Let’s add lithium powder

Lithium: a page from Theodore Gray’s book The Elements

Initial condition

• Hydrogen gas, H₂(g): 1.00 mol
• Helium gas, He(g): 1.00 mol

Last week, our vessel contained a mixture of hydrogen and helium gases. No chemical reactions have occurred so far, but that is about to change. Today, we’ll add 1.00 mole of lithium powder to the mixture and observe our first chemical reaction.

What does lithium look like?

Lithium is a soft, silvery metal with the consistency of Parmesan cheese. Lumps of lithium can be cut with a knife and it’s so light that it floats on oil. It would float on water as well if it weren’t for the violent reaction that would take place. Lithium is very well-known by science students for its ability to react with water, producing hydrogen gas and an alkaline solution of lithium hydroxide.

There’s no water in our vessel so the above reaction won’t actually take place. We’ve only got hydrogen gas and helium gas inside. Let’s see if our powdered lithium reacts with either of those gases.

Will the lithium powder react in our vessel?

Yes! Lithium reacts with hydrogen gas very slowly. One paper by NASA cited a reaction occurring at 29°C but the yield and rate were both very low. Because I want to initiate as many reactions as possible in this experiment, I’m going to heat my vessel to 99°C by immersing it in a bath of hot water. According to the NASA paper, this temperature would give my reaction a 60% yield after two hours.

Lithium hydride is beginning to collect in the bottom of my 10-litre vessel. It’s a grey-to-colourless solid with a high melting point.

How much of each substance do we now have in the vessel?

First, we need to know which reagent is limiting. We can calculate this by using the following rule:

Let’s substitute the values into the expression for all the reactants in this reaction: Li(s) and H₂(g).

If the yield was 100% (i.e. a complete reaction), I’d expect to make 1.00 mole of lithium hydride. However, we’re only going to get 0.60 moles because according to the NASA paper, the yield of this reaction is only 60% at my chosen temperature.

Let’s do an ‘ice’ table to find out how much of each reactant reacts, and hence how much of each substance we have left in our reactor vessel.

units are mol	2Li	H2	2LiH
I (initial)	1.00	1.00	0
C (change)	-0.60	-0.30	+0.60
E (equilibrium)	0.40	0.70	0.60

By the end of our reaction, we’d have:

• H₂(g): 0.70 mol
• He(g): 1.00 mol
• Li(s): 0.40 mol (still solid: it melts at 180.5 degrees)
• LiH(s): 0.60 mol

What does 0.60 mol LiH look like?

Let’s use the density formula to try find out how many spoonfuls of LiH we’ve created.

We’ve made 6.11 millilitres of lithium hydride powder! That’s a heaped teaspoon of LiH.

What’s the resulting pressure in the vessel?

Our elevated temperature of 99°C will have caused a considerable pressure increase inside the vessel.

That’s 5.2 atmospheres (atm) of pressure, which is quite high. A typical car tyre is about 2 atm for comparison.

What if the vessel exploded?

BANG. The contents of the vessel, after they’ve rained down on an unsuspecting crowd, would react explosively with the water and other compounds in our bodies to produce caustic lithium hydroxide and toxic lithium salts. I recommend stepping away from the vessel and behind a thick safety screen at this point. Even though our imaginary vessel is quite strong, we better put on a lab coat and safety glasses as well—just in case.

Conclusion after adding lithium powder

• H₂(g): 0.70 mol
• He(g): 1.00 mol
• Li(s): 0.40 mol (still solid: it melts at 180.5 degrees)
• LiH(s): 0.60 mol
• Pressure = 525.5 kPa
• Temperature = 99°C

Next week, we’ll add 1.00 mole of beryllium to the vessel and see what happens.

Reference: Smith, R. L.; Miser, J. W. (1963). Compilation of the properties of lithium hydride. NASA