ELECTROLYSIS WITH REACTIVE ELECTRODES

The discharge of ions during electrolysis can be influenced by the types of electrodes used.

Electrodes help in the conduction of electrons into and out of the cell as well as provide the surface for electrode reactions. They are made up of electric conductors like metals or graphite. They may be in the form of rods or as a surface coating on the rods of other material or as a coating on the inside surface of the electrolytic cell.

Some electrodes are inert (do not take part in electrolytic reaction) they are just conductors of electricity to transfer electrons. Platinum or carbon electrodes are examples of inert electrodes.

While other electrodes are reactive (which may influence the ionic discharge). They be oxidized and are made from most of the other metals. These electrodes are oxidised before anions.

One example of reactive electrodes changing the ionic discharge will be the electrolysis of Copper (II) sulphate using copper electrodes.

Recall: If the electrolysis of copper (II) sulphate is done using carbon electrodes, oxygen is released at the anode and copper is deposited at the cathode.

4^th EXAMPLE:

THE ELECTROLYSIS OF COPPER SULFATE SOLUTION USING COPPER ELECTRODES

Copper is a good conductor of electricity, and is used extensively to make electrical wiring and components. The extraction of copper from copper ore is done by reduction with carbon. However, the copper produced is not pure enough for use as a conductor, so it is purified using electrolysis.

In this process, the positive electrode (the anode) is made of the impure copper which is to be purified. The negative electrode (the cathode) is a bar of pure copper. The two electrodes are placed in a solution of copper(II) sulfate.

Reaction at ANODE:

· It¢s the copper anode that is the crucial difference than electrolysing copper sulfate solution with an inert (carbon/graphite/platinum) electrode.

· However, neither OH⁻ nor SO₄²⁻ ions are discharged as the electrode is an active (copper) electrode.

· The copper electrode loses electrons more readily than OH⁻ and SO₄²⁻ ions.

· Each Cu atom loses two electrons to form Cu²⁺ ion.

· The copper anode will dissolve away to form Cu²⁺ ions and enter the solution: Cu(s) → Cu²⁺(aq) + 2e−

Reaction at CATHODE:

· Copper ions from the solution are deposited at the cathode as copper atoms:

Cu²⁺ + 2e− → Cu(s)

· The cathode of pure copper gets thicker as the anode loses mass.

· The impurities, which contain precious metals, collect at the bottom of the cells.

Overall Reaction:

· Concentration of copper (II) sulphate (electrolyte) remains unchanged.

· Copper (II) sulphate solution remains blue in colour because Cu deposited = Cu dissolved. Both involve a two electron transfer so it means mass of Cu deposited = mass of Cu dissolving for the same quantity of current flowing (flow of electrons). You can check this out by weighing the dry electrodes before and after the electrolysis has taken place.

· Copper from the anode is transferred from the anode to the cathode.

Additional Use:

· This method can be used to refine copper.

· Impure copper is used as the anode while a piece of pure copper is used as the cathode.

· Pure copper from the impure copper anode will dissolve into the electrolyte, and pure copper is then deposited onto the copper cathode.

· Impurities are left on the anode, which will then fall off to collect below the anode.