Balanced Redox Equations for OCl- Titration with S2O32-

[jumbogala]

Homework Statement

OCl- is reacted with I- in acidic solution to generate I₂.

The I₂ generated is then titrated using a solution of Na₂S₂O₃.

If you look at the sum of the two reactions above, information about S₂O₃- can be used to determine the amount of I₂ and thus the amount of ClO-.

Give the relevant balanced equations for the titration of OCl- with S₂O₃^2-. Including all redox half reactions there are three.

Homework Equations

N/A

The Attempt at a Solution

First I figured out the two equations. I think they are:
OCl- + H₂O + 2I- ---> Cl- + 2OH- + I₂

I₂ + 2S₂O₃^2- ---> 2I- + S₄0₆^2-

Are those correct? Is it different because it's in acidic solution? The OH- make me think I did this in basic solution, but I'm not sure.

Then I added the two equations together to find:

OCl- + H₂O + 2S₂0₃^2- -----> Cl- + 2OH- + S₄0₆^2-

If the last equation is correct, I don't see how that works. There's no I₂ in the equation. The question says I need to go from thiosulfate ion, to I₂, then to OCl-... hmm. What am I doing wrong?

Thanks!

 

[Borek]

First I figured out the two equations. I think they are:
OCl- + H₂O + 2I- ---> Cl- + 2OH- + I₂

I₂ + 2S₂O₃^2- ---> 2I- + S₄0₆^2-

OK with me.

OCl- + H₂O + 2S₂0₃^2- -----> Cl- + 2OH- + S₄0₆^2-

If the last equation is correct, I don't see how that works. There's no I₂ in the equation. The question says I need to go from thiosulfate ion, to I₂, then to OCl-... hmm. What am I doing wrong?

At first approximation - nothing. Iodine is only an intermediate product, used to transfer electrons in a controlled manner, as reaction between hypochlorite and thiosulfate is a messy one, with numerous byproducts and dubious stoichiometry.

 

[Blueshift5]

Your attempt at the solution is close, but there are a few errors. First, the second equation should be written as follows:

I2 + 2S2O32- + 2H+ ---> 2I- + S4O62- + H2O

This accounts for the acidic solution by including the H+ ions. Also, in the first equation, the OH- ions should not be present since the reaction is taking place in acidic solution. The correct equation is:

OCl- + 2I- + 2H+ ---> Cl- + I2 + H2O

When you add the two equations together, you should get:

OCl- + 2S2O32- + H+ ---> Cl- + 2I- + S4O62- + H2O

This equation shows the overall reaction that takes place when OCl- is titrated with S2O32-. The thiosulfate ions are used to titrate the I2 that is generated from the first reaction, and the remaining OCl- is converted to Cl- ions. This equation can be used to determine the amount of OCl- present in the original solution, based on the amount of S2O32- needed to titrate the I2. Overall, the key to balancing redox equations is to make sure that the number of electrons lost in the oxidation half reaction is equal to the number of electrons gained in the reduction half reaction.