Section 1 i) Summary

Electrolysis is defined as chemical decomposition produced by passing an electric current through a liquid or solution containing ions. This process by which we can obtain the individual elements from an ionic compound has truly revolutionised the world.

Ionic compounds do not conduct electricity while solid. This is because the ions are not free to move. When aqueous, the ions are dissociated and therefore are able to move freely. When molten, the bonds between the ions are broken and they are able to move freely. Electricity is able to pass through because there are free-moving charged particles that can carry current.
Ionic compounds have very high melting points, and because of this the solid is often dissolved in water to create an aqueous electrolyte. When molten ionic compounds are electrolysed, the rules are simple: Metal goes to the cathode, Non-metal to the anode. But with aqueous solutions, it is less simple.

At the anode: If the non-metal is a halogen, it will form at the anode. If it is not, hydrogen gas will form: 2H+ + 2e- --> H2

At the cathode: If the metal is a low reactivity metal, it will coat the cathode. If it is a high reactivity metal, oxygen gas will form: 4OH- --> O2 + 4H2O + 4e-


The amount of a substance removed can be calculated by how much charge has passed through.
1 mole of electrons is equal to 1 faraday, which is 96500 coulombs.
First, you have to know the ionic half equation, the current passing through, and the time it takes.
Then, you can calculate the charge with this equation:
Charge (in coulombs) = Current (in amps) x Time (in seconds)
Q = I T
Divide the charge you have just calculated by 96500 to get the number of faradays, which is how many moles of electrons transferred.
Then look at the equation to see how many moles of electrons there are for each mole of the product, and divide accordingly.
If you are trying to find the mass, multiply by Mr, and you will have mass in grams
If you are trying to find the gas volume, multiply by 24000 to get volume in cm3.
See examples in the specification here

Industry
One of the most common industrial uses of electrolysis is the electrolysis of brine, or sodium chloride. This is called chlor-alkali industry.

The products of this reaction are used for many different things:

Chlorine: killing bacteria, making bleach, making hydrochloric acid, etc.

Hydrogen: used in making ammonia, making margarine, etc.

Sodium hydroxide: Making soap, paper, ceramics, etc.

The ionic half-equations are:

2Cl- --> Cl2 + 2e-

2H+ + 2e- --> H2

Because two gases are formed, a diaphragm is put in place between the two electrodes to prevent them from mixing and reacting.