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Topic: Sythesis of Iodine Solution  (Read 8782 times)

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Offline CKabes

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Sythesis of Iodine Solution
« on: December 16, 2011, 10:34:17 AM »
A mixture of Potassium Permanganate (KMnO4) and Potassium Iodide (KI) are mixed with solid Sodium Hydrogen Sulfate (NaHSO4) and a few ml's of water are added. The Solution is then heated and a purple vapor (Iodine) travels thrugh the curved glass pipe affixed at the top of the testube, into a solution of Alcohol. Then Alcohol becomes brown, and tests positive for Iodine.

What's going on here?

KMnO4 + KI + NaHSO4 --> ???

Offline Dan

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Re: Sythesis of Iodine Solution
« Reply #1 on: December 16, 2011, 11:16:24 AM »
So, you know one of your products is iodine...

You are in an acidic solution (sodium hydrogen sulfate [a.k.a. sodium bisulfate] is a fairly strong acid). Which oxidation state is Mn(VII) reduced to in acidic solution?
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Offline CKabes

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Re: Sythesis of Iodine Solution
« Reply #2 on: December 16, 2011, 12:05:02 PM »
I believe permanganates in acid are reduced to Mn2+

Offline Borek

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Re: Sythesis of Iodine Solution
« Reply #3 on: December 16, 2011, 03:47:49 PM »
Good, you also observe iodine vapors - that means you already should know both half reactions.
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Offline CKabes

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Re: Sythesis of Iodine Solution
« Reply #4 on: December 16, 2011, 05:16:08 PM »
Ok, so

KMnO4 + 2KI + NaHSO4 + H2O --> I2 + H2O (because of the permangante in acid) + Mn2+ + NaHSO4 (this doesn't play a role in the reaction except to change the ph maybe?) + ??

What happens to the potassium?

And if the acid gives up it's hydrogen, what are we left with?

-I also think maybe this is two reactions or more?-

Offline Borek

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Re: Sythesis of Iodine Solution
« Reply #5 on: December 16, 2011, 06:53:09 PM »
Do you know what a net ionic reaction is?
« Last Edit: December 17, 2011, 03:40:50 AM by Borek »
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Re: Sythesis of Iodine Solution
« Reply #6 on: December 16, 2011, 08:39:34 PM »
NaHSO4 (this doesn't play a role in the reaction except to change the ph maybe?) + ??

It does play a role - you have identified that Mno4- is reduced to Mn2+, but what happened to the oxygen?

Hint: What's the half-reaction for the reduction of permanganate in acid?
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Offline CKabes

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Re: Sythesis of Iodine Solution
« Reply #7 on: December 17, 2011, 08:55:51 AM »
8 H+ + MnO4- + 5 eāˆ’ ā†’ Mn2+ + 4 H2O

We're getting there!  :) but now what to do with the potassium? Could it form K2SO4? and then we're only left with a sodium compound, if that were the case.
And what is a net ionic reaction?

Offline CKabes

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Re: Sythesis of Iodine Solution
« Reply #8 on: December 17, 2011, 09:03:03 AM »
Ok, I'm sure this is wrong, but so far, this is what I would hypothosise:

2KMnO4 + 14KI + 16NaHSO4 --> 2Mn2+ + 8H2O + 8K2SO4 + 8Na2SO4 + 7 I2

Offline Borek

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Re: Sythesis of Iodine Solution
« Reply #9 on: December 17, 2011, 09:28:04 AM »
Ok, I'm sure this is wrong, but so far, this is what I would hypothosise:

2KMnO4 + 14KI + 16NaHSO4 --> 2Mn2+ + 8H2O + 8K2SO4 + 8Na2SO4 + 7 I2

You are right it is wrong - it is not balanced. Not only the mass is conserved (which means atoms must be balanced), also the charge is conserved (so the charge on the left must be identical to the charge on the right). Apparently this is not the case - you have +2 on the right, but 0 on the left. In the net ionic reaction Mn2+ would be a perfectly valid product, but as you are listing uncharged compounds as everything else, you need to find a counterion to list Mn2+ as an electrically neutral salt.

Net ionic reaction is the one that doesn't list all spectator species (ones that don't change during reaction). In the net ionic reaction you don't care about exact compounds reacting, but about ions reacting. For example

NaOH + HCl -> NaCl + H2O

is an overall reaction, while

H+ + OH- -> H2O

is a net ionic reaction. Note that if you mix any strong acid with any strong base counterions (like Na+ and Cl- in this case) don't change - before and after reaction they are freely floating in the solution. Net ionic reaction shows what is really happening, ignoring everything else.
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Offline CKabes

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Re: Sythesis of Iodine Solution
« Reply #10 on: December 17, 2011, 09:46:12 AM »
hmmm,

The first counter Ion that comes to mind would be Oxygen,
Manganese(II) could from Manganese Oxide right? MnO? But there would need to be and Oxygen ion present.

What about Manganese(II) Sulfate? There is already a sulfate ion present, would that work?

Offline Borek

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Re: Sythesis of Iodine Solution
« Reply #11 on: December 17, 2011, 07:43:29 PM »
Oxygen as a counterion in water solution? No way.

Manganese(II) sulfate is a reasonable product.
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Offline CKabes

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Re: Sythesis of Iodine Solution
« Reply #12 on: December 21, 2011, 01:35:49 PM »
Could somebody please provide me with the final equation?

Offline Borek

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Re: Sythesis of Iodine Solution
« Reply #13 on: December 21, 2011, 04:17:22 PM »
Could somebody please provide me with the final equation?

No. But you were given so many hints it is the highest time yo try to solve it. You know iodine is being produced, you know you should expect MnSO4 between products - that's already half of the work.
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