Sunday 31st (Sodium Boom n beyond)

Spent the morning very happily going over the potassium footage.  The whole thing is a symphony of physics and chemistry that eventually I decided it would take too long to explain, so I just put up the explosion footage.

It had been raining in Ridgecrest and this clearly was  a freak event.  The standard greeting in Rigercrest between the notably fat folks seems to be ‘hot enough for you’, and not without reason, I think it’s been over 100C every day I’ve been here.  However for the afternoon I decided I would give blowing up the sodium a go.  Now I have lots of sodium, and the scale and geometry is different.  Eventually I decided the way to do it was to get a stick over a reasonable sized vat, which meant that I could lift up the sodium from a distance, have it swing over the vat, then be lowered in.  Sodium is significantly less dense than water, and so would need to be weighted.  The sticks were typically about 10g, so eventually I wrapped up about 4 sticks in aluminium foil, attached weights to them and lowered them in to the vat from a distance.

Initially the reaction seemed really quite slow, to the point where I was wondering how this was going to pan out.  Indeed it took over 10 seconds before the first ‘explosion’ (compared to about 1second for the potassium experiment).  I, for my part am mostly certain this is related to the metal boiling.  However, when it really blew up (and did so quite convincingly) it really made a mess of the container.  Again, the hydrogen burning wasn’t really even a relevant factor, it’s just the adiabatic expansion of the gas released by the reaction.  In this case 40g gives about 40L of gas.  Thats a lot of gas to release in a 5L container!  On inspecting the aftermath it became clear that large chunks of sodium had been thrown clear of the reaction, not just unreacted, but unmelted!  This for me was a stunning observation!  It shows that whatever the reaction that takes place, the rate that heat is generated at the surface is MUCH higher than the rate the heat can be conducted away from the surface.  Sodium is an absolute bitch for making a mess!  Small amounts of sodium had been sprayed all over the tripod, and stripped the paint.  They had then picked up water to make conc. NaOH, draincleaner which will first turn you skin into soap, before chemically burning you.  The stuff was everywhere!  Cleaned up as best I could (again I was on the inaccessible dirt road to nowhere in my little ravine).  Headed back to LA.  Stopped off to see Lisa n Howard n pick up a shower, which I more than needed.  Initially i wanted to get up onto mnt Wilson early, for sunset, but got chatting to Lisa and that never happened.  It was full dark before I set out on the nightmare trek across LA by dark.  It didnt help that I managed to set the GPS wrong, and ended up on the long drive through the horrific twistiy winding roads of the Wilson range (rt 2).  There had been rain here too, and while the road was all but deserted, top speed was about 40 mph and ‘tire killing rocks’ materialized out of the gloom on a regular basis.  Driving that 20 miles was an incredibly intense experience, ultimately driven by ‘do u want it or not’.  I did, I wanted to get up on mnt wilson and do something on light pollution and LA.  I got there about midnight, and got my piccies!

Potassium explodes without Hydrogens help

Thus far almost all the stuff I’ve done on the alkali metals has been on ~250 mg quantities.

Self preservation says, “Start Small!”, and with good reason, the reactions of alkali metals with water are known to scale in treacherous fashions.

In the first instance I went for 1.6 g of potassium and 3.5g of sodium.

Note here there is not the slightest hint of green gas with the potassium!

Now the potassiums the one I’m going to focus on for the moment, simply as when this is played back at about 1/100ths speed it becomes clear that the hydrogen burning plays essentially no part in the violence of this reaction.

Fascinatingly the explosion appears to go in two stages here!

So potassium, molecular weight ~40, so 1.6 g is 0.04moles. 2moles of potassium gives a mole of H₂ , so this reaction will produce about half a liter of hydrogen.  Just for scale the jug is about 3 liters.  It’s also fascinating to see the ‘springiness’ around this generated gas.

Why is Potassium gas Green?

So my interest was really spiked here when I found that when reacting with water, potassium gives off a green gas!

While the green of potassium gas has been known about for over a century, being found in Encyclopedia Britianica articles as far back as 1911, the origin of that green color is proving a little more elusive.

If you know what you are doing it’s relatively easy to observe.  250 mg of potassium is ‘the right’ scale to work with.  Much smaller and it’s all over before you get a chance to do anything, much bigger and you run into problems with the metal exploding, or the hazards of hydrogen build-up.  All you need is a heavy glass vessel of about the dimensions of a wine glass.  Insert a burning acetone taper through a small hole in the top of the vessel and to burn out all the oxygen, and then drop in your potassium.  The green color is easily visible.

I’m tempted to propose it’s due to the solvated electron, which is a hot research topic at the moment as it’s part of the principal mechanism by which radiation damage happens to DNA.  It’s known to be stable and blue in liquid ammonia, but survives only picosecond in water.

A bead of Sodium Potassium alloy in anhydrous liquid ammonia. The blue colouration is due to the presence of solvated electrons

It’s an interesting suggestion, but I think the observation that you get the green color when there is no water around pretty much kills this idea.

So how do you get a green gas.  Well the only other metallic gaseous vapor I’ve seen was mercury which I once boiled to test the calibration on a thermocouple.  That has no color at all that I can remember.

Well as you will recall virtually all the gases are colorless, e.g. all the Noble gases (helium, argon etc) and all the first period gases (nitrogen, oxygen, and mostly fluorine).  However the heavier halogens have colored vapors that get more heavily colored as you go down the group.  Chlorine, light green, bromine, brown etc.

Chlorine (left) and bromine (right).

The reason these gases have these colors is the same reason the sky is blue, Rayleigh scattering.  That’s related to the polarizability of the molecule.  This is a very different mechanism from the electronic transitions that give the classical flame colors of the alkali metals!

It’s well known that all the alkali metals have stable bound states as diatomic molecules, similar in electronic configuration to hydrogen.  IF K₂ had a similar polarizability to Cl₂, it may well be green like chlorine!  Regrettably finding the polarizability K₂ is not as easy as it sounds.  A significant difference between H₂ and K₂ is the bond energy.  H₂ has a bond energy of about 400kJ/mol, while K₂ has a bond energy of ~50kJ/mol.  For reference, the hydrogen bond, the thing that holds water together as a liquid, has a bond energy of ~20kJ/mol.  If you heat any bound state and eventually the species will gain enough energy to separate and become individual species.  With water this happens at about 100 ^oC (boiling, ~400 K).  For hydrogen it’s about 3000 K.  You can actually do the real calculations, it’s just I’m too lazy at the moment, and so I’m just going to do a linear extrapolation between these two.  That gives K₂ breaking up at about 2-300 ^oC.

Well that would fit nicely with green gas being evolved at lower temperature, but as the temperature rises, the diatomic species break up, and the relevant polarizability of the molecule, and hence the Rayleigh scattering, and hence the color is lost.

Great, so if this is a working hypothesis, then the diatomic metals should match their corresponding halogen right?  The bummer is sodium.  Sodium gives off a blue vapor when it boils.  Fluorine is almost colorless.  ARSE!

The game is not over yet!

I’ve decided I need to see sodium vapor for myself, but how to do it with only the junk I have to hand!  What I really need is a nice small sealed silica tube that can take temperatures over a thousand degrees C.  Hmmmm, thinking, thinking…..