Reactions of chlorine, bromine and iodine with aluminium
In this demonstration experiment, aluminium reacts with each of three halogens, chlorine, bromine and iodine. Spectacular exothermic redox reactions occur, accompanied by flames and coloured ‘smoke’, forming the solid aluminium halides:
2Al + 3X2 → 2AlX3 (X = Cl, Br and I)
Read our standard health & safety guidance
Lesson organisation
This is a class demonstration that shows the spectacular reactivity of three non-metals from Group 7 with a metal.
These experiments must be done in a fume cupboard as both the reactants and products are hazardous. Teachers attempting this demonstration for the first time are strongly advised to do a trial run before doing it in front of a class.
Each experiment should take about 5 minutes.
Apparatus and chemicals
Eye protection
Thick chemically-resistant gloves such as marigold industrial blue nitrile
Access to a fume cupboard
The teacher will require:
Mortar and pestle
Heat resistant mat, 30 x 30 cm
Watch glasses, about 10 cm diameter, 2
Reduction tube (see note 1)
Test-tubes, 3
Test-tube rack
Teat pipette
Filter paper
Spatula or wooden splint
Bosses, clamps and stands
Chemicals for one demonstration:
Aluminium foil, a few cm2
Aluminium powder (Highly flammable, Contact with water may liberate hydrogen), 0.3 g
Liquid bromine (Corrosive, Very toxic), 1 cm3 (see note 2)
Solid iodine (Harmful), 2 g
Silver nitrate solution, about 0.1 mol dm3 (Low hazard but stains skin and clothing at this concentration), about 10 cm3
A little deionised water in a washbottle
Chlorine generator (Toxic, Irritant) Generating gases - scroll down for chlorine
Sodium chlorate(I) solution (14% (w/v) available chlorine) (Corrosive), about 100cm3
Hydrochloric acid, 5 mol dm-3 (Corrosive), about 50 cm3
Technical notes
Aluminium powder (Highly flammable, Contact with water may liberate hydrogen) Refer to CLEAPSS Hazcard 1
Liquid bromine (Corrosive, Very toxic) Refer to CLEAPSS Hazcard 15A, SRA04 The aluminium/bromine reaction
Solid iodine (Harmful) Refer to CLEAPSS Hazcard 54A
Silver nitrate solution (Low hazard at concentration used) Refer to CLEAPSS Hazcard 87 and Recipe card 58
Chlorine (Toxic, Irritant) Refer to CLEAPSS Hazcard 22A and Recipe card 26
Sodium chlorate(I) solution (Corrosive) Refer to CLEAPSS Hazcard 89
Hydrochloric acid (Corrosive) Refer to CLEAPSS Hazcard 47A and Recipe card 31
1 The reduction tube should be fitted with a one-holed rubber stopper fitted with short length of glass tubing and attached to the chlorine generator. Alternatively an 8–10 cm length of wide bore glass tubing with a stopper fitted with a short length of glass tubing at each end could be used – see diagram.
2 Wear suitable protective gloves (thick, chemically resistant) when handling liquid bromine. Have 500 cm3 of 1 mol dm-3 solution of sodium thiosulfate available to treat any spillages on the skin.
Procedure
HEALTH & SAFETY: Wear eye protection and gloves
Reaction of Al + Cl2
a Set up the chlorine generator in a fume cupboard. Make sure it is securely clamped.
b Loosely crumple a piece of aluminium foil (10 x 5 cm) so that it will just fit inside the reduction tube and push it into the tube. Attach the tube to the generator with a short length of rubber tubing and clamp it in position at the end nearest to the generator, so that the aluminium foil can easily be heated using a Bunsen burner – see diagram.
c Generate a gentle stream of chlorine by dripping the hydrochloric acid slowly on to the sodium chlorate(I) solution, and allow it to pass over the aluminium. When the green colour of the chlorine gas fills the reduction tube, start heating the aluminium foil with a Bunsen burner at the end nearest to the chlorine supply. Once the aluminium is hot, a bright glow will suddenly appear where it starts to react with chlorine.
d Remove the heat. The bright glow should spread along the aluminium. If not, heat again, and increase the flow of chlorine gas. A lot of white ‘smoke’ – aluminium chloride – is produced, some of it condensing as a white powder on the walls of the reduction tube and the rest streaming out of the hole in the end of the tube. When the reaction is over, stop the chlorine supply and remove the heat.
e When the reduction tube has cooled down, disconnect it and, still in the fume cupboard, scrape a little of the white powder into a test-tube. Add a little deionised/distilled water to the solid to dissolve it. Care: the reaction between anhydrous aluminium chloride and water can be quite vigorous – an audible hiss can often be heard - see Teaching Notes.
f Test a drop of the solution with Universal indicator paper. It is strongly acidic. Test the remainder with a little silver nitrate solution. A white precipitate forms, showing the presence of chloride ions.
