# Elemental iodine from Potassium Iodide - Reaction Equations

by Astrum
Tags: elemental, equations, iodide, iodine, potassium, reaction
 P: 273 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$\rightarrow$ 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
 Sci Advisor P: 3,376 The Iodine gets oxidized: $\rm 2 I^-\rightarrow I_2+2e^-$ Hydrogen peroxide gets reduced: $\rm H_2O_2 +2e^-+2H^+\rightarrow 2 H_2O$ The protons on the LHS stem from the hydrochloric acid, that's why you add acid.
 P: 273 I see, hydrogen peroxide is a oxidizing agent, I seemed to have missed that. Would $H_{2}O_{2}$ react with KI in the absence of any hydrogen ions? The net equation would be $2I^{-}+2K^{+}+2H^{+}+2Cl^{-}+H_{2}O_{2}\rightarrow I_{2} + 2KCl + 2H_{2}O$ ?
P: 3,376

## Elemental iodine from Potassium Iodide - Reaction Equations

 Quote by Astrum Would $H_{2}O_{2}$ 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.
P: 269
 Quote by DrDu 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.
 P: 273 I'm wondering why the $H_2O_2$ doesn't oxidize the chlorine anion as an equal amount to the oxidation of iodide. This process probably produces some amount of $Cl_2$.
 P: 5 Chloride is significantly harder to oxidize than iodide. Look up the redox potentials.

 Related Discussions Biology, Chemistry & Other Homework 1 Biology, Chemistry & Other Homework 1 Biology, Chemistry & Other Homework 3 Biology, Chemistry & Other Homework 1 Chemistry 3