C2aL10  Free Ions

Key Words

Decompose - break down into simpler substances.

Electrodes - conducting materials that dip into the electrolysis solution.

Electrolysis - using electricity to break down compounds of reactive elements.

Giant lattice - lots of alternating positive and negative ions of an ionic compound that go on to form a crystal.

Ionic - compound made of ions.

Ions - charged atoms.

Grade E

Ionic compounds form long range giant lattice structures consisting of millions and millions of positive and negative ions.

Ionic compounds usually dissolve in water, and the ions are freed from the giant lattice.

When an ionic compound is melted, ions are freed from the giant lattice.

A solid ionic compound does not conduct an electric current.  With free ions it is possible for an electric current to flow.  Lead bromide will melt if heated.  If a current is passed through the molten material (the electrolyte), the lead bromide will decompose:

PbBr_{2 (l)} ® Pb _(l)  + Br_{2 (g)}

This is called electrolysis.  Electrolysis is the only way to separate reactive metals like aluminium and zinc from their compounds. 

Grade C

In electrolysis the electrolyte can be:

• molten;
• in a solution.

In either case, two electrodes are placed in the electrolyte and a direct current from a power pack is passed through it:

• The positive electrode is the anode, and attracts the negative ions to it.

• The negative electrode is the cathode and attracts the positive ions to it.

Grade A

If we decompose molten sodium chloride by electrolysis, we get sodium and chlorine gas.  In fact sodium is highly reactive and would oxidise rapidly.

If we do the same with sodium chloride in aqueous solution, we get hydrogen from the cathode and chlorine from the anode.  This is because we have these ions:

Na⁺ _(aq)   Cl^-  _(aq)  H⁺ _(aq)  OH^- _(aq)

The sodium is more reactive than the hydrogen, so hydrogen is given off at the electrode.  Hydroxide ions are more reactive than the chloride ions, so chlorine is given off.