I have to do an experiment where I will find out what the influence of the concentration on the rate of a homogenous reaction is.

We are asked to create 2 solutions, respectively A and B:

$$\ce{KI + Na2S2O3 + Starch + water}$$


$$\ce{H2O2 + H2SO4 + water}$$

As long as A and B aren't added together, the reaction cannot start.

Now what I'm supposed to do is make 10 different A and B solutions and add these together and measure the time it takes for the solution to turn blue (Iodine + Starch as an indicator).

Now, my problem is that the following questions are also asked:

"Write down the different reactions that take place"
This seems dubious to me but might not be.

e.g. Does dissolving into ions also count as a reaction?
Does the Starch (for which I have no given chemical formula) react in solution A?
I've also read online that adding $\ce{H2O2}$ and $\ce{H2SO4}$ creates a "Piranha" solution ($\ce{H3O+ + HSO4- + O}$ or $\ce{H2SO5 + H2O}$, I've found both online but I don't know which is correct here.)

In conclusion I just don't know which reactions I should give (Everything that happens in A and B isolated from eachother or everything that happens when I add A and B together? Either way, I'm also not sure what the reactions would be because, for instance, I don't have any certainty on what the chemical composition of the Starch is.)

Is the reaction a multistep reaction or a single step reaction? Why? If it's a multistep reaction, write down the partial reactions

Assuming that adding B to A is "The" reaction, how do I find out if there are multiple steps or not? How do I find these steps?

Given that this question is asked already makes me think that it is in fact a multi-step reaction.


I've also read online that adding $\ce{H2O2}$ and $\ce{H2SO4}$ creates a "Piranha" solution [...]

This is true - in principle. In reality, Piranha solution is made from concentrated sulfuric acid and 30% hydrogen peroxide.

In the case of your experminent, you will most likely use diluted sulfuric acid and aqueous solutions of hydrogen peroxide with a much lower concentration. Consequently, the Piranha effect will not play a role in your experiment, neither is the oxidizing power of sulfuric acid relevant here.

On a side side note, treating a sample with conc. $\ce{H2SO4}$ is a method for the qualitative determination of iodide: Upon heating, one can observe the violet vapour of iodine.

In your case, you need the sulfuric acid to keep the pH value low. Under these conditions, $\ce{H2O2}$ acts as an oxidant:

$$\ce{H2O2 + 2 H+ + 2 e- -> 2 H2O}$$

Does dissolving into ions also count as a reaction?


Does the Starch (for which I have no given chemical formula) react in solution A?

Starch is a mixture of two natural polymers (amylopectin and amylose), both made from D-glucose. Amylopectin doesn't play a role here, but it doesn't disturb either and it's just cheaper to let you work with the natural mixture.

It's the helical amylose that is relevant here - and it acts as a reagent.

What actually happens in the reaction is the following:

$$\ce{2 I- -> I2 + 2 e-}$$ $$\ce{I2 + I- -> I3-}$$

The linear triodide ion forms a strongly coloured inclusion complex with the amylose in the starch.

  • $\begingroup$ This should be the accepted answer. $\endgroup$ May 4 at 6:37

You could look up "iodine clock reactions" and get the straight skinny on the chemistries and their kinetics. You might also look up "Theory of Experimentation" and see how confounded variables cannot be teased apart by varying them individually. Instruct your instructor.



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