A2 salters Kinetics of harcourt essen reaction help! Watch

The Best1
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I am just going to explain what I think happens in this reaction, and if anyone knows about this experiment and thinks I am doing something wrong, please let me know

So I take two beakers, beakers A and B.

I add the iodide ions solution (Potassium iodide I think) to beaker A with hydrogen peroxide. In beaker B, I add sodium thiosulphate solution with starch and then I mix beaker B with A. As soon as I mix them together, I start my stopwatch, and stop timing it when the solution turns from colourless to blue/dark colour.

The concentration of iodine added divided by the time taken gives me the rate of reaction of that concentration. Plotting a graph of rate of reaction against concentration, I can tell what order the iodine is.

That's what I know so far, one question I have is, since I add the beaker B to A, it is supposed to measure the iodine (being produced from the first reaction of iodide and hydrogen peroxide) reacting against thiosulphate ions. If I already add hydrogen peroxide and iodide ions together, doesn't iodine molecules already start being produced? So when I add the thiosulphate ions, won't there be iodine molecules already in the beaker which will react? So how is rate of reaction exactly measured?
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username1445490
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This is not how you do this reaction
Beaker A should contain hydrogen peroxide and Beaker B Potassium iodide and thiosulphate with a little starch
You mix A and B together and the peroxide oxidises the iodide to iodine but the sodium thiosulphate immediately reacts with the iodine and turns it back to iodide. When all the thiosulphate has reacted the iodine formed does not get turned back to iodide and turns to a blue black colour due to the starch.
You time how long it take for the blue black colour to form and so you are measuring the rate at which the thiosulphate is reacting.
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The Best1
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(Original post by Madasahatter)
This is not how you do this reaction
Beaker A should contain hydrogen peroxide and Beaker B Potassium iodide and thiosulphate with a little starch
You mix A and B together and the peroxide oxidises the iodide to iodine but the sodium thiosulphate immediately reacts with the iodine and turns it back to iodide. When all the thiosulphate has reacted the iodine formed does not get turned back to iodide and turns to a blue black colour due to the starch.
You time how long it take for the blue black colour to form and so you are measuring the rate at which the thiosulphate is reacting.

HOLY CRAP this makes 100 times more sense now! That was the biggest flaw I saw in my own explanation, thanks so much for that!!
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The Best1
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(Original post by Madasahatter)
This is not how you do this reaction
Beaker A should contain hydrogen peroxide and Beaker B Potassium iodide and thiosulphate with a little starch
You mix A and B together and the peroxide oxidises the iodide to iodine but the sodium thiosulphate immediately reacts with the iodine and turns it back to iodide. When all the thiosulphate has reacted the iodine formed does not get turned back to iodide and turns to a blue black colour due to the starch.
You time how long it take for the blue black colour to form and so you are measuring the rate at which the thiosulphate is reacting.
one more question though, how much sodium thiosulphate (since I use a fixed amount for the different experiments I do) should I use in relative to how much iodide ions I use etc, I need help determining how much?
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username1445490
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See http://en.wikipedia.org/wiki/Iodine_...xide_variation
This expt sheet explains it http://www.rsc.org/learn-chemistry/r...mp00000822/pdf
and uses thiosulphate and iodide molar ratio of 1:1
also see http://www.kbcc.cuny.edu/academicdep...5_kinetics.pdf
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The Best1
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(Original post by Madasahatter)
See http://en.wikipedia.org/wiki/Iodine_...xide_variation
This expt sheet explains it http://www.rsc.org/learn-chemistry/r...mp00000822/pdf
and uses thiosulphate and iodide molar ratio of 1:1
also see http://www.kbcc.cuny.edu/academicdep...5_kinetics.pdf
Thanks so much for this, I have some small questions if you can please answer

1- Do we strictly use the same amount of thiosulphate ions added for every experiment, while changing the hydrogen peroxide/iodide ions concentration for each experiment?

2- It says an acidic solution is needed in the reaction between Iodide ions and hydrogen peroxide- why? Can I use HCl or sulfuric acid?

3- What is a catalyst? It says in my school sheet I need to find the order of reaction of the catalyst. What is the catalyst in this reaction?

4- Just to clarify, I make solution A in a volumetric flask of potassium iodide, water and thiosulphate ions added. I then make five different solutions in five beakers of different hyodrogen peroxide concentration and starch solution. Then one by one, I add a fixed amount of solution A into each beaker, and time how long it takes to become dark colour. Is this right?

Also, how should I prepare the hydrogen peroxide concentration?

Thanks so much for your help, I really appreciate it, if there is any way I can repay, please ask
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username1445490
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(Original post by The Best1)
Thanks so much for this, I have some small questions if you can please answer

1- Do we strictly use the same amount of thiosulphate ions added for every experiment, while changing the hydrogen peroxide/iodide ions concentration for each experiment?

2- It says an acidic solution is needed in the reaction between Iodide ions and hydrogen peroxide- why? Can I use HCl or sulfuric acid?

3- What is a catalyst? It says in my school sheet I need to find the order of reaction of the catalyst. What is the catalyst in this reaction?

4- Just to clarify, I make solution A in a volumetric flask of potassium iodide, water and thiosulphate ions added. I then make five different solutions in five beakers of different hyodrogen peroxide concentration and starch solution. Then one by one, I add a fixed amount of solution A into each beaker, and time how long it takes to become dark colour. Is this right?

Also, how should I prepare the hydrogen peroxide concentration?

Thanks so much for your help, I really appreciate it, if there is any way I can repay, please ask
1. As you are measuring the time taken for the thiosulfate to be used up the amount of it must be the same each time.
2. The solution should be acidic because of the half equations being
H2O2 +H+ + 2e --> 2H2O
2I- -> I2 +2e
So H+ ions are needed or it won't work. I would use H2SO4 but HCl is fine.
3. There is no catalyst present in the normal reaction but Fe2+ ions act as a catalyst so if you keep the concentrations of everything constant and vary the concentration of Fe2+ ions then you can investigate the effect of the catalyst concentration and determine its order.
4. In beaker A have KI, thiosulfate and starch and in beaker B just H2O2.
Keep the concentration of H2O2 constant and vary the concentration of KI by using water to dilute it and keep the volume constant.
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