Tag Archives: hydrogen

Let’s add carbon (graphite) powder

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‘Carbon’ page from Theodore Gray’s amazing book: The Elements

Today, we’re going to add 1.00 mole of carbon to our vessel. After adding boron last week, we left our vessel locked at 350 °C and with a pressure of 638 kPa. These reactions are taking place at 350 °C and constant volume (exactly 10 litres). Pressure inside the vessel will therefore change over time.

Allotropes of carbon

Carbon has various allotropes (structural arrangements of an element). Diamond is extremely strong and highly unreactive, while graphite is soft and brittle. The differences are all due to the type of bonding between carbon atoms. In diamond, carbon atoms are bonded by four strong covalent bonds with the surrounding atoms in a strong, hard three-dimensional ‘network lattice’. Graphite owes its softness and brittleness to the fact that its carbon atoms are bonded by only three strong covalent bonds in a two-dimensional ‘layer lattice’. Individual layers are very strong, but the layers can be separated by just the slightest disturbance. Touching graphite lightly onto paper will remove layers of carbon atoms and place them onto the page (such as in a pencil). Using a diamond the same way would likely tear the paper instead.

For this reason, I’m going to put graphite into the vessel instead of diamond. Diamond is so strong and inert that it’s unlikely to do any interesting chemistry in our experiment. Graphite, on the other hand, will.

The following seven chemical reactions will take place after adding carbon (graphite) powder

As soon as the carbon powder enters the vessel, it will begin to react with the following three species as follows:

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The ethyne produced in the third reaction will then react with any lithium and beryllium remaining in the vessel as follows:

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The hydrogen gas produced by the above two reactions will then react with lithium and carbon (if there’s any left) as follows:

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These reactions have the potential to all occur at the same time. Tracking them properly would require calculus and lots of kinetics data including the activation energy of each reaction and the rate constant for each equation. Quick searches on the National Chemical Kinetics Database yields no results for most of these equations, which means we won’t be able to use a computer model to calculate exact quantities of each product. Instead, I’m going to run a computer simulation using Excel that makes the following three assumptions:

  • all these reactions occur at the same rate;
  • all these reactions are first-order with respect to the limiting reagent;
  • all these reactions are zeroth-order with respect to reagents in excess.

The results will be a close approximation of reality – they’ll be as close to reality as we can get with the data that’s available.

Here are the results of the simulation

Here’s a graph of the simulation running for 24 steps. Exactly one mole of carbon powder is added at step 5.

Periodic Table Smoothie - Let's Add Carbon Carbon quickly reacts to form lithium carbide, beryllium carbide and two organic molecules: methane and ethyne
Carbon quickly reacts to form lithium carbide, beryllium carbide and two organic molecules: methane and ethyne

Summary of results

The results are incredible! We’ve made ethyne and methane, both of which have the potential to do some really interesting chemistry later on. I’m hoping that we can make some more complex organic molecules after nitrogen and oxygen are added – maybe even aminoethane – let’s see.

Hydrogen has also re-formed. I’m hoping that this gas lingers for long enough to react with our next element, nitrogen: we might end up making ammonia, NH3(g).

You may have noticed that I removed the “boron-lithium system” from the vessel. The 0.178 moles we created are now stored separately and will not be allowed to react any further. With such little literature about the reactivity of B3Li, it’s impossible to predict what compounds it’ll form later on. B3Li is so rare that doesn’t even have a Wikipedia page.

Pressure in the vessel increases to 718 kPa after carbon is added
Pressure in the vessel increases to 718 kPa after carbon is added
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Mass of sludge in the vessel changes after adding carbon

Here’s what’s present in the vessel after adding carbon

Substance Moles present after 500 ‘steps’
He(g) 1.00000
Be(s) 0.51435
LiH(s) 0.27670
Li2C2(s) 0.27165
B2H6(g) 0.23300
Be2C(s) 0.17470
H2(g) 0.14267
BeC2(s) 0.13625
CH4(g) 0.00949

We also have 0.178 moles of B3Li stored separately in another vessel.

