Chemguide: Core Chemistry 14 - 16

The electrolysis of aqueous solutions of ionic compounds using non-inert electrodes

This page looks in detail at the electrolysis of copper(II) sulfate solution using copper electrodes and silver nitrate solution using a silver anode.

If you haven't recently done so you should first read the page about the electrolysis of solutions using inert electrodes - particularly looking at the electrolysis of copper(II) sulfate and silver nitrate solutions.

The electrolysis of copper(II) sulfate solution using copper electrodes.

Start by watching this bit of video which shows a quick lab demonstration.

At the cathode

Copper(II) ions and hydrogen ions arrive. Copper is below hydrogen in the reactivity series and so is discharged in preference to the hydrogen. A layer of brown copper builds up on the cathode.

Cu²⁺(aq) + 2e^-     Cu(s)

At the anode

Sulfate ions and hydroxide ions (from the water) arrive but neither is discharged.

Instead, copper atoms in the electrode release electrons and go into solution as Cu²⁺(aq) ions.

Cu(s)     Cu²⁺(aq) + 2e^-

The net effect

The copper(II) sulfate solution is unchanged. For every copper(II) ion discharged at the cathode, a new one is released into solution at the anode.

All that happens is a transfer of copper from the anode to the cathode.

The bit of video you just watched shows this happening.

Washing, drying and weighing the cathode also needs a lot of care so that none of the coating is dislodged.

Using this to purify copper

The next bit of video explains how this is done.

The only thing the video leaves out is an explanation of what happens to ions of metals above copper in the reactivity series (like zinc) when they get into the solution.

In fact nothing much happens to them! They will be attracted to the cathode, but aren't discharged when they get there. The copper(II) ions are discharged instead because they are lower in the reactivity series.

However, if the concentration of zinc ions gets too great, then it is possible some of these might discharge as well, making the pure copper cathode impure again. That's why the solution is changed from time to time.

Using a similar reaction in silver plating

In silver plating, you use a pure silver anode and whatever metal object you are trying to plate as the cathode.

It doesn't matter that you might be using a copper cathode rather than a silver one. All that happens at the cathode is that it gets covered in silver.

The electrolyte is silver nitrate solution.

At the cathode

Silver ions and hydrogen ions arrive. Silver is below hydrogen in the reactivity series and so is discharged in preference to the hydrogen. A layer of silver builds up on the cathode.

Ag⁺(aq) + e^-     Ag(s)

At the anode

Nitrate ions and hydroxide ions (from the water) arrive but neither is discharged.

Instead, silver atoms in the electrode release electrons and go into solution as Ag⁺(aq) ions.

Ag(s)     Ag⁺(aq) + e^-

The net effect

The silver nitrate solution is unchanged. For every silver ion discharged at the cathode, a new one is released into solution at the anode.

All that happens is a transfer of silver from the anode to the cathode.

The next bit if video shows this happening.