Reaction Rate experiment: Potassium Permanganate and Hydrogen Peroxide

[neilparker62]

TL;DR Summary

Relationship between reaction rate and concentration when Potassium Permanganate reacts with excess Hydrogen Peroxide in acidic medium.

I am picking up on an old thread having revisited the associated experiment.

https://www.physicsforums.com/threa...ganate-and-hydrogen-peroxide-solution.992713/

10ml of Permanganate solution (0.0135 mol/liter) were reacted with 10ml of Peroxide and 10ml of vinegar. The peroxide was in excess so all the permanganate reacted changing from purple to colourless. This colour change was timed for various dilutions of the permanganate. For example, one run would consist of 8ml stock permanganate and 2ml distilled water reacted with the 10ml peroxide and 10ml vinegar. Constant volume of 30ml was thus maintained.

Reaction rate was determined as [KMnO4] / t . and plotted against [K Mn O4]. I'm not sure how valid it is to plot x/t vs x but that's what we did yielding following data and graph. Well it seems to show a pretty good exponential fit and my question is where does that come from? Have trawled the internet but can't find anything similar except maybe the well know sodium thiosulphate reaction rate experiment.

https://www.desmos.com/calculator/fdkytmcxgy

 

[adf89812]

google rate laws graphs

 

[Dullard]

I'm assuming that your question is 'big-picture.'
The curve shape is the result of the fact that the instantaneous rate of reaction is controlled by how much of the limiting reactant remains - and that you are monitoring that specific reactant. If you did the same experiment (monitoring permanganate) with a much-reduced peroxide concentration (making it the limiter), you'd see a different relationship.

 

[neilparker62]

I'm assuming that your question is 'big-picture.'
The curve shape is the result of the fact that the instantaneous rate of reaction is controlled by how much of the limiting reactant remains - and that you are monitoring that specific reactant. If you did the same experiment (monitoring permanganate) with a much-reduced peroxide concentration (making it the limiter), you'd see a different relationship.

I think the aim was to have the peroxide well in excess so that you would definitely see the purple colour disappear and could time how long that took. If the concentration of peroxide and of acetic acid (from vinegar) is kept constant , then rate should be proportional to [KMnO4].^2 - second order ? Did get something suggesting a straight line graph plotting 1/t vs [KMnO4] or 1/[KMnO4] vs t.