Blue copper(II) chloride solution is electrolysed using carbon electrodes, as shown. A pinkish-brown layer slowly builds up on one electrode. Which product ...
Question 1 Report
Blue copper(II) chloride solution is electrolysed using carbon electrodes, as shown. A pinkish-brown layer slowly builds up on one electrode. Which product is deposited on the cathode during this electrolysis?
Answer Details
Copper(II) chloride solution contains blue \(\mathrm{Cu^{2+}}\) ions and \(\mathrm{Cl^-}\) ions. The positive copper ions move to the negative electrode and gain electrons there, \(\mathrm{Cu^{2+}} + 2e^- \rightarrow \mathrm{Cu}\), so the pinkish-brown layer building up on the cathode is copper. Its colour matches the metal.
Chlorine is made at the positive anode by \(2\mathrm{Cl^-} \rightarrow \mathrm{Cl_2} + 2e^-\) and is a pale green gas, so it could not form a solid layer. Hydrogen is not discharged because copper is below hydrogen in the reactivity series, and oxygen would need hydroxide ions to be discharged in preference to the far more easily oxidised chloride.
Use the colour clue: a pinkish-brown deposit at a cathode is almost always copper.
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