Monday, 22 February 2016

Coulomb explosions in alkali metals

The reaction of alkali metals with water is one of the all-time classic chemistry demonstrations: a pellet of sodium or potassium is dropped onto the surface of water, where it reacts to produce hydrogen gas and a lot of heat. The heat ignites the hydrogen and melts the metal into a little ball which then jumps around on top of the water trailing flames and steam.

The basic chemistry is a reduction-oxidation reaction, that is, a reaction involving exchange of electric charges. Water isn't just a bunch of neutral water molecules H2O. It also contains fragments in the form of positively charged hydrogen ions H+ and the negatively charged remainder called hydroxide OH-.

H2O --> H+ + OH-

The alkali metals are holding onto their last negatively charged electron very weakly, and give it up very easily to the positively charged hydrogen atom. The neutral potassium atoms become positively charged potassium ions, and the positive hydrogen ions become neutral hydrogen atoms:

K + H+ --> K+ + H

Those neutral hydrogen atoms can then pair up to form hydrogen molecules, while the charge of the new positive potassium ion is balanced by the negative hydroxide ion:

K+ + OH- --> KOH (in solution)
H + H --> H2 (gas)

This is, of course, a gross simplification, but it gives you an overall idea of the processes involved. The upshot is that neutral hydrogen forms a gas that is ignited by all the heat released by the reaction, leaving behind potassium hydroxide KOH, also known as potash. This is an alkali.

However one of the great pleasures of science is checking to see what really happens. A group of researchers at the Academy of Sciences of the Czeck Republic and the Technische Unversitat Braunschweig studied the reaction between potassium and water using a high-speed camera, and they found that the metal bursts apart in a ball of spikes:

What's going on here? Well, remember that the first step in the reaction is that each potassium atom loses an electron to the water. That leaves all the atoms at the surface of the potassium with a positive charge. Similar electric charges - like matching poles on magnets - repel each other. Potassium is a pretty soft metal, so once all those surface atoms turn into charged ions, and before they have a chance to dissolve into the water, the repulsion between the ions is enough to cause the ball to blast apart into spikes. This is called a "Coulomb explosion" - Coulomb from the Coulomb force between electrical charges, and explosion because it's an explosion. The researchers performed highly sophisticated computer simulations to verify that this was indeed what's happening - no mean feat, given that they have to accurately consider the movement of electrons, metals, and water molecules.

Coulomb explosions show up in other parts of chemistry. A Mass Spectrometer which measures the masses of charged molecules flying through the instrument, so you can figure out what they are, or measure how much of a known molecule is in a mixture. However first you have to get the molecules out of the mixture so they're floating around freely, and put an electric charge on them. A classic way of doing this is to run a stream of your mixture into a powerfully electrically charged nozzle. By dumping a big charge on the mixture, you not only give the molecules the right electric charge, but you cause all of the different molecules to repel each other rapidly until you've got just individual molecules floating around in space.

I did some research work on that process in my undergraduate years. It's always great to see how universal these processes are.

Bringing this obviously ancient draft post up to date, here's Periodic Videos putting a light alkali metal - lithium - into 7-Up. The results are less spectacular, because the reaction is less aggressive.

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