Fig. 1 shows part of a membrane cell used at a water-treatment works. Concentrated sodium chloride solution, also called brine, flows into the cell. A direc...
Question 1 Report
Fig. 1 shows part of a membrane cell used at a water-treatment works. Concentrated sodium chloride solution, also called brine, flows into the cell. A direct current passes through the electrodes. Chlorine gas is collected at the positive electrode. The membrane allows sodium ions to pass through but keeps most products in separate sections. The alkaline solution leaving the negative electrode is used in some cleaning processes.
(a) Name the gas formed at the positive electrode. [1]
(b) Give the ionic half-equation for the reaction producing this gas. [2]
(c) Complete the statement about the negative electrode: hydrogen is formed from water, and the remaining solution contains sodium hydroxide. Give the ionic half-equation for hydrogen formation. [3]
(d) Explain why the membrane is needed in this electrolysis cell. [4]
Answer Details
(a) Chlorine is formed at the positive electrode. [1]
(b) Chloride ions lose electrons at the positive electrode:
\[\mathrm{2Cl^-\rightarrow Cl_2+2e^-}\]
[2]
(c) At the negative electrode, water gains electrons:
\[\mathrm{2H_2O+2e^-\rightarrow H_2+2OH^-}\]
This forms hydrogen and hydroxide ions. Sodium ions in that compartment combine with hydroxide ions in solution to give sodium hydroxide. [3]
(d) The membrane keeps chlorine separate from hydrogen, preventing their mixing. It also prevents chlorine reacting with sodium hydroxide. However, it allows sodium ions to move to the negative-electrode compartment, maintaining charge balance so current can continue. Any four stated marking points gain [4].
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