Question

Determine the rate of reaction between potassium iodate (KIO3) and sodium sulphite (Na2SO3) with the use of starch indicator

Answer 1

Reaction between Potassium Iodate (KIO3) and Sodium Sulphite (Na2SO3)

Reaction of KIO3 and Na2SO3 is depend upon the reaction of these two reactants.

In this process sodium sulphite will act as a reducing agent to reduce potassium iodate to iodide .

while using  starch to test this reaction you realise that starch changes colour to blue colour .

Answer 2

Determine the rate of reaction between potassium iodate (KIO3) and sodium sulphite (Na2SO3) with the use of starch indicator.

we know, in acidic medium, potassium iodate reduced to iodide by sodium sulphite . reactions are follows as : 

$\textbf{step 1}$ : $O_3^-+3SO_3^{2-}\rightarrow I^-+3SO_4^{2-}.....\textbf{(slow)}$
$\textbf{step 2}$ : $IO_3^-+5I^-+6H^+\rightarrow 3I_2+3H_2O......\textbf{(fast)}$
$\textbf{step 3}$ : $I_2+SO_3^{2-}+H_2O\rightarrow SO_4^{2-}+2I^-+2H^+......\textbf{(very fast)}$

in step 1 , it is clearly seen that potassium iodate is reduced to iodide ions . iodide ions, thus formed , oxidised to iodine by the reaction with more iodate ions. 

in step 2, in this step, formed iodine reacts immediately with sulphite ions forming iodide ions . when sulphite ions are completely consumed , the liberated iodine won't be consumed and would give blue color, if starch is present .Thus, the above reaction can be monitored by adding a known but limited volume of sodium sulphite solution and starch solution. This is an example of clock reaction as the rate of the reaction is estimated by the time taken for the appearance of blue colour.