Halogen reactions with iron
In this demonstration experiment, iron wool is heated in the presence of chlorine gas and the vapours of bromine and iodine. Exothermic redox reactions occur, causing the iron wool to glow. Iron(III) halides (FeX3) are formed as coloured solids:
The vigour of the reactions corresponds to the order chlorine> bromine> iodine, showing the trend of decreasing reactivity of the elements down Group 7.
To view a video clip of these demonstration experiments, click here.
These experiments must be carried out in a fume cupboard as both the reactants and products are hazardous. Teachers attempting this demonstration for the first time are strongly advised to carry out a trial run before doing it in front of a class.
Time allowed should be at least 20 min, depending on the amount of discussion and testing of the products between each experiment.
In addition to using this demonstration to show the relative reactivity of the halogens, the reaction of chlorine or bromine with iron could be used on its own to show the reaction between a reactive non-metallic element and a metal.
The approximate quantities of chemicals below are sufficient for one demonstration:
Iron wool, 3 tufts about 1 g mass each (Note 2)
Hexane (HIGHLY FLAMMABLE, HARMFUL), 100 cm3 (Note 2)
Liquid bromine (VERY TOXIC, CORROSIVE), 0.5 cm3
Sodium thiosulfate solution, 1 M, 500 cm3
Iodine solid (HARMFUL), 0.5 g
Silver nitrate solution, approximately 0.1 M, 10 cm3
To generate chlorine:
Potassium manganate(VII) (OXIDISING AGENT, HARMFUL), 10g (Note 3)
Concentrated hydrochloric acid (CORROSIVE), 100 cm3 (Note 3)
Deionised or distilled water
Refer to Health & Safety and Technical notes section below for additional information.
Eye protection for teacher and students, protective gloves for teacher
Access to a fume cupboard
Apparatus to set up a chlorine generator
Reduction tube (Note 1)
Beakers (100 cm3), 3
Teat pipette (preferably glass, with a narrow tip)
Test-tubes, 3 small, and a test-tube rack
Bunsen burner and heat-proof mat
Bosses, clamps and stands
Health & Safety and Technical notes
Wear suitable eye protection (goggles) and work in a fume cupboard throughout each stage of this demonstration. Iron wool, Fe(s) - see CLEAPSS Hazcard.
Hexane, C6H14(l), (HIGHLY FLAMMABLE, HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Liquid bromine, Br2(l), (VERY TOXIC, CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard. See Standard technique: Handling liquid bromine and preparing bromine water. Wear suitable protective gloves when handling bromine and have at least 500 cm3 of 1M sodium thiosulfate readily available to treat any spillages.
Sodium thiosulfate solution, Na2S2O3(aq) - see CLEAPSS Hazcard.
Iodine solid, I2(s), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Silver nitrate solution, AgNO3(aq) - see CLEAPSS Hazcard. Although LOW HAZARD at this concentration, the solution can still stain skin, clothes and some bench materials.
To generate chlorine gas, Cl2(g), (TOXIC, DANGEROUS FOR THE ENVIRONMENT) - see Standard techniques: Generating, collecting, and testing gases and refer to CLEAPSS Hazcard and CLEAPSS Recipe Book.
Potassium manganate(VII), KMnO4(s), (OXIDISING, HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Concentrated hydrochloric acid, HCl(aq), (CORROSIVE) - see CLEAPSS Hazcard.
1 The reduction tube should be fitted with a one-holed rubber stopper fitted with short length of glass tubing. Alternatively, an 8 to 10 cm length of wide-bore glass tubing with a stopper at each end fitted with a short length of glass tubing could be used. See diagram below.
2 The finest grade iron wool is best since it provides the maximum surface area. Iron wool is often sold with a thin layer of grease on its surface to stop it rusting. Working in a fume cupboard, the layer of grease can be removed by dipping the iron wool in hexane (or alternative) a few times. The solvent must be allowed to completely evaporate.
