Higher and foundation tier
Electrolysis (electro:electricity and lysis: splitting) as its name suggests is using electricity to break up Ionic compounds into the elements that make them up. Ionic compounds are made up of positively charged metal ions and negatively charged non-metal ions. Metals when they react tend to lose electrons and end up forming ions with a positive charge and non-metals when they react gain electrons and end up forming ions with a negative charge.
All ionic compounds have a giant lattice structure with
strong bonds between the metal and non-metal ions. The ionic lattice for the compound sodium bromide is shown opposite.
It contains sodium ions,
Na+ and bromide ions, Br- which having opposite
strongly attracted to each other. The ions are
held tightly within the lattice structure and are
NOT free to move. This
however creates a bit of a problem!
In order for electrolysis to work electricity must be able to flow through the substance, and SOLID ionic compound are electrical insulators, they do not conduct electricity. In order to conduct an electrical current the ions must be free to move and this is simply not the case in SOLID ionic compound. However if the solid ionic compoun is melted or dissolved in water then the giant ionic lattice is broken down and the ions are free to move, this means that molten ionic compounds (liquids) and solutions of ionic compounds will conduct electricity.
We can easily demonstrate this by placing solid sodium bromide in a crucible with two electrodes connected to a
battery pack. A bulb is included in the circuit, if it lights then an electrical current is flowing. To begin with
the bulb is unlit but as the sodium bromide melts and becomes molten the
ions within the structure are free to move, the
bulb now begins to glow as an electrical curren flows.
If you look carefully at the electrodes in the image below you will see that a brown, bleachy smelling gas is produced at the positive terminal (the anode) and a silvery grey metallic solid coats the negative electrode (the cathode).
Electrolysis basically turns ions back into atoms. This means forcing electrons back onto metal ions and turning them back into metal atoms, reducing them, and at the same time pulling electrons back-off non-metal ions, oxidising them and turning them back into non-metal atoms. The apparatus used above can be drawn out in a simple way to show what is happening in the cell (see image below). The electrodes in the cell are made of graphite, an inexpensive but good electrical conductor. Graphite also plays no part in any of the reactions which take place during electrolysis, that is it is inert and so it will not interfere with the electrolysis experiment. The graphite electrode connected to the negative terminal of the battery is called the cathode and the electrode connected to the positive terminal of the battery is called the anode. They are dipped in the electrolyte (a solution which conducts electricity), the molten sodium bromide.
The cell or battery releases electrons from the negative terminal, these electrons travel to the cathode. However once there the positively charged sodium ions pick them off and are reduced.
|molten ionic compound||cathode product||cathode half-equation||anode product||Anode half-equation||observations|
|potassium oxide||potassium||K+ + e → K||oxygen||2O2- → O2 + 4e||bubbling at anode as oxygen released.|
|lithium chloride||lithium||Li+ +e → Li||chlorine||2Cl- → Cl2 + 2e||green/yellow gas produced at anode. Smells of bleah and bleaches litmus paper white. Silvery metal produced at cathode.|
|copper iodide||copper||Cu2+ +2 e → Cu||oxygen||2I- → I2 + 2e||violet coloured vapours produced at the anode. Cathode coated in a bronze/brown coloured metal.|