[Chem]About a bromine clock reaction

Question

5Br- + BrO3- + 6H+ ----- 3 Br2 + 3 H2O
C6H5OH + 3Br2 → C6H2Br3OH + 3HBr
When phenol is completely reacted, the free bromine formed will decolorize the methyl orange indicator.
After the solution changes to colorless,
why the solution turns to yellowish-brown precipitate
and have a nasty smell after a period of time?

Answer 1

The precipitate is not yellowish-brown. Actually, the solution changes from colourless to yellowish-brown, and a colourless precipitate is formed.

5Br- + BrO3- + 6H+ → 3 Br2 + 3 H2O ..... (*)
C6H5OH + 3Br2 → C6H2Br3OH + 3HBr ..... (**)

When phenol is completely reacted, reaction (**) stops. Now, the Br2 formed in reaction (*) dissolves in the solution. When the concentration of Br2 is high enough, the solution looks yellowish-brown. Besides, a very small amount of Br2 can even evaporate to cause the nasty smell.

The colourless precipitate is C6H2Br3OH, which is only slightly soluble in water.