Electrolysis to Extract Metals

Page 1

Basic Principles of Electrolysis

Electrolysis: Chemical decomposition produced by passing an electric current through a liquid or solution containing ions.

Electrolyte: An ionic substance with freely moving ions. Electrolytes conduct electricity.

Cation: A positively charged ion

Anion: A negatively charged ion

Electrolysis uses electricity to break down an electrolyte.

Two electrodes (usually a carbon or a metal) are connected to a d.c. current and placed in the electrolyte.

The positive electrode is called the anode

The negative electrode is called the cathode

The ions migrate towards the electrode with the opposite charge

Cations collect at the cathode

Anions collect at the anion

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Electrolysis of Copper Sulfate

During the electrolysis:

Copper atoms in the anode lose two electrons to become copper ions

These ions are attracted to the cathode, where they gain two electrons and are deposited as pure copper

Anode loses mass, cathode gains mass

Impurities from the ions collect as a sludge beneath the anode

Half equations:

Anode: \(Cu(s) \rightarrow Cu^{2+} (aq) +2e^-\)

Cathode: \(Cu^{2+} (aq) +2e^- \rightarrow Cu(s)\)

Electroplating

A technique using electrolysis to coat one metal with a thin layer of the other

In this case the electrodes are referred to as non-inert.

If this experiment were to be repeated using carbon or platinum electrodes, then the electrodes would be intert.

The cathode would show a salmon pink discolouration which is a thin layer of copper atoms.

When using copper electrodes, include variable resistor in experimental set up

Wash electrodes in distilled water and then dip them in propanone before use

Record the masses of the dry electrodes before the start of the experiment

Page 3

Manufacture of Aluminium

Very reactive metallic elements are extracted from ores by electrolysis

Because they are higher up in the reactivity series than carbon, they cannot be displaced from their oxides by carbon

The usual source of aluminium is the ore, bauxite.

This is treated chemically to leave a white aluminium oxide powder, \(Al_2O_3\), commonly known as alumina

Alumina has a very high melting point .

Therefore, in order for it to be electrolysed, it has to be dissolved in molten cryolite

Acts as a solvent for the alumina

Cryolite also has a high melting point

Cathode:

Attracts Al ions where they gain electrons to form aluminium metal

\(Al^{3+}+3e^- \rightarrow Al\)

Anode:

Attracts oxide ions

Lose electrons and form oxygen gas

Oxygen gas reacts with carbon anodes, so they have to be replaced from time to time

\(2O^- \rightarrow O_2 + 4e^-\)