Concept:In brine electrolysis with a mercury cathode, sodium ions are discharged instead of hydrogen because mercury has a high hydrogen overvoltage.
Explanation:At the graphite anode, concentrated chloride ions are preferentially oxidized.
The anode reaction is:
2Cl−→Cl2+2e−.
So chlorine gas is produced at the anode.
At the mercury cathode, sodium ions are reduced to sodium metal.
The cathode reaction is:
Na++e−→Na.
The sodium dissolves in mercury to form sodium amalgam.
Hydrogen gas is not liberated at the cathode because mercury prevents water from being reduced.
Therefore, the anode product is chlorine and the cathode product is sodium.
Answer:C. chlorine and sodium.