Brainiac’s alkali metals were FAKED!

Creative commons material at:

On the road I will be putting a journal up of my tour of the US on:

So why does 7 grams of lithium release more energy than 130 g of cesium reacting with water.

Well 7 g of Li and 130 g of Cs contain about the same number of nuclei. Now you could just do the experiment of dunking both of these into water, and seeing which generates the most heat. Its a little impractical due to the cost of cesium. The violence of the reaction probably wouldnt be that big of a problem. Cesium melts at almost room temperature, so you just put it in a syringe and inject it into water.

However one of the nice things about chemistry is that it allows us to calculate things. In this case we can calculate the relative energy of Cs and Li dissolving in water.

Li(m) + H2O = Li+(aq) +.5H2 +OH-(aq)

Cs(m) + H2O = Cs+(aq) +.5H2 +OH-(aq)

Well as we are doing a comparison, there are components that are energetically equivolent in both these equations so we can cancel them.

Li(m) = Li+(aq)

To calculate this we need to know three values, the energy to atomize the metal, the energy to strip an electron off it, and the energy to dunk that atomized ion into water at infinite dilution. Thankfully chemists are nerdy types and all these values are known.

Put the numbers in and you find that Li(m) and Cs(m) reacting with water are virtually energetically identical. However in the case of equal masses of Li and Cs, the lithium will release about 20x as much energy (as you have 20x as many atoms).

So why does one explode and not the other. I think several factors are relevant.
1) the area of water: metal contact. Lithium floats v. high in the water, and only areas of contact generate heat. Cesium sinks like a rock.
2) Cesium boils a lot easier than lithium.
3) Ill bet, but dont know, that Cs has a much lower surface tension. This means its far easier for cesium to get a standing wave on its surface that encloses an drop of water (I think causing the explosion). If the metal has a very high surface tension this cannot happen without much bigger drops.

Bottom line is, I’ve got to get the cameras closer to the action to work it out, and regrettably that’s not going to happen for months.

I’m heading out on the road, and trading the roof of bricks and mortar for that of dark night and bright stars.

The guys doing the rubidium are University of Nottingham.



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