RSC - Advancing the Chemical Sciences


Education

 

The reaction between aluminium and bromine


Demonstrations to capture the student's imagination, by Adrian Guy of Blundell's School. In this issue: the reaction between aluminium and bromine 

Aluminium-bromine reaction

My PGCE tutor at the University of Exeter, Keith Simpson, showed me this spectacular demonstration of the highly exothermic reaction between aluminium foil and bromine performed in a gas jar.

The Al-Br2 reaction 

Before attempting this demonstration practise the experiment using smaller quantities first. CLEAPSS recommends a small-scale procedure suitable for use in schools: working in a fume cupboard, add 1 ml of bromine from a plastic pipette to 0.25 g aluminium foil in a borosilicate boiling tube. Then, if you want to, gradually increase the quantities used to those suggested. For any variation on the method described here, follow the safety tips and always do your own risk assessment. 

Kit 

  • Gas jar and lid; 
  • 10 ml of bromine; 
  • 2 g Aluminium cooking foil; 
  • Heavy-duty nitrile gloves; 
  • Safety goggles; 
  • Bromine spillage kit (500 ml of 1 M sodium thiosulfate solution); 
  • Fume cupboard - with external ducting (not portable). 

Procedure 

Pour no more than 10 ml of liquid bromine into a gas jar. Tear and weigh a piece of aluminium foil ca 2 g and loosely form this into a ball so it fits into the gas jar. Drop the aluminium foil into the bromine and put the lid on the jar. Gently swirl the contents for ca five seconds so that the bromine mixes with the aluminium. Close the fume cupboard window and stand well back.  

After a few seconds you will see faint red/orange sparks, after which the reaction proceeds quickly, filling the fume cupboard with white aluminium bromide smoke and bromine vapour accompanied by a colourful display of flames and sparks. The reaction takes ca 30s to complete, during which the gas jar and its contents become very hot. Do not add water to the hot gas jar immediately after the reaction has stopped because an explosive reaction with the hot aluminium bromide is likely. Wait ca five minutes for the gas jar and contents to cool and then carefully squirt a gentle stream of water from a 250 ml distilled water bottle down the inside of the gas jar to dissolve any AlBr3 formed on the jar walls. Finally, fill the gas jar with 1 M sodium thiosulfate solution to reduce any unreacted bromine to bromide ions. Dispose of the contents down the sink with plenty of water. 

Special tips 

Performing this reaction in a darkened room improves the effect dramatically. Don't worry if the gas jar lid or even the jar itself breaks because any spilt bromine will evaporate if left in the fume cupboard for ca one hour, and residual stains can be carefully dissolved with 1 M Na2S2O3 solution and then washed down the sink with plenty of water. 

Safety

Bromine vapour is toxic by inhalation and the liquid causes severe burns to eyes and skin. Safety goggles and chemically-resistant, heavy-duty nitrile gloves must be worn to handle Br2 and the experiment must be done in a fume cupboard. Always have 500 ml of 1 M sodium thiosulfate solution available to deal with any spillage and safe disposal of excess bromine. Aluminium bromide causes burns and its dust particles are dangerous to the eyes. Aluminium bromide reacts vigorously and exothermically with water, releasing HBr fumes which cause severe burns and are toxic by inhalation. When disposing of aluminium bromide work in a fume cupboard and wear protective goggles and gloves. 

Teaching goals 

At GCSE, I use this demonstration in conjunction with the reactions between aluminium powder and ground iodine on a heat mat (initiate reaction with a drop of water), and between a 2 g red-hot ball of aluminium foil dropped into a gas jar of chlorine gas to illustrate the relative reactivity and physical properties of the halogens and their salts. I also use this as an opportunity to emphasise the Greek origins of the halogens' names. 

At A-level, the demonstration offers a valuable opportunity to discuss the hydrolysis of the hydrated aluminium ions. The exothermic reaction between the aluminium and liquid bromine is: 

2Al(s) + 3Br2(l)  right arrow 2AlBr3(s) 

The aluminium bromide produced dimerises in the gas phase forming white Al2Br6(s) smoke. A violent reaction occurs when aluminium bromide reacts with water, forming hydrated aluminium ions [Al(H2O)6]3+ and bromide ions, Br-. The hydrated aluminium ions then rapidly hydrolyse the water ligands:  

Al(H2O)63+(aq) + H2O(l)  right arrow Al(H2O)5(OH)2+(aq) + H3O+(aq)  

This forms an acidic solution of HBr, which is evolved as a gas, owing to the heat generated by this hydrolysis.