|Red Phosphorus and Bromine|
Phosphorus is a nonmetal which exists in several allotropic modifications. Phosphorus atoms within phosphorus molecule are connected with each other differently. Consequently, different allotropic modifications, so called allotropes, possess quite different properties. For instance, red and white (which is sometimes referred to as yellow) phosphorus, with structures shown bellow, are the main two allotropes of phosphorus.
Contrary to red phosphorus which is a stable at room temperature, with melting point of about 400oC, white phosphorus melts at about 44oC and is the most reactive form of phosphorus – it is very flammable and pyrophoric (self-igniting), glows greenish in dark and is extremely poisonous. Bonding in red P is stronger than white P; though red phosphorus is more stable than white, it is still reactive.
In this experiment a vigorous reaction with liquid bromine is shown. Red phosphorus ignites in contact with bromine and combusts with an intensive yellowish-orange flame, producing a lot of white smoke. During this violent reaction phosphorus(III) bromide prepared by treating phosphorous with bromine is formed as the main reaction product, according to the following equation:
Red phosphorus ignites and starts burning as soon as it comes in contact with bromine vapor. As can be observed from the reaction enthalpy, phosphorus(III) bromide synthesis is exothermic, which means that heat is released during the reaction. Liberated heat is sufficient to cause bromine and formed PBr3 evaporation, but also causes red phosphorus to ignite and combust in air yielding white fumes of phosphorus pentoxide. On the pictures bellow you can see all products that were left on the glass after reaction mixture cooled down - yellow crystals of PBr5, light yellow surfaces belong to PBr3 and the dark matter is unreacted red phosphorus.
Side reaction of phosphorous combustion is represented with the following equation:
Bromine also reacts with phosphorus(III) bromide giving light yellow PBr5. Phosphorus pentabromide decomposes above 100oC to yield phosphorus tribromide and bromine. An excess of phosphorus can be used in order to prevent side product formation:
Phosphorus(III) bromide is a very hygroscopic liquid that readily hydrolyzes in moist air, evolving corrosive hydrogen bromide:
This experiment is similar to the one with aluminium and bromine. Main differences would be the color of the flames (sparks of aluminium were followed by reddish and bright white light), also reaction with phosphorus is immediate, as opposed to aluminium with bromine.
It is possible to use chlorine gas instead of bromine in the experiment, with similar, but the reaction is not that impressive. The reaction of red phosphorus with chlorine gas is shown in another experiment on this website.
Because of a high toxicity of bromine and reaction products, the experiment must be performed in a fume hood. Be very careful when adding bromine to phosphorous because the flame catches instantly. Reaction performed is very violent and a lot of corrosive and poisonous fumes are produced. With red phosphorus being flammable you should not conduct this reaction without the supervision. Also none of the experiments mentioned above should be scaled up for added effects.