In general, when we put potassium into water, it will produce potassium hydroxide and hydrogen:

$\ce{2K + 2H2O ⟶2KOH + H2}$

However isn't the following reaction also possible?

$\ce{2K + H2O ⟶K2O + H2}$

Why isn't the oxide produced?

  • 6
    $\begingroup$ That's a chemical equation, not a formula (just nitpicking). Welcome to the site! People may say "$\ce{K2O}$ might be transiently formed", but it's too unstable to be isolated, especially in water. We might as well say that $\ce{KOH}$ is directly formed. Side note: A chemical equation can always be balanced, but that is no indication of whether the reaction is a fairy tale or not. $\endgroup$ Jun 5 '19 at 9:46
  • $\begingroup$ Thanks for your answering. I am really appreciate your answer and manner. You even spot some mistakes in the question and let me have a good chance to learn. Thanks :) $\endgroup$ Jun 5 '19 at 11:10
  • $\begingroup$ Thermodynamics comes into effect. $\endgroup$ Jun 6 '19 at 10:05

The reactions are ongoing this way:

Relatively free electrons of potassium reduce water:

$$\ce{2 e- + 2 H2O -> H2 + 2 OH-}\tag{1}$$

That leaves metal positively charged.

Liquid ammonia, if exposed to alkali metal, reacts with electrons much slower than water, forming a dark blue solution of solvated electrons. As electrons progressively kick out protons from ammonia, forming hydrogen, the solution finally turns to a colourless solution of NaNH2.

But back to water.

The potassium ions get hydrated, decreasing the charge.....

$$\ce{K(s)^{n+} -> K(s)^{(n-m)+} + m K+} \tag{2}$$

forming $\ce{KOH}$ solution in form of mixture of hydrated ions $\ce{K+ + OH-}$

But heavy potassium ions cannot keep pace with light and fast electrons and the drop of melted metal progressively gains positive charge and finally ends up by - as authors call it - Coulombic explosion.

The hydrogen gets eventually ignited by microsparcs due the charge instability even before the explosion. As "Terminator T1000-like" spikes of liquid metal eventually pierce isolating vapour+hydrogen layer, getting into contact with ignitable hydrogen-air mixture.

It was recently theoretically predicted by quantum chemistry simulation for several dozens of alkali atoms by Czech chemist Pavel Jungwirth And col. Chemistryworld-Alkali metal explosion explained

They have experimentally verified it by high speed 10000 f/s camera, using sodium/potassium alloy forming an eutectic with low melting point.

I knew that from the popular science radio broadcast interview, finding backwards some reference for it.

See also their article in Nature ( which I forgot about and found later):

Coulomb explosion during the early stages of the reaction of alkali metals with water

Abstract Alkali metals can react explosively with water and it is textbook knowledge that this vigorous behaviour results from heat release, steam formation and ignition of the hydrogen gas that is produced. Here we suggest that the initial process enabling the alkali metal explosion in water is, however, of a completely different nature. High-speed camera imaging of liquid drops of a sodium/potassium alloy in water reveals submillisecond formation of metal spikes that protrude from the surface of the drop. Molecular dynamics simulations demonstrate that on immersion in water there is an almost immediate release of electrons from the metal surface. The system thus quickly reaches the Rayleigh instability limit, which leads to a ‘coulomb explosion’ of the alkali metal drop. Consequently, a new metal surface in contact with water is formed, which explains why the reaction does not become self-quenched by its products, but can rather lead to explosive behaviour.

  • $\begingroup$ This is very insightful. Never thought about such details of reaction mechanism. We take so many things for granted in science. But how to prove the equation number 1 i.e. "Relatively free electrons of potassium reduce water:"? $\endgroup$
    – M. Farooq
    Jun 6 '19 at 4:41
  • $\begingroup$ @M. Farooq Note that eq 1 was supposed even earlier. The article was about mechanism of droplet explosion and hydrogen ignition. Hehe, what about testing of flying curvature of metal droplets in strong electrostatic field ? :-) $\endgroup$
    – Poutnik
    Jun 6 '19 at 4:47
  • $\begingroup$ You know about Taylor cone, even water jet becomes a spray in a strong electric field. $\endgroup$
    – M. Farooq
    Jun 6 '19 at 4:50
  • $\begingroup$ That reminds me Kelvin water dropper. When the apparatus gets charged enough, streams of drops start to diverge, repulsed by the same charge. $\endgroup$
    – Poutnik
    Jun 6 '19 at 6:03
  • $\begingroup$ @M. Farooq See also Nature article link. About the electrons, consider blue solution of solvated electrons in liquid ammonia with alkali metal, as ammonia reacts with electrons much slower than water, finally forming a colourless solution of NaNH2. $\endgroup$
    – Poutnik
    Jun 6 '19 at 6:56

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