TheStudent.
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Hi there,

I'm investigating the iodine clock reaction and i'm changing the concentrations of:
Hydrogen peroxide
Sulfuric acid
Potassium iodide
Sodium thiosulfate

However, i've been told that i'm not supposed to change the concentration of sodium thiosulfate. Does anyone know why? I have found out that it is second order, so i'm now confused as to whether or not it appears in the rate equation.

Thank you in advance
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TheStudent.
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Am I allowed to bump this soon?
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chembob
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I don't think the thiosulfate will appear in the rate equation, as far as I'm aware it's there to reduce any I2 formed back into I-
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TheStudent.
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(Original post by chembob)
I don't think the thiosulfate will appear in the rate equation, as far as I'm aware it's there to reduce any I2 formed back into I-
Oh right >_< so i've changed it for no reason. I guess I could use it to explain how reducing agents don't appear in rate equations? or should I not mention it at all?

Or to prove that increasing concentration increases rate? I've also talked about how its a limiting raegant, so could use it to prove that too.. not sure if its actually relevant though
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chembob
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(Original post by TheStudent.)
Oh right >_< so i've changed it for no reason. I guess I could use it to explain how reducing agents don't appear in rate equations? or should I not mention it at all?

Or to prove that increasing concentration increases rate? I've also talked about how its a limiting raegant, so could use it to prove that too.. not sure if its actually relevant though
I'd wait till anyone else has an opinion in this; is this A level coursework?
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(Original post by chembob)
I'd wait till anyone else has an opinion in this; is this A level coursework?
Yep A2 level, ocr salters spec B
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(Original post by TheStudent.)
Yep A2 level, ocr salters spec B
Ok, I never did anything like this for my A level coursework, I've only done this reaction during labs at uni although we were titrating the reaction mixture at the same time
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(Original post by chembob)
Ok, I never did anything like this for my A level coursework, I've only done this reaction during labs at uni although we were titrating the reaction mixture at the same time
Why were you titrating it
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paulbridger
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(Original post by TheStudent.)
Why were you titrating it
It's the same reaction, just a different way of recording the results, via aliquots.
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(Original post by paulbridger)
It's the same reaction, just a different way of recording the results, via aliquots.
Ok so what i've understood from this:

" After mixing, record the exact time that a dark color appears in the solution and remains upon mixing. Immediately add a 1.0 mL aliquot of 0.4 M Na2S2O3 with a 1 mL plastic syringe. Mix the solution and wait for the dark color to appear again. When it reappears record the time and add another aliquot of 0.4 M Na2S2O3. Continue through 4 or 5 of these cycles. "

Is that, in the context of the reactants i'm using, I will need to continually add a set volume of sodium thiosulfate in order to find the number of moles of thiosulfate ions used up before the colour change can occur. Slightly confused, but is that sort of correct?
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paulbridger
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(Original post by TheStudent.)
Ok so what i've understood from this:

" After mixing, record the exact time that a dark color appears in the solution and remains upon mixing. Immediately add a 1.0 mL aliquot of 0.4 M Na2S2O3 with a 1 mL plastic syringe. Mix the solution and wait for the dark color to appear again. When it reappears record the time and add another aliquot of 0.4 M Na2S2O3. Continue through 4 or 5 of these cycles. "

Is that, in the context of the reactants i'm using, I will need to continually add a set volume of sodium thiosulfate in order to find the number of moles of thiosulfate ions used up before the colour change can occur. Slightly confused, but is that sort of correct?
Erm, I think you're doing the reaction backwards to how I know.
So there is iodine being formed from the iodide ions. I think you are using a set volume of thiosulphate as that isn't the reactant you are measuring, which is iodine. I don't want to say much more and mislead you as I'm only an A-level student myself :P
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So i'm currently using 0.005moles of sodium thiosulfate. This concentration makes it change blue-black at X time. If I carry on adding 0.005 moles of sodium thiosulfate each time it changes colour, will it go back to colourless and then blue black again?
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(Original post by TheStudent.)
Ok so what i've understood from this:

" After mixing, record the exact time that a dark color appears in the solution and remains upon mixing. Immediately add a 1.0 mL aliquot of 0.4 M Na2S2O3 with a 1 mL plastic syringe. Mix the solution and wait for the dark color to appear again. When it reappears record the time and add another aliquot of 0.4 M Na2S2O3. Continue through 4 or 5 of these cycles. "

