Let’s add lithium powder

Lithium: a page from Theodore Gray's book The Elements
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.

image022.png

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.

image013.png

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:

image023.png

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

image026
image028
image030

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.

image035
image037
image039
image041
image043

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.

image002
image050
image016

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

3 thoughts on “Let’s add lithium powder

  1. On the Mohs scale of hardness, we have 0.4, 0.5 and 0.6 MPa for Li, Na and K, respectively. But the close numbers can be deceptive; lithium is noticeably harder to cut than sodium, which in turn can’t be flattened as easily as K can under the gentle press of a hammer head.

    Like

  2. Love your project, great idea and very well presented!!
    Maybe a little disclamer to this week’s edition would be good though, noting that your 0.4 mol of Li(s) will not actually idle around and wait for what will happen next week. The limitation of the reaction with hydrogen obviously only applies for a duration of 2 hours, but not all readers might get that. However, for the sake of this thought experiment I of course agree some pure Li should remain at this stage.🙂

    Like

    1. Thanks for clarifying… I was actually assuming no time elapsed from one week to the next – I’m writing this as if the elements are added in quick sequence!

      I’ll get around to writing oxygen at some point… there are so many reactions that take place, though… it’ll be quite complicated.

      Like

Talk to me

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s