Chlorine, hydrogen and sodium hydroxide are used as source materials for various other widely used compounds. The world requires these chemicals in large quantities - 39 million tonnes of chlorine, 36 million tonnes of hydrogen and 41 million tonnes of sodium hydroxide per year. This demand is met by the electrolysis of sodium chloride. Sodium chloride is found in large quantities, particularly in the ocean, and can be easily extracted and purified. Sodium chloride is an ionic compound formed with the metal sodium and the non-metal chlorine. Below shows the dot-cross diagram for the formation of sodium chloride:.
Diagram 1: Formation of sodium chloride .
To convert sodium chloride into hydrogen, chlorine and sodium hydroxide, we must electrolyse sodium chloride. The sodium chloride is dissolved in water and two electrodes with a potential difference are placed into the solution. Two reactions occur when this process is applied; one at the anode (positive electrode) and one at the cathode (negative cathode). Chlorine ions are attracted to the anode as the chlorine ion is negatively charged and is therefore attracted to a positive charge. At the electrode the chlorine ion is discharged, becoming a chlorine atom and free electrons. At the cathode, water molecules are combined with electrons to form hydrogen and hydroxide ions. The reactions are shown below:.
2Cl‾ (aq) Ã Cl2 (g) + 2e‾.
2H2O (l) + 2e‾ Ã H2 (g) + 2OH‾ (aq).
"Oxidation is the loss of electrons- . In the above anode reaction we can see that electrons are lost making this an oxidising reaction. Reduction is the opposite of oxidation; reduction is the gain of electrons. The above cathode reaction shows that electrons are gained to form hydrogen and hydroxide ions, identifying this as a reduction reaction.
As both of these reactions occur at the same time, the whole reaction can be called a reduction-oxidation reaction (redox reaction).