pumps in concentrated sodium chloride solution, also called brine. It uses a membrane cell so that substances formed at opposite electrodes do not mix. Chlorine gas is sent to a nearby water-treatment unit, where it is used to disinfect water.
| measurement after 20 minutes | value |
|---|---|
| volume of chlorine collected at the positive electrode | 240 cm3 |
| volume of hydrogen collected at the negative electrode | 238 cm3 |
| temperature of sodium hydroxide solution leaving cell | 31 degrees C |
Table 1
Fig. 1 shows the cell used for the record in Table 1.
(a) Give the name of the gas made at the positive electrode. [1]
(b) Describe why the membrane is needed in this cell. [2]
(c) Use the ions in brine to give the half-equation at the positive electrode. [2]
(d) Describe how hydroxide ions are formed at the negative electrode. [2]
(e) Use Table 1 to state whether the volumes of the two gases support a 1:1 volume ratio. [2]
(f) Give one reason why the temperature of the solution may be checked during production. [2]
Answer Details
Membrane cell electrolysis of brine
- (a) The gas at the positive electrode is chlorine. [1]
- (b) The membrane keeps chlorine separate from the sodium hydroxide solution and hydrogen. This prevents chlorine reacting with sodium hydroxide to form bleach/chlorate-containing products. [2]
- (c) At the positive electrode, chloride ions are oxidised: \[\mathrm{2Cl^- \rightarrow Cl_2 + 2e^-}\] [2]
- (d) Water gains electrons, so it is reduced. The half-equation is: \[\mathrm{2H_2O + 2e^- \rightarrow H_2 + 2OH^-}\] Thus hydroxide ions form at the negative electrode. [2]
- (e) Yes. The volumes, 240 cm3 chlorine and 238 cm3 hydrogen, are very similar. The 2 cm3 difference is reasonably explained by gas loss or measurement uncertainty. [2]
- (f) Temperature may be checked because it affects reaction/production rate and high temperature can damage equipment or the membrane. Maintaining temperature can also keep product quality and concentration consistent. Any two valid reasons. [2]
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