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Elemental iodine from Potassium Iodide - Reaction Equations

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Astrum
#1
Jul25-13, 02:05 AM
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P: 274
In the reaction to get I2 from an aqueous solution of KI.

The process. Take the aqeuous KI, and pour concentrated hydrochloric acid, followed by hydrogen peroxide.

K++I-+H++Cl-+H2O[itex]\rightarrow[/itex] KCl + HI + H2O

The reaction with H2O2 is perplexing.

I'm a physics student trying to do chemistry, I'm sure I made some mistake in writing out that chemical equation (I forgot if ions have to be separated in the equation, when in a solute).

What are the specifics of this chain of reactions? The elemental iodine will precipitate out of solution. The H2O2 must be oxidizing something in the K+ + Cl- + H+ + I- + H2O
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DrDu
#2
Jul25-13, 03:47 AM
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The Iodine gets oxidized:
[itex]\rm 2 I^-\rightarrow I_2+2e^- [/itex]
Hydrogen peroxide gets reduced:
[itex]\rm H_2O_2 +2e^-+2H^+\rightarrow 2 H_2O [/itex]
The protons on the LHS stem from the hydrochloric acid, that's why you add acid.
Astrum
#3
Jul25-13, 04:59 PM
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P: 274
I see, hydrogen peroxide is a oxidizing agent, I seemed to have missed that.

Would [itex]H_{2}O_{2}[/itex] react with KI in the absence of any hydrogen ions?

The net equation would be [itex]2I^{-}+2K^{+}+2H^{+}+2Cl^{-}+H_{2}O_{2}\rightarrow I_{2} + 2KCl + 2H_{2}O[/itex] ?

DrDu
#4
Jul26-13, 02:33 AM
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Elemental iodine from Potassium Iodide - Reaction Equations

Quote Quote by Astrum View Post
Would [itex]H_{2}O_{2}[/itex] react with KI in the absence of any hydrogen ions?
The net equation is correct. As soon as you use water as a solvent there will always be hydrogen ions around due to the autoprotolysis of water. I am not sure whether the reaction would also take place in an alkaline medium.
hilbert2
#5
Jul26-13, 04:08 AM
P: 327
Quote Quote by DrDu View Post
I am not sure whether the reaction would also take place in an alkaline medium.
In alkaline solution of ##H_{2}O_{2}## the actual oxidant is the perhydroxyl ion ##HO^{-}_{2}##, for which we have the redox half-reaction

##HO^{-}_{2}+H_{2}O+2e^{-} \rightarrow 3OH^{-}## ##E^{0}## = +0.87 V

The redox potential is higher than that of iodine (+0.59 V), so the reaction should also happen in alkaline medium.
Astrum
#6
Jul26-13, 06:39 PM
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P: 274
I'm wondering why the [itex]H_2O_2[/itex] doesn't oxidize the chlorine anion as an equal amount to the oxidation of iodide.

This process probably produces some amount of [itex]Cl_2[/itex].
Corribus
#7
Jul26-13, 08:10 PM
P: 5
Chloride is significantly harder to oxidize than iodide. Look up the redox potentials.


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