Reacting aluminium and iodine

This experiment must be done as a demonstration, in a fume cupboard. The experiment takes only 5 minutes if the iodine is ground and weighed in advance.

You should try the experiment in advance if you have not done it before.

Equipment

Apparatus

• Eye protection
• Access to a fume cupboard (note 1)
• Mortar and pestle
• Tin lid (or other metal container) (note 2)
• Heat resistant mat
• Teat pipette

Apparatus notes

1. Check that the fume cupboard works before starting. Tape a length of tissue paper or ribbon onto the sash to see that air is drawn in.
2. A tin lid is an excellent piece of apparatus for this reaction, but any other piece of flat metal can be used. The reaction produces a lot of heat, so protecting the floor of the fume cupboard is essential.

Chemicals

• Aluminium powder (fine) (HIGHLY FLAMMABLE), about 0.1 g
• Iodine (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), about 0.4 g
• Warm water

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection. The demonstration must be done in a fume cupboard.
• Aluminium powder (fine), Al(s), (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC001A.
• Iodine, I₂(s), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC054.
• Sometimes the mixture fails to react and must be disposed of safely. It has been reported that in some cases the reaction can occur with a delay of several hours which can be hazardous. If you are disposing of a mixture which has not yet reacted pour this into an excess of 1 M sodium hydroxide in a fume cupboard and leave until it all dissolves before resuming normal disposal.
• The quantities specified in this reaction should not be exceeded unless the school has an Explosives Certificate issued by the police.

Procedure

1. Finely grind 0.4 g of iodine in the mortar.
2. Carefully mix the iodine with 0.1 g of aluminium powder and place the mixture in a small mound on the tin lid.
3. Put one or two drops of warm water onto the top of the mound using the teat pipette. There can be an induction period before the reaction starts but if there appears to be nothing happening add another one or two drops of water. A little detergent in the water assists wetting.
4. When the reaction starts, clouds of purple iodine vapour are released as heat is generated. At this point the fume cupboard should be switched on, as iodine vapour is toxic. The mixture then bursts into flame, producing a white smoke together with the iodine vapour, and leaving a glowing, white residue of aluminium iodide.

Teaching notes

It is important to try this experiment before doing it as a demonstration, as different samples of aluminium powder can react differently. The induction period for some samples can be quite long. However, this is an impressive and spectacular demonstration, proving that water can be a catalyst. It also shows that aluminium is a very reactive metal, and that its usual unreactive nature is due to the surface oxide layer.

The chemical properties of iodine are very similar to those of bromine and chlorine. However, its reactions are far less vigorous. It can also act as an oxidant for a number of elements such as phosphorus, aluminium, zinc and iron, although increased temperatures are generally required. Oxidation of finely dispersed aluminium with iodine can be initiated using drops of water. The reaction is strongly exothermic, and the excess iodine vaporises, forming a deep violet vapour.

The reaction is:

2Al(s) + 3I₂(s) → Al₂I₆(s)

Anhydrous aluminium halides, such as the aluminium iodide produced here, react vigorously with water, sometimes violently if freshly prepared and still hot, releasing fumes of corresponding hydrogen halide. The residue should be disposed of in a fume cupboard, after allowing it to cool completely, by adding small amounts to 1 M sodium carbonate solution, allowing the reaction to subside between additions. The resulting slurry can then be disposed of with plenty of water.