Ask A Scientist© Chemistry Archive

name         Alison S.
status       student
age          18

Question -   Can you explain in full detail the mechanism of the
iodine clock reaction and the electron transfer process that takes place?
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Hi Alison!
I imagine that you are aware of the experiment full sequence since it
is a simple and quite well known Lab item for chemistry cinetic
  demonstration . There are some variations, let us deal with the
commonest one:
1) You begin with 2 solutions A and B
Sol. A: KIO3 (potassium iodate); dissolve 2.0 g of KIO3 to 1 L of water
Sol. B: dissolve 2.0 g soluble starch  in 500ml boiling water, cool and add:
            0.40g NaHSO3 (sodium bisulfide) + H2SO4 (sulfuric acid) 6M
            or ~ 1.7 mL concentrated H2SO4. Dilute to 1 Liter.

You can test the concentration effect and the temperature effect
2) Concentration effect : dilute concentration of A and leave concentration
of B
     Prepare different  dilutions of A in test tubes always with 10 ml. Now
add 10 ml
    10 ml of solution B. See the effect...The reactions must be timed from
the time you mix the solutions and the data noted.

3) Temperature effect:  Use 10 ml of both solutions and mix them at quite
     different temperatures. See the effect...

At both experiments you must note very carefully the time when the mixed
solution turns completely blue. (the initial A and B  are colorless)

Reactions:

2 H (+)  + 5HSO3(-)  + 2 IO3(-)  =       I2 + 5 HSO4 (-)  + H2O

H2O + HSO3 (-) + I2   =  2 I(-)  + HSO4(-)  + 2 H ( +)


When the point is reached where all the bisulfite is used:

2I(-)  = I2(s)

I2 (s) + I(-) (aq)    =   I3 (-) (aq)

I3(-) (aq)   +  starch    =   I3 (+)    +  starch complex (blue color)

  I think that from there it will be easy for you to work out
the electrons transfer...

And thanks for using NEWTON!
Mabel
(Dr. Mabel Rodrigues)
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