C2aL11  Electrolysis

Key Words

Half equations - chemical equations that show the changes at each electrode.

Impurities - unwanted substances that are mixed in with the substance we want.

Oxidation - losing electrons.

Products - substances formed by a reaction.

Redox - reaction in which oxidation and reduction occur.

Reduction - gaining electrons.

Grade E

When we pass an electric current through a molten electrolyte like sodium chloride, we see the following.

Sodium forms at the cathode (negative electrode) and chlorine at the anode (positive electrode).

We can look at what is happening at each electrode.

We see that each sodium ion picks up one electron to make a neutral sodium atom.  It has gained an electron, and we say that the sodium has been reduced.

At the same time, each chloride ion loses an electron to make a chlorine atom.  Since chlorine gas consists of molecules made of two atoms, two chloride ions are involved.  Since they have lost electrons, we say that the chloride ions have been oxidised to form chlorine atoms.

Any reaction in which substances are oxidised while other substances are reduced is called a redox reaction.

We can sum up what happens at each electrode using symbol equations called half equations:

Na⁺ + e^- ® Na

2Cl^- ®  Cl₂ + 2e^-

Now let's see what happens in the sodium chloride solution.  In this electrolysis, we get chlorine gas at the anode as expected, but NOT sodium at the cathode.  Instead we get hydrogen.

This is because the sodium ions are more reactive than the hydrogen ions.  They stay in solution, while the hydrogen ions gain the electrons:

2H⁺_(aq) + 2e^- ® H_{2 (g)}

Chlorine ions are less reactive than hydroxide ions.  So they are given off at the anode. 

The sodium ions and the hydroxide ions left in the solution form sodium hydroxide.  So the products of the electrolysis of a sodium chloride solution are:

• hydrogen;

• chlorine;

• sodium hydroxide.

  Sodium metal is NOT reacting with the water to form sodium hydroxide.

Grade C

If we decompose molten lead bromide (PbBr₂) by electrolysis, the half equations are:

Pb²⁺ + 2e^- ® Pb

2Br^- ®  Br₂ + 2e^-

• The lead ions have been reduced to lead atoms

• The bromide ions have been oxidised to bromine atoms.

When sodium chloride is electrolysed,  the products of the electrolysis of a sodium chloride solution are:

• hydrogen - used as a fuel, making ammonia, making margarine, as a gas in weather balloons;

• chlorine - used to make bleaches, chlorinated solvents, making chloroethene (for PVC), extracting bromine from sea water;

• sodium hydroxide - making soap and detergents, paper making, purification of bauxite (an aluminium ore).

Copper is purified using electrolysis:

The half equations are:

Cu _(s) ®  Cu²⁺_(aq) + 2e^-

Cu²⁺_(aq) + 2e^- ® Cu _(s)

This may seem rather pointless, but copper needs to be very pure.  Metal impurities like zinc,  which are more reactive than copper, lose electrons  and go into solution.   Metal impurities that are less reactive drop out and form a sludge.

Grade A

Pure water is a bad conductor.  A small amount of sulphuric acid will improve the conductivity.  We call this acidulated water. 

Water will decompose by electrolysis according to the half equations:

2H⁺_(aq) + 2e^- ® H_{2 (g)}

4OH^- _(aq) ® 2H₂O _(l) + O_{2 (g)}

So water becomes hydrogen and oxygen gases.