Electrolysis

Electrolysis:

The process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

Used to extract metals higher than carbon in the reactivity seires

Electrodes: A conductor which allows current to flow through an electrolyte

Electrolyte:  A chemical compound that dissociates in solution into ions

Positive

Anode

Negative

Is

Cathode          

Postive electrodes are called anodes. The ions are called anions. Reduction occurs at the anode.

Negative electrodes are called cathodes. The ions are called cations. Oxidation occurs at the cathode.                                                                                                     

In order to carry out electrolysis the compound must either be molten or in solution so that the ions can move to the relevant electrodes.

The electrolye has to be an ionic compound, so there are charged ions.

Covalent compounds can not be electrolysed as they are not ions and therefore do not have a charge.

Molten compound

If the compound is molten it is easy to determine what will be formed at each electrode. Non metals go to the anode . Metals go to the cathode.

Solution

If the compound is solution then the water also forms ions. If the metal is less reactive than hydrogen then metal forms at the cathode. If it is more reactive than the hydrogen forms at the cathode and the metal stays in solution.

Aluminium is extracted from an ore called Bauxite which contains aluminim oxide. 

Bauxite is mixed with cryolite to reduce the temperature required to keep the ore molten. However very high temperatures are still required so furnaces are used to heat the mixture. 

The aluminium ions are reduced and gain 3 electrons whilst the oxygen is oxidiesed and loses 2 electrons.

.

Brine is a solution of sodium chloride in water.

Brine contains sodium, chlorine, hydrogen and hydroxide ions.

When electrolysed it forms hydrogen at the cathode, chlorine at the anode and sodium hydroxide in solution.

Uses for products

Chlorine is used to kill bacteria (drinking water, swimming pools) and to make bleach, disinfectant and plastic.

Hydrogen is used to make margarine and hydrochloric acid.

Sodium hydroxide is a strong alkali and has many uses in soap, papermaking , neutrilising acis ad controlling pH. 

Electroplating is to:

• protect a metal
• make something more attractive
• increase an object more resistant to scratching
• cheaper

The solution contains ions of the of the plating metal

Half equations for electroplating with zinc

• Anode: Zn ---> Zn+2 + 2e-
• Cathode: Zn2+ + 2e- ----> Zn