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I recently added a potassium hydroxide, $\ce{KOH}$, pellet to a small droplet of hydrogen peroxide (30 %), $\ce{H2O2}$. You could clearly see the potassium hydroxide dissolving in the droplet when instantaneously the solutions began to fizz strongly (boil and/or release oxygen gas, I'm not sure) and release a lot of heat.

I'm not sure what happened, but I've got a few ideas of what might have happened:

  • There is some kind of reaction between the $\ce{KOH}$ and $\ce{H2O2}$.
  • The $\ce{KOH}$ released a lot of energy when dissociating into the aqueous solution and then boiled the water, maybe reaching the temperature of 150.2 °C in which it also triggered the decomposition of $\ce{H2O2}$ into water and oxygen gas.
  • On Wikipedia I read:

    The rate of decomposition increases with rising temperature, concentration and pH.

    So when $\ce{KOH}$ is dissolved, the solution has a high pH and this might trigger the decomposition of hydrogen peroxide

I did notice after a pH test that the plastic lid on which I had condensed some of the evaporated water had a very high pH, so this would most likely contain some dissolved potassium hydroxide.

In case a reaction happened, what could that specific reaction be?

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    $\begingroup$ I have no time to answer, but I am pretty sure that $\ce{KOH}$ is capable of abstracting a proton from hydrogen peroxide, leaving protonated dioxygen ($\ce{HO2-}$), which should be a very strong acid. $\endgroup$ – Martin - マーチン May 5 '14 at 14:59
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    $\begingroup$ @Martin While $\ce{KOH}$ can certainly deprotonate $\ce{H2O2}$, the resulting hydroperoxide anion $\ce{HO_2^{-}}$ isn't a strong acid. $\ce{H2O2}$ has a $pK_a$ of 11.75 in water according to Wikipedia, and the second ionization is probably several orders of magnitude harder. $\endgroup$ – Nicolau Saker Neto May 5 '14 at 23:35
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    $\begingroup$ @Nicolau You are very right, I confused it with $\ce{HO2+}$ - as I said I was in a hurry. $\endgroup$ – Martin - マーチン May 6 '14 at 15:20
  • $\begingroup$ your cap is basic because of small aerosolized water droplets, not some gas phase product. $\endgroup$ – A.K. Dec 13 '15 at 14:23
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According to THE HOMOGENEOUS BASE-CATALYZED DECOMPOSITION OF HYDROGEN PEROXIDE J. Phys. Chem., 1961, vol. 65 , pages 304–306:

$\ce{H2O2 + OH- <=> HO2- + H2O}$

$\ce{H2O2 + HO2- ->}$ [6-membered ring with 2 "H"s bridging 2 "O-O"s, with another H bonded to one of the "O"s] $\ce{-> H2O + O2 + OH-}$

It is clear that the rate will go through a maximum when the peroxide is 50% ionized, that is $[\ce{H2O2}] = [\ce{HO2-}]$.

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  • There is some kind of reaction between the $\ce{KOH}$ and $\ce{H_2O_2}$

There was definitely a reaction.

  • The $\ce{KOH}$ released a lot of energy when dissociating into the aqueous solution and then boiled the water, maybe reaching the temperature of 150.2 °C in which it also triggered the decomposition of $\ce{H_2O_2}$ into water and oxygen gas.

Water cannot reach a temperature above 100 °C unless it is pressurized.

Yes, $\ce{H_2O_2}$ can disassociate rather quickly under the right conditions. An enzyme called catalase, can break up $\ce{H_2O_2}$ at an astounding rate. If you add an amount you can barely see, like the size of a period at the end of a sentence, to 1 mL of 30% $\ce{H_2O_2}$ in a test tube, the oxygen production will push the liquid out of the tube.

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