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For example, in the iodine and starch reaction, if I use 20 mL of a set concentration of each, will it be slower to completion if I use 40 mL of each at the same concentration?

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    $\begingroup$ Depends, how you use them. $\endgroup$ – Mithoron Jun 9 '18 at 21:18
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    $\begingroup$ In a simple bimolecular reaction between A and B with rate constant k the rate = k[A][B], where [A] is the concentration of A, and [B] for B, so if the concentration is the same the volume does not matter if other things remain constant, temp , pressure etc. $\endgroup$ – porphyrin Jun 12 '18 at 7:52
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If you define "completion" by a certain percentage of conversion then no, they will behave exactly the same, provided that things like cooling and stirring are the same in both systems.

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  • $\begingroup$ Problem is if you scale up process, those things can't be exactly the same. $\endgroup$ – Mithoron Jun 9 '18 at 22:41
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Here some elements to add to your reflection:

  • Some chemicals (among them, many bio-polymers or proteins) can highly change fluid properties (mainly viscosity and surface tension) when dissolved, like you can see when solubizing Starch;
  • Sheer stress affects viscosity of non-Newtonian fluids (moderate concentrations of starch are not Newtonian fluids, looks like jelly), and then chemical advection and diffusion. Thus it can affect reaction rate by modifying mass transfer rate;
  • Sheer stress may change protein or polymer conformations (think about Starch which is a polymer) and then can affect their physical or chemical properties (eg.: it can produce foam a complex polyphasic system, or denature protein).
  • Dark-purple color you see when mixing Iodine with Starch comes from a very specific complex of Iodine-Starch chains;
  • Practically, you will apply sheer stress differently to a tank about 1L than 1000L.

Regarding to that aspects, it explains why sheer stress may impact reaction rates, in your case: Iodine Starch complex coordination reaction rate.

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