Question in Chem text book,plz help.

Question

the progress of the reaction
5Br-(aq)+BrO3-(aq)+6H+------>3Br2(aq)+3H2O(l) is studied in the laboratory.Potassium bromide, potassium bromate, sulphuric(VI) acid,phenol and methyl range indicator are mixed and the time required for the red to disappear is recorded.
a)Why is there a red colour found in the mixture?
b)Why does the red colour disappear after some time?

Answer 1

a)
Methyl orange is an acid-base indicator. In an acidic medium (due to the presence of the sulphuric acid), methyl orange is red in colour.

b)
Bromine is formed in the following equation :
5Br-(aq)+BrO3-(aq)+6 H+(aq) → 3Br2(aq) + 3H2O(l) ..... (*)
Firstly, the bromine formed in reaction (*) is removed by the (small fixed amount of) phenol.
C6H5OH(aq) + 3Br2(aq) → C6H2Br3OH(aq) + 3HBr(aq)
When all phenol is removed, the “free” bromine produced in reaction (*) will bleach the methyl orange, and thus the red colour disappears.

2007-05-16 22:53:44 補充：
This is an experiment of " clock reaction ". The red colour changes to colourless suddenly.