alkanes chemical properties

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.

metal atom - electronspositively charged metal ion, M+
non-metal atom + electrons negatively charged non-metal ion, NM-

sodium bromide lattice 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 charges are 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 of molten sodium bromide

Explaining electrolysis

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.

electrolysis of 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.

Na+(l) + e Na(l)
sodium ions gain an electron and are reduced, they turn back into sodium atoms. The cathode will be coated in a thin layer of sodium metal. The electrolyte is hot, well above the melting point of the sodium, so the sodium produced will be a liquid. At the positive anode the negatively charged bromide ions will have electrons stripped from them. The bromide ions are oxidised to form bromine atoms. However bromine gas is a diatomic substance (see section on the halogens) and two of the bromine atoms will join to form a bromine molecule. We can show what happens at the anode in an ion-electron half-equation.
Br-(l) Br(g) + e
However bromine is a diatomic molecule so we need to multiply this equation by a factor of two:
2Br-(l) Br2(g) + 2e
This means that to balance the number of electrons lost by the bromide ions the half-equation for the cathode needs to be multiplied by 2:
2Na+(l) + 2e → 2Na(l)
The sodium ions gain electrons and are reduced at the cathode while at the anode the bromide ions lose electrons and are oxidised to form bromine gas. This is shown in the diagram below:

electrolysis of sodium bromide

Migration of ions during electrolysis

We can observe the movement of ions during the electrolysis of coloured compounds, for example copper chromate dissolves to form a green solution. If this green solution is added to a U-tube and electrolysed as shown in the diagram below we observe a pale blue colour at the cathode, the cathode is also covered in a brown furry solid. This brown solid covering the cathode is copper metal which is produced when the copper ions (Cu2+) are reduced at the cathode.
Cu2+(aq) + 2e Cu(s)
While at the positively charged anode a yellow colour and some bubbling is observed. electrolysis of copper chromate Copper chromate is an ionic compound containing blue copper ions (Cu2+) ions and yellow chromate ions (CrO42-). These two ions mix and the resulting solution is green. However when the solution is electrolysed the positively charged (Cu2+) ions are attracted to the negatively charged cathode while the yellow CrO42- ions are attracted to the positively charged anode. This simple demonstration is a good piece of evidence for the presence of ions in a solution.

Key points

Consider the anode and cathode products for the electrolysis of the following molten ionic compounds. The table also gives the ion-electron half equations (higher tier only) for the reactions at the electrodes and any relevant observations.
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.

Practice questions

Check your understanding - Questions on electrolysis