Elemental iodine from Potassium Iodide - Reaction Equations

  1. 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
     
    Last edited: Jul 25, 2013
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  3. DrDu

    DrDu 4,463
    Science Advisor

    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.
     
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  4. 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] ?
     
  5. DrDu

    DrDu 4,463
    Science Advisor

    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.
     
  6. 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.
     
  7. 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].
     
  8. Chloride is significantly harder to oxidize than iodide. Look up the redox potentials.
     
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