A student electrolysed two solutions of sodium chloride, one dilute and one concentrated, each with carbon electrodes and a direct current. Table 10.1 is fo...
Question 1 Report
A student electrolysed two solutions of sodium chloride, one dilute and one concentrated, each with carbon electrodes and a direct current. Table 10.1 is for recording the products.
| solution electrolysed | product at cathode | product at anode |
|---|---|---|
| dilute sodium chloride | ? | ? |
| concentrated sodium chloride | ? | ? |
(a) Complete Table 10.1 to show the product formed at each electrode for each solution. [4]
(b) State how the product at the anode differs between the two solutions. [1]
(c) Suggest why the product at the cathode is the same for both solutions. [1]
Answer Details
This question compares the electrolysis of dilute and concentrated sodium chloride solution. The cathode product is the same, but the anode product changes with concentration, which is a common exam trap.
(a) The completed table:
| solution electrolysed | product at cathode | product at anode |
|---|---|---|
| dilute sodium chloride | hydrogen | oxygen |
| concentrated sodium chloride | hydrogen | chlorine |
Dilute: cathode = hydrogen [1], anode = oxygen [1]. Concentrated: cathode = hydrogen [1], anode = chlorine [1].
(b) The anode product differs: the dilute solution gives oxygen, while the concentrated solution gives chlorine [1]. When the chloride ions are concentrated enough, chloride is discharged in preference to the hydroxide ions from water.
(c) The cathode product is hydrogen in both cases because hydrogen (from water) is less reactive than sodium, so hydrogen ions are discharged in preference to sodium ions, \( \mathrm{2H^+ + 2e^- \rightarrow H_2} \) [1]. The very reactive sodium stays in solution as Na+ ions.
Exam tip: for a reactive metal such as sodium, potassium or calcium in aqueous solution, the cathode always gives hydrogen, not the metal.
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