Electrolysis Of Copper (II) Sulphate Solution

This experiment enables students to carry out the electrolysis of copper(II) sulfate solution and to link their findings with the industrial electrolytic refining of copper.
 

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This class experiment can be done by students working either in pairs or threes.

CHEMICALS

Aqueous copper(II) sulfate, about 0.5 M, 200 cm³

Copper strips (optional), 2 (Note 3)

Small pieces of emery paper

Refer to Health & Safety and Technical notes section below for additional information.

APPARATUS

Eye protection

Each group of students will require:

Beaker (250 cm³)

Graphite electrodes (about 5 mm diameter), 2

Retort stand and clamp to hold electrodes (Note 1)

DC power supply (6 volt)

Light bulb (small, 6 volt, 5 watt) - optional
(Note 2)

Leads and crocodile clips 

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Read our standard health & safety guidance

Wear eye protection. Students must wash their hands at the end of all practical work. 

Copper(II) sulfate solution, CuSO₄(aq) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. At the suggested concentrations, the copper(II) sulfate solution is LOW HAZARD If the concentrations are increased, the solutions must be labelled with the correct hazard warnings. Copper(II) sulphate solution is HARMFUL if concentration is equal to or greater than 1M.

1 There are several ways of securing the graphite electrodes. Using a retort stand and clamp is probably the most convenient. They can also be fixed using Blutac on to a small strip of wood resting on the top of the beaker.

2 A bulb can be included in the circuit to indicate that there is a flow of current.

3 As an extension to the basic experiment, strips of copper can be used in place of the graphite rods.

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a Ask the students to set up the cell as shown. They should watch for any activity on each of the electrodes, and write down their observations.

The cathodes can be cleaned using emery paper.

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Students should see a deposit of copper forming on the cathode. This will often be powdery and uneven. You should explain that, if the current used is much lower, then the solid coating is shiny, impermeable and very difficult to rub off; this process forms the basis of electroplating.

Bubbles of gas (oxygen) are formed at the anode.

Cathode reaction: Cu²⁺(aq) + 2e^- → Cu(s)

Anode reaction: 2H₂O(l) → O₂(g) + 4H⁺(aq) + 4e^-

With carbon (graphite) electrodes, the oxygen usually reacts with the anode to form CO₂. If copper is used for the electrodes, the copper anode dissolves. The reaction is the reverse of the cathode reaction.

The results of this experiment can lead to a discussion about electroplating and the electrolytic refining of copper.

It can be instructive to allow students to copper-plate metal objects supplied by the school and previously tested for their suitability. Personal items should not be used. In many cases, an alternative redox reaction often takes place before any current is actually passed. This happens for instance in items made of metals above copper in the reactivity series. It is wise not to complicate electrolytic deposition with chemical displacement - valued articles can be effectively ruined.

Extension experiments for copper refining

1 After doing the electrolysis as described above, the electrodes can be interchanged. Students can then see the copper disappearing from the surface of the copper-coated anode

Cu(s) → Cu²⁺(aq) + 2e^-

This leads to a discussion as to why, during electrolytic refining:

• the anode consists of an unrefined sample of the metal
• the cathode is made of pure copper or a support metal such as stainless steel.
   

2 The electrolysis can be done using two weighed copper strips. This is to confirm that the mass gained at the cathode is equal to the mass loss at the anode.