Is that, in the context of the reactants i'm using, I will need to continually add a set volume of sodium thiosulfate in order to find the number of moles of thiosulfate ions used up before the colour change can occur. Slightly confused, but is that sort of correct?
That's pretty much the method we used

The dark colour appears due to the formation of I2. Adding the thiosulfate reduces it back to I- ions (i.e. the starting material). Yes you have to add the set amount of thiosulfate every time the colour appears, but you should able to work out the moles of thiosulfate added from the concentration and the volume added (I think- obviously I don't know the entire context of your experiment)
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(Original post by paulbridger)
Erm, I think you're doing the reaction backwards to how I know.
So there is iodine being formed from the iodide ions. I think you are using a set volume of thiosulphate as that isn't the reactant you are measuring, which is iodine. I don't want to say much more and mislead you as I'm only an A-level student myself :P
Woops lol that wasn't my own method, I got it from this website when I was trying to figure out what a aliquot was:
http://icn2.umeche.maine.edu/genchem...k/iodine1.html

Thats the titration method, and I was just trying to get to grips with it in case I decide to use it to support my own results on why sodium thiosulfate increases the rate of reaction.

I have changed the concentration of KI, h2o2 and sulfuric acid already, but I decided to extend that to sodium thiosulfate as well
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(Original post by TheStudent.)
So i'm currently using 0.005moles of sodium thiosulfate. This concentration makes it change blue-black at X time. If I carry on adding 0.005 moles of sodium thiosulfate each time it changes colour, will it go back to colourless and then blue black again?
Yes (As long as there's still peroxide left)
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(Original post by chembob)
Yes (As long as there's still peroxide left)
(Original post by chembob)
That's pretty much the method we used

The dark colour appears due to the formation of I2. Adding the thiosulfate reduces it back to I- ions (i.e. the starting material). Yes you have to add the set amount of thiosulfate every time the colour appears, but you should able to work out the moles of thiosulfate added from the concentration and the volume added (I think- obviously I don't know the entire context of your experiment)
Ok well I'm using 5x10^4 moles of sodium thiosulfate atm. Do you think I should use the titration method to support why sodium thiosulfate increases the rate of reaction? The reason being more moles of ions will need to be consumed before the iodine is produced, and colour change occurs?

Will I have to do this titration from scratch, or could i just bluff it by using the results I already have from changing its concentration and simply working out the number of moles consumed each time?
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(Original post by TheStudent.)
Ok well I'm using 5x10^4 moles of sodium thiosulfate atm. Do you think I should use the titration method to support why sodium thiosulfate increases the rate of reaction? The reason being more moles of ions will need to be consumed before the iodine is produced, and colour change occurs?

Will I have to do this titration from scratch, or could i just bluff it by using the results I already have from changing its concentration and simply working out the number of moles consumed each time?
Have you just read about the titration method or are you actually meant to be using it?
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(Original post by chembob)
Have you just read about the titration method or are you actually meant to be using it?
LOL i've only read about it, but we're allowed to do whatever we like, no restrictions. You get marks for being 'creative' so i'm considering using the titration method.
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(Original post by TheStudent.)
LOL i've only read about it, but we're allowed to do whatever we like, no restrictions. You get marks for being 'creative' so i'm considering using the titration method.
Do the titration! I did that last week for my Edexcel coursework, so can help you out if you need it. It's really easy. You have one big conical flask with the reaction mixture in. You then remove aliquots and quench the mixture at time intervals, using NaHCO3. Once you have about 5/6 smaller conical flasks of the quenched reaction mixture you titrate them to see how the concentrations of the iodine. From that you can work out the order, and therefore the rate of reaction (:
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(Original post by paulbridger)
Do the titration! I did that last week for my Edexcel coursework, so can help you out if you need it. It's really easy. You have one big conical flask with the reaction mixture in. You then remove aliquots and quench the mixture at time intervals, using NaHCO3. Once you have about 5/6 smaller conical flasks of the quenched reaction mixture you titrate them to see how the concentrations of the iodine. From that you can work out the order, and therefore the rate of reaction (:
My reaction takes about 20 seconds to change to a blue-black colour complex with the standard concentrations i'm using. So will it even work? :s

Ahhhhh i'm so confused now, I thought you add aliquots not remove them.

I'm not sure if this will work with the iodine clock reaction... i mean, what will I be titrating them with?

So confused with your method i've written the chemical substances im using above.. did you use this method with the iodine clock reaction?
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