Higher and foundation tiers

Electrolysis

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. Recall that 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 while non-metals when they react gain electrons and end up forming ions with a negative charge. We can summarise this as:

metal atom - electrons → positively charged metal ion, M⁺

non-metal atom + electrons → negatively charged non-metal ion, NM^-

Giant ionic lattices

3d model of the sodium bromide lattice. All ionic compounds have a giant lattice structure made up of positively charged metal ions and negatively charged non-metal ions with strong bonds between the metal and non-metal ions. The ionic lattice for the ionic compound sodium bromide is shown opposite. It contains positively charged sodium ions (Na⁺) and negatively charged bromide ions (Br^-) which having opposite charges are strongly attracted to each other. The ions are held tightly within the giant ionic lattice structure (shown opposite) 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 compound 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 (see image below). If a bulb is included in the circuit then it will light up when an electrical current is flowing. To begin with the bulb is unlit but as the sodium bromide begins to melt and becomes molten the metal and non-metal ions within the structure are free to move and the bulb now begins to glow as an electrical current 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 showing the formation of bromine gas at the anode and sodium metal at the cathode.

Explaining electrolysis

Electrolysis basically turns ions back into atoms. This means forcing electrons back onto positively charged metal ions and turning them back into neutral metal atoms; reducing them. At the same time electrons are pulled off the negatively charged non-metal ions; oxidising them and turning them back into non-metal atoms. The apparatus shown in the diagram 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 cell is called the cathode and the electrode connected to the positive terminal of the cell is called the anode. They are dipped in the electrolyte (a solution which conducts electricity); in this case the molten sodium bromide.

electrolysis of sodium bromide The cell or battery releases electrons from the negative terminal and these electrons travel to the cathode. However once there the positively charged sodium ions pick them off and are they reduced according to the equation below:

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 the sodium metal. However most of the sodium is likely to be seen as a liquid metal floating on top of the electrolyte, since the temperature of the electrolyte will be well above the melting point of the sodium metal formed.

At the positive anode the negatively charged bromide ions will have electrons stripped from them, these electrons will move onto the graphite anode and head back to the cell or battery to complete the circuit. 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 newly formed 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) → Br_2(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:

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 and 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 (CrO₄^2-). 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 CrO₄^2- 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

Metals are always produced at the cathode during electrolysis
Non-metals are always produced at the anode.
Reduction occurs at the cathode and oxidation occurs at the anode.
Electrolysis is a redox process.

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	2O^2- → 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 bleach 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.

Electrolysis

metal atom - electrons → positively charged metal ion, M⁺

non-metal atom + electrons → negatively charged non-metal ion, NM^-