A student electrolysed 50 cm³ of blue aqueous copper(II) sulfate using two carbon electrodes and a direct current. The student was asked to record the obser...
Question 1 Report
A student electrolysed 50 cm³ of blue aqueous copper(II) sulfate using two carbon electrodes and a direct current. The student was asked to record the observations at each electrode and any change to the solution.
(a) Describe what the student would see at the cathode. [2]
(b) Describe what the student would see at the anode. [2]
(c) Describe a test the student could use to show that the gas at the anode is oxygen, and give the result. [2]
(d) State and explain what happens to the blue colour of the solution during the experiment. [2]
Answer Details
This question tests the products of electrolysing aqueous copper(II) sulfate with inert carbon electrodes. Copper ions are discharged at the cathode and oxygen is released at the anode, so the blue solution gradually loses its colour.
(a) At the cathode the student sees a pink / reddish-brown solid [1] that coats or is deposited on the electrode; this is copper metal [1]. The reaction is \(Cu^{2+} + 2e^{-} \rightarrow Cu\): copper ions gain electrons and are discharged in preference to hydrogen because copper is below hydrogen in the reactivity series.
(b) At the anode the student sees bubbles of a colourless gas [1] being given off; this gas is oxygen [1]. Hydroxide ions from the water are discharged: \(4OH^{-} \rightarrow O_2 + 2H_2O + 4e^{-}\).
(c) To show the anode gas is oxygen [2]: insert a glowing splint into the gas [1]; the splint relights / bursts into flame [1]. Relighting a glowing splint is the specific test for oxygen.
(d) The blue colour fades / becomes paler [1] because copper(II) ions (\(Cu^{2+}\)) are removed from the solution as copper is deposited at the cathode [1]. The blue colour is due to \(Cu^{2+}\) ions, so as they are used up the solution becomes less blue.
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