Tuesday
July 22, 2014

Homework Help: chemistry

Posted by jacob on Monday, February 5, 2007 at 1:29am.

The balanced equation doesnt seem to help because the thiosulfate is not in the equation, I think it is preventing the reaction from going backwards. THe equation is:
2iodide ion + hydrogen peroxide + 2 H3O --> elemental iodide + 4 H2O
within the reaction there is also deionized water, an acetate buffer, 0.3 M KI, 0.02 M Na2S2O3 (Sodium thiosulfate) and starch. and then hydrogen peroxide is added to that solution.
It says the thiosulfate removes the elemental iodide as it is formed so I think that is preventing the reaction from going backwards.
I dont know the amount of the product and am just given the amount of each solution, because it is a rate problem, I am supposed to determine the rate experimentally but I dont understand how you can find the amount of thiosulfate reacted when I am only given sodium thiosulfate.
Is it possible to seperate the sodium from the sodium thiosulfate? I think the sodium from the sodium thiosulfate reacts with the buffer as it becomes a acetic acid-sodium acetate buffer.
I am assuming that to get this I must use either the rate law equation or the activation energy equation but those dont seem to help any.

How can you find the amount of moles reacted in a rate problem. I am given the molarity of sodium thiosulfate ( 0.002 M) and I am to find the amount of thiosulfate ion reacted in moles. I also have the temperature and the length of the reaction. Can I seperate the thiosulfate ion from the sodium thisulfate with stoichiometry?
Any help is appreciated

I would look to the balanced equation. Do you know the amount of product?

Thiosulfate removes the elemental iodine as follows:
S2O3^-2 + I2 ==> S4O6^-2 + 2I^-
Yes, you may separate thiosulfate ion from sodium thiosulfate.
Na2S2O3(s) + H2O ==> 2Na^+(aq) + S2O3^-2(aq)

You are correct that the acetate is there to buffer the solution but your reasoning is not correct. The original equation you wrote contains H3O^+. That is the ion being controlled by the NaC2H3O2 (sodium acetate) and not the sodium ion.
You can calculate the amount of thiosulfate used by knowing how much I2 was formed. I hope this wlll get you started.

how can you find out how much I2 is formed
I think that that is the second step after I find the amount of thiosulfate reacted.
I know the molarity of KI and Na2S2O3 and the volume of each of those solutions so I can get the moles or grams of those but how can I get them when they are a combination of 2 elements?

You don't have enough information in all of the posts for us to know exactly what you are doing or how you are doing it. So we are trying to answer questions piece meal. Have you balanced the equation? I think it should be
2H^+ + H2O2+ 2I^- ==> I2 + 2H2O

As soon as the I2 is formed it is reduced back to I^- by thiosulfate. When does the starch indicator turn? What time are you measuring? Is the H2O2 standardized? If not, is anything standardized? If the H2O2 has been standardized, that is what is used to determine the amount of I2 released.

The starch indicator turns blue at various times because there are various amounts of each solution. The H2O2 is standardized in the first couple of trials so I guess that I can use that

Yes, if the H2O2 concentration is known then I2 will be known.

ok thanks i think i got it

So what is the rate reaction?

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