3 Double-check that the acid you are using to generate chlorine is concentrated hydrochloric acid. Several accidents have occurred when teachers have inadvertently used concentrated sulfuric acid.
a Place a 1 g tuft of cleaned iron wool in the reduction tube so that it is well spread out. Leave at least a 1 cm gap between the stopper and the iron wool.
b Connect the reduction tube to the chlorine generator with a short length of rubber tubing. Clamp it in position over a Bunsen burner.
c Pass a slow stream of chlorine over the iron wool from the chlorine gas generator. Do this by allowing the hydrochloric acid to drip slowly on to the potassium manganate(VII). After a few seconds, it should be possible to see the greenish colour of the chlorine gas filling the reduction tube, as all the air is expelled.
d The iron wool may ignite without any heating. If not, gently heat at the end nearest to the chlorine generator until the wool does ignite (no further heating should be required).
e A vigorous reaction will occur and the glow will spread along the wool in the tube, producing clouds of brown iron(III) chloride. Some of this may emerge as a smoke from the end of the reduction tube.
f Continue passing chlorine over the iron wool until no further reaction occurs. Stop the chlorine supply and allow the tube to cool.
g When cool, disconnect the reduction tube and rinse a little of the product into a clean beaker with some distilled water. Pour some of this solution into a clean test-tube and test with a few drops of silver nitrate solution. A white precipitate of silver chloride will form, confirming the presence of chloride ions.
Bromine See Standard techniques: Handling liquid bromine and preparing bromine water.
a Wear suitable protective gloves and take care to avoid spillage when handling liquid bromine. It is CORROSIVE and VERY TOXIC. Transfer about 0.5 cm3 liquid bromine into one of the boiling tubes, using the teat pipette.
Care is needed to avoid spillage - the density and volatility of the bromine cause it to drip very easily from the pipette. Keep the bromine container and the mouth of the test-tube close together. Replace the lid of the bromine container immediately.
b Using tongs or tweezers, place a 1 g tuft of cleaned iron wool into the boiling-tube so that it is well spread out and almost fills the boiling-tube. Leave a 2 cm gap between the iron wool and the surface of the liquid bromine.
c Clamp the test-tube near the top and at an angle of about 45° – see diagram.
d Heat the test-tube, gently at first, with a yellow-tipped blue flame (air hole on Bunsen burner slightly closed). Do this by moving the flame slowly between the bottom half of the iron wool and the bromine. As the bromine vapour starts to rise up into the iron wool, heat the wool more strongly.
e Remove the heat when the wool starts to glow due to the heat of the reaction. Note the extra heating required to get this reaction started compared to the reaction involving chlorine.
The iron will become coated with yellow-brown iron(III) bromide, and a brown ‘smoke’ may escape from the mouth of the test-tube.
f When the reaction appears to be over, use tongs or tweezers to remove some of the remaining iron wool from the test-tube.
g Rinse the iron wool in a few cm3 of deionised/distilled water in a small beaker. Pour out some of the resulting solution into a clean test-tube and test with a few drops of silver nitrate solution. Formation of a cream precipitate of silver bromide confirms that bromide ions are present.
a Transfer about 0.5 g of solid iodine (HARMFUL) into one of the boiling tubes. Place a 1 g tuft of cleaned iron wool in the test-tube and clamp it as before.
b Working in a fume cupboard, heat the test-tube with a yellow-tipped blue flame (air hole on Bunsen burner slightly closed). Heat gently at first by moving the flame slowly between the bottom half of the iron wool and the iodine.
c As the purple iodine vapour starts to rise up into the iron wool, heat the wool more strongly. Remove the heat when the reaction causes a dull glow – see Additional notes below. Some red-brown iron(III) iodide should form.
d When the reaction appears to be over, remove some of the remaining iron wool from the test-tube with tweezers and rinse it in a few cm3 of deionised/distilled water in a small beaker.
Pour some of the resulting solution into a clean test-tube and test it with a few drops of silver nitrate solution. Formation of a yellow precipitate of silver iodide confirms that iodide ions are present.
Additional teaching notes, hints and tips
The order in which the experiments are done is a matter of choice, but it is probably best to leave the most reactive halogen (chlorine) to last, to end with a vigorous reaction - and confirm a class prediction?
The reaction with iodine is much less vigorous than that with bromine and it may be difficult to see a glow at all. A couple of trial experiments beforehand may be necessary to get the right balance between heating the iodine and getting the iron hot enough for a reaction to start. If the iron is heated too vigorously, it may start to glow from reaction with the oxygen in any air that may still be present in the test-tube.
The general equation for the reactions involved is:
2Fe(s) + 3X2(g) → 2FeX3(s) (X = Cl, Br and I)
Health and Safety checked November 2006
Page last updated on 03 January 2012