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My teacher performed an experiment and made us watch.

He prepared 3 conical flask containing 50 mL mixture of sodium thiosulphate, potassium iodide and acetic acid. He also had 3 measuring cylinders containing 12.5 mL, 25 mL and 50 mL of hydrogen peroxide of the same concentration. He added few drops of universal indicator to each of the flask. Then he added the $\ce{H2O2}$ to them as well and started timing.

The general idea is that the indicator is going to change colour from orange to green, which is the colour when he stopped timing. The flask added with 50 mL $\ce{H2O2}$ had the shortest time recorded.

It is intuitive that higher volume of $\ce{H2O2}$ added will increase the speed of reaction but I don't know how to explain it. In kinetics we learn the effect of concentration on rate of reaction, but i cannot use this idea to explain because the concentration does not change! All three solutions of $\ce{H2O2}$ added has the same concentration because they are undiluted (He did not add distilled water to the $\ce{H2O2}$ solutions to make them the same volume).

So how can I explain why the use of higher volume (50 mL) will make the reaction go faster?

An example of the experiment can be viewed through youtube : https://www.youtube.com/watch?v=Tlj5RvCSpLw

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    $\begingroup$ It is $\ce{H2O2}$ in the reaction solution, not the concentration in the reagent. The reaction volumes were 62.5, 75 and 100 ml. $\endgroup$ – MaxW Jan 8 '16 at 17:13
  • $\begingroup$ All three solutions of H2O2 added has the same concentration because they are undiluted. So how can I explain why the use of higher volume (50mL) will make the reaction go faster? $\endgroup$ – user24479 Jan 8 '16 at 17:24
  • $\begingroup$ As Max said, it doesn't matter and how concentrated is reagent before using it or how much is it, but what is the concentration in reaction mixture. $\endgroup$ – Mithoron Jan 8 '16 at 17:32
  • $\begingroup$ Consider the reaction from this perspective: you have three solutions of H2O2, 12.5mL 25mL and 5omL. You dilute each of them with 50mL of your thiosulphate solution. Which flask now has the highest H2O2 concentration? $\endgroup$ – Spontification Jan 8 '16 at 22:13
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The concentration of $\ce{H2O2}$ doesn't change but when you have more of it in the reaction (50 ml vs. 12.5 ml) there are more moles of $\ce{H2O2}$ meaning more collisions speeding up the reaction.

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