# Equation for the reaction of iodide to iodine using hydrogen peroxide under acidic conditions?

I'm building a home lab around All Lab, No Lecture Illustrated Guide to Home Chemistry Experiments and I'd like to understand the chemistry that I'm doing a bit better. For example, here's an iodine experiment.

1. Weigh out 2.0 g of potassium iodide and transfer it to a test tube.

2. Add about 1.5 mL of distilled water to the test tube and swirl to dissolve the potassium iodide.

3. Add 1.5 mL of concentrated hydrochloric acid (or about 1.8 mL of hardware store muriatic acid) to the test tube and swirl to mix the solutions.

4. Add about 10 mL of drugstore 3% hydrogen peroxide. The solution immediately turns dark brown as the iodide ions are oxidized to elemental iodine, which precipitates out.

5. Swirl the test tube to suspend the iodine and pour the liquid through a funnel with a folded piece of filter paper to capture the iodine crystals.

6. Rinse the iodine crystals on the filter paper several times with a few mL of distilled water. The rinse solution appears brown from dissolved iodine, but iodine is not very soluble in water, so you're not losing much of your yield.

7. Spread out the filter paper on a watch glass or saucer and allow the crystals to dry thoroughly. Iodine gradually sublimates (passes directly from solid to gaseous form) at room temperature, so don't leave the crystals exposed to air any longer than necessary to dry them.

8. Once the crystals are dry, transfer them to a sealed storage bottle or vial.

It's easy enough to follow and I know I have $$\ce{KI, H2O, HCl, H2O2}$$ with a result of $$\ce{I2\, +\, ???}$$ but then what?

I've had HS Chemistry, skipped Chem in college :(, so I know how to balance equations, convert from g or ml to moles, etc, but where can I learn to translate an experiment into equations without having to ask others (Textbooks or online resources are ideal, going and taking chem at a local uni. is a distant second, since I work fulltime)?

Currently I'm sticking to inorganic chem, so that should simplify things a bit.

I'm currently studying EdX's Solid State Chemistry, so I hope that'll help some.

• This is the reaction equation that you're looking for: $\ce{2 HCl + 2 KI + H2O2 -> I2 + 2 KCl + 2H2O}$ (source:sciencemadness.org/talk/viewthread.php?tid=10202 , 2nd post) – user2117 May 16 '14 at 18:21
• Thank you. That was my "bonus" question. I suspected that the results were $$KCl,H_2O$$, but I wasn't sure. My main question still stands. Where can I learn to make that leap instead of always looking it up? Also, how do you do subscripts without using LaTeX? – Joshua Olson May 16 '14 at 18:30
• @solarmist - we have the mhchem package for MathJax. Learn more here and here – Ben Norris May 16 '14 at 19:17
• @LievenB - that equation is wrong. There isn't any unionized hydrochloric acid because it is a strong acid. It would be more reflective of chemical fact to write instead $H_3^+O$. – Dissenter May 16 '14 at 19:23
• @Dissenter That's the essence of my question where can I learn to make that kind of call or determine the configuration of the product of a reaction? – Joshua Olson May 17 '14 at 1:26

The oxidation equation is as such: $\ce{2I- -> I2 + 2e-}$
The reduction equation is as such: $\ce{2H+ + H2O2 + 2e- -> 2H2O}$
Therefore, the overal equation is as such: $\ce{2H+ + H2O2 + 2I- -> 2H2O + I2}$