Reaction of Al + Br2
a Tear or cut some aluminium foil into several small pieces about 2 x 2 mm in size. Carefully pour 1 cm3 of liquid bromine onto a watchglass on a heat resistant mat in a fume cupboard. Sprinkle a few pieces of aluminium foil on to the surface of the bromine. Within a minute there are flashes of flame and a white ‘smoke’ of aluminium bromide is formed, together the orange vapour of bromine. Carefully hold another watchglass over the reaction to condense some of the ‘smoke’ on to its surface as a solid.
b Wash any aluminium bromide collected in this way off the watchglass into a test-tube using a little deionised water (Care: see Teaching Notes below). Test the solution with indicator paper and silver nitrate solution as above. The solution is acidic and a cream precipitate of silver bromide is formed.
Reaction of Al + I2
a Weigh out 2 g of iodine, which should be dry, and grind it to a powder in a fume cupboard, using a mortar and pestle. Place the powdered iodine on a filter paper on a dry heat resistant mat and add 0.3 g of aluminium powder to it. Mix the two solids together in the fume cupboard using a wooden splint – do not grind them together. When they are thoroughly mixed, pour the mixture into a heap on the heat resistant mat or in a metal tray, such as a tin lid, positioned in the middle of the fume cupboard.
b To start the reaction, use a teat pipette to place a few drops of water on the mixture. After a time lag, the water begins to steam and clouds of purple iodine vapour are given off, indicating that an exothermic reaction has started. After a few more seconds sparks are given off and the mixture bursts into flame. When the reaction subsides, a white residue of aluminium iodide remains. Scrape a little of this into a test-tube (Care: see Teaching Notes below), add some deionised water and filter if necessary. Test the solution with indicator paper and silver nitrate solution as above. The solution is acidic and a yellow precipitate indicates the presence of iodide ions.
Teaching notes
These reactions make quite spectacular demonstrations in themselves, the bromine + aluminium reaction even more so in a partly darkened room. Classroom management in semi-darkness (Practical Physics advice)
The demonstrations can be used to show the reaction between reactive non-metallic elements and a fairly reactive metal to form compounds, or as part of the study of the reactions of the Group 7 elements. Here the apparent order of reactivity is not that predicted from their position in the Group (that is chlorine → bromine → iodine). This is because of the different physical states of the three halogens, and the different surface area of the aluminium as a powder or foil. This can be used to make an important point about ‘fair’ comparisons of reactivity.
These reactions also serve to show that aluminium is in fact a more reactive metal than it appears in everyday use. The protective oxide layer of aluminium has to be penetrated by the halogens before the reactions can start, hence the delays, and the need for water to assist the two solid elements getting into contact, in the case of aluminium and iodine.
The clouds of iodine vapour released when aluminium and iodine react can stain the inside of a fume cupboard. Teachers may prefer to demonstrate this reaction outdoors, if possible.
The anhydrous aluminium halides are vigorously hydrolysed (sometimes violently if freshly prepared and hot, as here) by water, giving off fumes of a hydrogen halide and a forming an acidic solution of the aluminium salt. To dispose of the solid residues, allow them to cool completely before adding in small amounts to 1 mol dm-3 sodium carbonate solution in a fume cupboard. Wait until the reaction has subsided before adding more solid. Dispose of the resultant slurry with plenty of water.
Health and Safety checked, October 2007
Web Links
Al + Cl2:
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/CLAL/PAGE1.HTM
Al + Br2:
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ALBR/PAGE1.HTM
Al + I2:
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA1/R1MAIN/CD1R1260.HTM#1270
3 out of 5
Use the Al - Br reaction in the CLEAPSS information. I personally use a loosly rolled cigar of aluminium foil, not individual balls of foil.It means my hands are not placed near the reaction chamber once it has started. I collect the aluminium bromide on the outer surface of a water-filled round-bottomed flask.
Remember that aluminium bromide explodes when it reacts with water. The dissolving of this compound needs to be done with caution.
Submitted by: Adrian Rosser on 10 November 2008