Electrolysis

Electrolysis is a chemical change caused by passing an electric current through a compound which is either molten or in solution. An electric current (in chemistry terms), is a flow of electrons or ions. An electrolyte is a substance that undergoes electrolysis. Electrolytes all contain ions. Ionic compounds, for example, are electrolytes. Electrolytes can only undergo electrolysis when molten or in a solution, where the ions are free to move. Covalent compounds are not electrolytes because they don’t contain ions.

Electrolysis can form new substances when ionic compounds conduct electricity. It is set up as so:

The electrodes are usually made of carbon because it is fairly un-reactive.
The positive electrode is called the anode.
The negative electrode is called the cathode.

A simple example of electrolysis involves molten (melted) lead (II) bromide:

So what happens?

Molten lead is found at the bottom of the cathode.

Bromine gas comes out of the anode.

When the power supply is switched off, no more bubbles are produced and everything else stops.

What the hell happened?

Since the lead (II) bromide (PbBr₂) is molten, its ions are free to move around.

The bromide ions are attracted to the positive electrode. The extra electron which makes the bromide ion negatively charged is deposited into the anode, thus, turning them back into neutral bromine atoms. These then covalently bond to join bromine atoms (i.e. bromine gas).

On the other hand, the lead ions gain back to electrons (it has a 2⁺ charge) and become normal lead atoms. These fall to the bottom of the container as molten lead.

The half-equation at the anode would be:

2Br^– -> Br₂ + 2e^–

What it basically means is that, two bromine ions are formed when one bromine molecule receives two electrons (to fill its shell). This is the format for all anions (ions that are negatively charged).

The half equation at the cathode would be:

Pb²⁺ + 2e^– -> Pb

All cation half-equations are of this form. Half-equations basically show the gaining and losing of electrons. This basically means that the lead (II) ions get two electrons to become a neutral lead atom.

With molten substances, the metal will be produced at the cathode and whatever it’s bonded to will be produced at the anode.

When you electrolyse aqueous solutions (not molten salts), things are much different because you have to consider the water molecules too. Water is a weak electrolyte but it can ionise to form hydrogen and hydroxide ions.

If the metal is more reactive than hydrogen, then hydrogen ions from water is discharged instead. These pair up to form hydrogen gas that escapes as bubbles.

If the metal is below hydrogen, you get the metal produced.

If you have solutions of halides (chlorides, bromides or iodides), you get the halogen (chlorine, bromine or iodine) produced.

With other negative ions such as sulphates, oxygen would be produced.

The electrolysis of sodium chloride solution (brine) does not give sodium and chlorine! Here’s the electrolysis:

Sodium is higher than hydrogen in the reactivity series, so hydrogen ions from the water in the sodium chloride solution is discharged instead at the cathode.

2H⁺ + 2e^– -> H₂

Chloride ions give up one electron each (chloride ion = 1- charge) and become chlorine atoms. These covalently bond to form chlorine gas and bubbles out of the solution at the anode.

2Cl^– -> Cl₂ + 2e^–

When all the chlorine has been removed from the solution, only hydroxide (OH^–) ions and sodium (Na⁺) ions are left, as well as some water. These combine to form sodium hydroxide solution (NaOH).

The electrolysis of sodium chloride solution is used to manufacture sodium hydroxide solution. The process is slightly different – it is electrolysed in a diaphragm cell:

The products are kept separated by the diaphragm. If the chlorine produced were to react to hydrogen, it would cause an explosion on exposure to sunlight or heat to give hydrogen chloride. Furthermore, if the chlorine were to react with the sodium hydroxide solution formed, it would form bleach. Uses of sodium hydroxide include:

Making bleach
Making soap
Making paper – NaOH breaks the wood down

Uses of chlorine include:

Sterilising water
Making hydrochloric acid
Making bleach

And… the electrolysis of copper sulphate solution:

Copper is lower than hydrogen and therefore, a coat of it forms at the cathode.

Cu²⁺ + 2e^– -> Cu

Oxygen gas is discharged from the hydroxide ions in the water because the sulphate ions are more stable.

4OH^– -> 2H₂O + O₂ + 4e^–

If you electrolyse the solution for longer, something else happens. The hydrogen ions are being discharged and remains in the solution. Similarly, sulphate ions are being discharged either. As a result, the solution turns into sulphuric acid (H₂SO₄) and it begins electrolysing:

Sulphate ions are being discharged from the acid so oxygen is discharged from the hydroxide ions instead.

4OH^– -> 2H₂O + O₂ + 4e^–

There are only hydrogen ions arriving at the cathode so they discharge as hydrogen gas.

2H⁺+ 2e^– -> H₂

Common Exam Question: Why is twice as much hydrogen produced than oxygen? For every four electrons that flow around the circuit, one molecule of oxygen and two molecules of hydrogen are produced.