Monday, 22 February 2016

inorganic chemistry - How can bromine water be called a solution?



Bromine water is a reagent which is used to test for unsaturation in organic compound. It is $2.8~\%$ bromine in water according to Wikipedia. But how is it made?


This link describes the procedure for making bromine water, in which it is stated to



simply decant the bromine vapors in water and it will form a solution which is yellow in colour.



But bromine can also react with water to form a mixture which includes hydrobromic acid ($\ce{HBr}$) and hypobromous acid ($\ce{HBrO}$).


$$\ce{Br2 + H2O -> HBr + HBrO}$$


(source)


So, if bromine reacts with water to form a mixture of acids, how can it be called a solution? Components of solution do not react with each other.



Answer




Much like with chlorine or iodine, the disproportionation of bromine can occur in pure water, but it is not fast. As you can see, the reaction generates acids, so without any base present the equilibrium is pretty close to the bromine side. You do need bases to disproportion bromine in any meaningful amount — much like chlorine which is generally quenched with $\ce{NaOH}$ solution to generate hypochlorite solutions.


So if you just put bromine and water together, they may react very slightly, but not much, and most of the bromine will sink to the bottom (assuming you used a lot) while the lesser part will dissolve in the water first. Since the concentration of $\ce{Br2}$ is much higher (and we’re talking orders of magnitude) that those of $\ce{Br-}$ and $\ce{OBr-}$, it is best to consider this a bromine solution and not a bromide/hypobromite one.


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