Chemistry 3.30 - 3.31 - Copper electrolysis and purification

One solution, two electrode materials

An electrolysis cell uses a direct current (DC) supply to drive chemical changes in an electrolyte. In this cell, the cathode is negative and the anode is positive. Copper sulfate solution contains mobile copper ions, $\ce{Cu^2+}$, sulfate ions and particles from water.

The electrode material matters. Graphite is treated as inert: it conducts charge but does not supply the main products. A copper electrode is active: copper atoms in the electrode can become copper ions and enter the solution.

The words cathode and anode name positions in the cell, not materials. A graphite rod and a copper strip can each be a cathode if connected to the negative terminal.

Following the copper

The visible changes make sense when the copper is followed at particle level.

With either electrode material, positive copper ions in the electrolyte are attracted towards the negative cathode. At its surface, the ions are discharged and become neutral copper atoms. Those atoms join the solid electrode, producing the reddish-brown coating and increasing its mass.

What happens at the anode depends on its material:

• At a graphite anode, the graphite does not replace the copper ions removed at the cathode. Oxygen bubbles form at the anode instead. As electrolysis continues, fewer copper ions remain, so the blue solution becomes paler.
• At a copper anode, copper atoms leave the solid and become copper ions in the electrolyte. This makes the anode lighter. At about the same time, copper ions from the solution become copper atoms at the cathode, making it heavier.

The copper-electrode cell therefore transfers copper from the anode, through the electrolyte's pool of copper ions, to the cathode. It is better not to imagine one named ion travelling all the way from one strip to the other. The measurable evidence is that the anode loses mass while the cathode gains a similar mass and the solution's copper-ion concentration stays approximately constant.

Core practical: collecting dependable evidence

The core practical has two linked sections. The graphite trial identifies products from observations. The copper trial measures how current affects electrode mass change.

Apparatus and arrangement

Use copper sulfate solution, a beaker, a low-voltage DC supply, connecting leads with crocodile clips and either two graphite rods or two copper strips. A small gas-collection tube and glowing splint allow the graphite-anode gas to be tested. The quantitative copper setup also needs an ammeter in series, a variable resistor, a stopwatch, emery paper, distilled water, labels and a balance reading to at least $0.01\,\mathrm{g}$.

Both electrodes dip into the same solution without touching. Connect the cathode to the negative terminal and the anode to the positive terminal. Keep the crocodile clips above the solution so the clips do not contaminate it.

Section A: observations with graphite electrodes

1. Add enough fresh copper sulfate solution to cover the lower parts of two clean graphite rods.
2. Secure the rods a few centimetres apart. Connect one to the negative terminal and the other to the positive terminal of the switched-off DC supply.
3. Switch on the supply and observe both electrodes. Record the location, colour and state of every product rather than writing only "a reaction happened".
4. The expected cathode evidence is a reddish-brown solid coating: copper. The expected anode evidence is bubbles: oxygen. If enough anode gas is collected in an inverted small tube, a glowing splint relights.
5. Switch off before removing or changing any connection. Record whether the solution becomes paler after sustained electrolysis.

Section B: current and mass change with copper electrodes

The investigable question is: how does current affect the mass change of copper electrodes during a fixed time?

For each current:

1. Rub two copper strips with emery paper to remove surface contamination. Label them anode and cathode, rinse with distilled water, allow them to dry fully, then record each initial mass.
2. Add a measured volume of fresh copper sulfate solution of the same concentration to the beaker. Fix both strips at the same depth and separation used for every run.
3. With the supply off, connect the labelled cathode to negative and the labelled anode to positive. Include the ammeter and variable resistor in series.
4. Switch on, start the stopwatch and adjust the variable resistor to the chosen current. Check and readjust throughout the $20\,\mathrm{min}$ so the current stays constant.
5. Switch off before removing the strips. Gently rinse them with distilled water. Do not wipe or scrub the new deposit, because that would remove copper.
6. Leave each labelled strip to dry completely, then measure its final mass using the same balance.
7. Repeat at least three times at that current. Repeat the complete method for all four currents, using freshly prepared conditions each time.

Four currents with three $20\,\mathrm{min}$ repeats are too long for one group in one lesson. A workable class design assigns the same current to at least three groups, assigns other currents to other groups and pools the results. Every group must use the same controlled method; otherwise differences between groups could be mistaken for an effect of current.

Safety that matches the hazards

• Wear eye protection. Copper sulfate solution at the concentration used in the school method is low hazard, but it can harm if swallowed and can irritate; avoid skin and eye contact, clean spills promptly, wash hands and follow local disposal instructions.
• Use only a low-voltage DC supply. Switch it off before moving leads or electrodes, and keep the electrodes apart: contact would short-circuit the cell, heat the circuit and invalidate the current.
• The safer drying route is to leave rinsed, labelled electrodes until fully dry. If a teacher directs rapid drying with propanone, use it in a fume cupboard, wear eye protection and keep it away from flames, sparks and hot equipment because propanone is highly flammable.