Tag Archives: elements

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.

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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.

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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:

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

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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.

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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.

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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

Let’s add hydrogen gas

Hydrogen: a page from Theodore Gray's book, The Elements
Hydrogen: a page from Theodore Gray’s book, The Elements

I’m going to add 1.00 mol of hydrogen gas, H2(g), to our 10-litre vessel. We’ll assume that the entire experiment is carried out at normal room temperature – let’s say it’s 20°C.

How much does a mole of hydrogen cost?

Hydrogen gas is a relatively cheap element, and my one mole of H2(g) would cost less than one cent at wholesale prices. That said, the shipping, handling and service fee would be a couple of orders of magnitude greater than the cost of the gas itself, and I’d probably need to give the store about a dollar for the privilege of taking one cent’s worth of hydrogen gas.

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What does a mole of hydrogen look like?

We’ll assume the temperature is 20°C and the atmospheric pressure is 102.3 kPa, which is what the Weather app on my phone is reading right now. After the hydrogen gas has been released from its high-pressure storage cylinder, my one mole of H2(g) would have a volume of 23.8 litres at these conditions. That’s about enough hydrogen gas to fill up a party balloon.

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Hydrogen is a colourless, odourless gas that’s lighter than air. It’d float upwards very quickly if I opened the valve in the store. I’m now going to squeeze all that gas into my 10-litre vessel.

What’s the resulting pressure of the vessel?

If I squeeze that 23.8 litres of hydrogen gas into my 10-litre vessel, the resulting pressure in the vessel must be greater than atmospheric pressure (1 atm) because I’ve compressed the gas. We can calculate the final pressure precisely by using the ideal gas law: PV=nRT.

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That’s significantly higher pressure than atmospheric pressure, which varies from 100 kPa to 102 kPa under normal weather conditions.

Interestingly, the pressure in the vessel, 243 kPa, is equal to 35.2 pounds per square inch (psi), which is the same as the recommended pressure for a car tyre.

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Other than making random movements inside the vessel, the hydrogen molecules won’t really do anything else.

How fast are the molecules moving about?

We can calculate the average speed of the molecules by using the following equation:

*Note that R is the gas constant, 8.31, and M is the molar mass in kg/mol

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The molecules are travelling at about 1760 metres per second (on average).

How much distance will the gas molecules travel before they collide with one another?

For this question, we need to calculate something called mean free path. The mean free path is the average distance we can expect each molecule to travel before it collides with another molecule. Mean free path is quite long in a vacuum, and very short at high pressure conditions. One of the formulae used to calculate mean free path, λ, is shown below.

**Note that in this formula, pressure (P) must be measured in pascals (Pa)- not kilopascals (kPa). We therefore need to multiply our kilopascal pressure by 1000 to convert it from kPa to Pa.

*** Note also that d is the diameter of the molecules being studied in metres. Wikipedia tells us that hydrogen molecules have a diameter of 120 picometres. I’ve used this value in the equation below.

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The molecules in our vessel collide with each other roughly every 260 nanometres. That’s tiny: it’s just a few percent of the width of a cell nucleus!

How often do the molecules collide?

Let’s go right back to Year 10 Physics for this one. The time between collisions will be equal to the average distance travelled between collisions divided by the average speed of the molecules:

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The molecules collide with each other roughly every 0.1478 nanoseconds.

How many times do the molecules bump into each other each second?

By taking the reciprocal of the average collision time, we can find out how many times the molecules collide with each other every second, on average:

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Each molecule in our ten-litre vessel makes 6.765 billion collisions per second with neighbouring molecules.

Apart from lots of uneventful particle collisions – a total of 4.07 decillion uneventful collisions per second to be precise – not much else is happening in our ten-litre vessel at this stage.

Conclusion

  • Hydrogen gas, H2(g): 1.00 mol

Next week, we’ll add some helium to the vessel and see what happens.

Periodic Table Smoothie

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Image from Randall Monroe’s excellent book, What If?: Serious scientific answers to absurd hypothetical questions

Yesterday, I was wondering what would happen if we mixed the entire periodic table of elements together in a blender. Unsurprisingly, it would explode, scattering radioactive dust and debris for miles around in a red-hot fireball formed from the simultaneous fission of the entire seventh row. The periodic table would only need to be the size of a matchbox in order for this explosion to happen.

Calculating exactly what would happen would be incredibly difficult. There are so many simultaneous reactions – including nuclear reactions – taking place that it’s almost impossible to predict the outcome in any more detail than “KABOOM”.

Making a real Periodic Table Smoothie  would be prohibitively expensive. You’d need 118 particle accelerators (costing $1 billion each) all pointing at the same target just to get single atoms of each element to collide at the same time. This is even more difficult than it sounds: those elements near the bottom of the periodic table (numbers 105 and above) are so unstable that they’d break down before they even reach the target. There are massive financial and physical challenges to mixing an entire periodic table up in a blender.

Instead of adding all the elements at the same time, I’ll be adding one element each week to an imaginary 10-litre vessel and documenting – as a theoretical exercise – what happens. Ultimately, we all know it’s going to explode at some point. But when will it do that? How many elements are we able to add before it finally explodes? Will we create anything interesting along the way?

This very idea was floated on Reddit’s AskScience forum in 2013 but nobody actually figured out (seriously) what would happen.

Join me next week to start the experiment.

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Elements 113, 115, 117 & 118 are now Confirmed!

Periodic table
Image courtesy of RSC

Article originally posted on rsc.org

Confirmation that four new elements – those with atomic numbers 113, 115, 117 and 118 – have indeed been synthesised has come from the International Union of Pure and Applied Chemistry (Iupac), completing the seventh row of the periodic table.

The groups credited for creating them – in Japan, Russia and the US – have spent several years gathering enough evidence to convince experts from Iupac and its physics equivalent, the International Union of Pure and Applied Physics, of the elements’ existence. All four are highly unstable superheavy metals that exist for only a fraction of a second. They are made by bombarding heavy metal targets with beams of ions, and can usually only be detected by measuring the radiation and other nuclides produced as they decay.

Continue reading on rsc.org…

3D Periodic Table with a G-Block (beta)

Here’s something you should all have in your Chemistry classrooms.

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This project was a mashup of two existing ones.

In 1969, Roy Alexander in the United States invented a helical periodic table with pop-out loops for the d-block and the f-block. It was by far the best way to visualise the periodic table at the time. His brilliant 1969 design patent is online here.

Then in 2010, Pekka Pyykkö in Finland created a periodic table that can accommodate elements up to Z=172, far beyond the range of elements that scientists can currently synthesise (currently Z=118 at best estimates). Pyykkö calculated the bizarre electron configurations of the g-block, and added these yet-to-be-discovered elements underneath the existing periodic table.

I blended these two ideas together and created a 3-dimensional periodic table with a g-block: Pyykkö’s elements in Alexander’s shape. The whole thing is about 30 cm high. I coloured all the elements by their electronegativities (where known).

Geography has globes, Biology has those limbless mannequins with their organs showing, and now Chemistry has this. It’s by far the most superior way to visualise the periodic table and electron configurations in 3D. It’s the only periodic table that puts all the elements in the right order without any gaps. It’s future-proof, fool-proof and waterproof.

Email me if you want to get your hands on one. Enjoy 🙂

A 3-dimensional periodic table with a g-block (front)
A 3-dimensional periodic table with a g-block (front)