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Topic: Redox Titration  (Read 1160 times)

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Offline mdk2121

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Redox Titration
« on: May 01, 2020, 05:54:57 PM »
I am a bit confused about a question in my textbook involving redox titrations. This is actually a sample problem that has been fully worked through in the book, so I am not trying to get its answer. However, I am confused about one particular step. Here is the question:

A group of students prepares to standardize a Na2S2O3 solution. 32 mL of the Na2S2O3 solution is titrated into 50 mL of a 0.01 M KIO3 solution to reach the equivalence point. They first titrate the KIO3 solution until it loses color, then add a starch indicator until the reaction is complete. The reaction proceeds in these two steps:

IO3-+ I-+ H+ ----->   I3-+ H2O

I3-+ S2O32-  -----> I-+ S4O62-

Determine the concentration of the sodium thiosulfate solution at the beginning of the experiment.##




I think I understand the basic logic here: We know that when we titrate a 32 mL solution of Na2S2O3  (unknown concentration) into 50 mL of 0.01 M KIO3 , we reach the equivalence point. To find the concentration of Na2S2O3 , we find the total number of moles IO3 at the equivalence point and then multiply it by the stoichiometric ratios to find the total moles of Na2S2O3 and then divide by the volume of Na2S2O3.

I realize that the two reactions must be balanced. The textbook notes that it is essential that students not fall into the trap of balancing the first equation as follows:

IO3-+ 2I-+ 6H+  -----> I3-+ 3H2O

I realize that this wrong because the charges don't add up correctly. However, the textbook just follows this up by telling us the correct balance:

IO3-+ 8I- + 6H+  -----> 3I3-+ 3H2O.            and

I3-+ 2S2O32-  -----> 3I-+ S4O62-     

What I don't understand is how we are supposed to figure out how to achieve this balance. I find myself trying to keep track of two different sets of variables (the stoichiometric balance and the charge balance) and can't conceive of a strategy for doing so that makes any sense.

I have a two part question. First, generally speaking, is there a strategy/approach for achieving this balance? In stoichiometric atom balancing, we generally save Hs to last, for example. Is there a roadmap one can apply for balancing when we have to balance atoms and charges?

And second, this question follows shortly after the textbook's discussion of balancing half reactions. However, I am struggling to see how this reaction links to half-reactions. Are the two equations above half reactions? Should I be balancing them as half reactions? (E.g. adding water to balance Os, adding H+s to balance Hs?) Or is that not at all the way I should be thinking about this?

So, in sum, I am trying to figure out (a) how to keep track of balancing atoms and charges, (b) whether balancing the half-reactions is the answer, (c) if so, how, and (d) if not, what in the world half reactions have to do with any of this!

Thank you so much!

(PS: I know that the answer to this problem is 0.094 M -- I promise I'm not trying to get the answer! Just trying to understand the process!)



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Re: Redox Titration
« Reply #1 on: May 02, 2020, 04:06:01 AM »
Balancing redox process is made of two steps - first, determine half reactions and balance them, second, add them to cancel out electrons. When doing half reactions the general strategy is to balance atoms first, then to use electrons to balance the charge. See for example https://www.chembuddy.com/?left=balancing-stoichiometry&right=half-reactions-method

IO3-+ 8I- + 6H+  -----> 3I3-+ 3H2O.            and

I3-+ 2S2O32-  -----> 3I-+ S4O62-

Both these reactions are complete and can be split into half reactions, although both can be a bit tricky. I am afraid the only way to learning how to is through doing many examples and gaining experience (but in general balancing redox reactions is not different from balancing by inspection, you just need to keep track of the charge as well).

Hint: in the first reaction iodate is being reduced to I2 and some iodide is oxidized to I2.
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Offline mdk2121

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Re: Redox Titration
« Reply #2 on: May 03, 2020, 03:01:12 PM »
Thank you! I just want to make sure I am following your response properly. Are you saying that it is possible to create half-reactions for these two reactions, but that it is a bit complicated? And that the more advisable route is to work through the numbers (as one would a normal stoichiometric balance while keeping in mind charge and total atom count) to get a feel for how to balance charge and atoms?

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Re: Redox Titration
« Reply #3 on: May 03, 2020, 06:18:35 PM »
Are you saying that it is possible to create half-reactions for these two reactions, but that it is a bit complicated?

Yes, especially the first one if often confusing for beginners, as both reduction and oxidation produce the same thing.

Quote
And that the more advisable route is to work through the numbers (as one would a normal stoichiometric balance while keeping in mind charge and total atom count) to get a feel for how to balance charge and atoms?

Not sure I understand what you are saying. As I wrote: the general way of balancing half reactions is to balance atoms first, then use electrons to balance charge. It is not a matter of this approach being 'advisable' or 'getting a feel', that's just how the method works.
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