The electrolysis of copper(ii) sulfate solution using inert electrodes
4 marks · NormalCopper(II) sulfate solution is electrolysed using inert electrodes for a long time.
Describe what happens to the colour of the solution.
Explain why these changes occur.
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(a) The blue colour fades and the solution becomes colourless.
(b) The blue colour is due to Cu^{2+} ions. As the electrolysis continues, these ions are removed from the solution to form copper metal at the cathode. When all the Cu^{2+} ions have been removed, the solution is left with mainly H^+ and SO_4^{2-} ions, which are colourless.
Common mistakes: Confusing which ions cause the colour
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Marking points
B1 (a) · 1 mark · Mentions blue colour fades/disappears
B2 (a) · 1 mark · States solution becomes colourless
M1 (b) · 1 mark · States blue colour is due to Cu^{2+} ions and these ions are removed
A1 (b) · 1 mark · States solution is left with H^+ and SO_4^{2-} (dilute sulfuric acid)
Worked solution
a) The blue colour of the solution gradually fades and the solution eventually becomes colourless.
b) The blue colour comes from copper(II) ions (Cu^{2+}). As electrolysis continues, Cu^{2+} ions are removed from solution and deposited as copper at the cathode. When all Cu^{2+} has been removed, the remaining solution contains H^+ and SO_4^{2-} ions, which is dilute sulfuric acid and is colourless.