Next week, we’ll add nitrogen and see what happens.

Let’s add boron powder

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‘Boron’ page from Theodore Gray’s book, The Elements

Boron is a metalloid: an intermediate between the metals and non-metals. It exists in many polymorphs (different crystal lattice structures), some of which exhibit more metallic character than others. Metallic boron is non-toxic, extremely hard and has a very high melting point: only 11 elements have a higher melting point than boron.

British scientist Sir Humphrey Davy described boron thus:

“[Boron is] of the darkest shades of olive. It is opake[sic], very friable, and its powder does not scratch glass. If heated in the atmosphere, it takes fire at a temperature below the boiling point of olive oil, and burns with a red light and with scintillations like charcoal” – Sir Humphrey Davy in 1809

Initial condition

Before we add the 1.00 mol of boron into our reaction vessel, we need to recall what’s already in there from our experiments so far:

  • H2(g): 0.70 mol
  • He(g): 1.00 mol
  • Li(s): 0.40 mol
  • LiH(s): 0.60 mol
  • Be(s): 1.00 mol

The temperature of our vessel is 99 °C and the pressure of the gaseous phase is 525.5 kPa.

Now, let’s add our 1.00 mol of boron powder.

Which reactions take place?

Boron reacts with hydrogen gas to produce a colourless gas called borane, BH3(g), according to the following equation[1]:

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Boron also reacts with lithium in very complex ways. If we heat the vessel up to 350 °C, we’d expect to see the formation of a boron-lithium system with chemical formula B3Li according to this equation[2]:

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Notice that now we’ve heated up our vessel to 350 °C to allow this reaction to happen, the lithium at the bottom of the vessel has melted.

Boron reacts with lithium hydride as well, but only at temperatures around 688 °C. With our vessel’s temperature set at 350 °C, we won’t observe this particular reaction in our experiment.[3]

Some allotropes of boron – in particular, the alpha allotrope that was discovered in 1958 – is capable of reacting with beryllium to form BeB12. Because we’re using beryllium powder, which has semi-random  symmetry, we won’t see any BeB12 forming in our vessel. Alpha-boron only exists at pressures higher than around 3500 kPa. At our moderate pressure of only 525.5 kPa, powdered (semi-random) boron will prevail and no BeB12 will form.[4]

For simplicity’s sake, let’s assume that the two reactions above take place with equal preference.

Boron powder reacts with hydrogen gas

Let’s do an ice table to find out how much borane we make.

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A quick n/ratio calculation shows us that the hydrogen gas is limiting in this reaction:

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We can expect all of the hydrogen gas to react with the boron powder.

units are mol 2 B 3 H2 2 BH3
I 0.50 0.70 0
C -0.466 -0.70 +0.466
E 0.0333 0 0.466

Borane is very unstable as BH3, and it would probably dimerise into B2H6(g). This is still a gas at 350 °C and is much more stable than BH3. For the rest of this experiment we’ll assume that our 0.466 mol of BH3 has dimerised completely into 0.233 mol of B2H6.

Boron powder reacts with lithium

With the molar ratios present in our vessel, at 350 °C, we’d expect to witness the formation of a boron-lithium system, with chemical formula B3Li.

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A quick n/ratio calculation shows that in this reaction, the boron powder is limiting.

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All of the remaining boron therefore reacts with lithium. To calculate exactly how much B3Li we’ve created, let’s do another ice table:

units are mol 3 B Li B3Li
I 0.533 0.40 0
C -0.533 -0.178 +0.178
E 0 0.222 0.178

What’s in our vessel after adding boron?

We have the following gas mixture in our vessel:

Helium gas, He(g): 1.00 mol

Helium is an inert noble gas that will probably remain in the vessel until the end of the experiment. It’s used in party balloons.

Borane gas, B2H6(g): 0.233 mol

We made this today. Borane is used in the synthesis of organic chemicals via a process called hydroboration. An example of hydroboration is shown below.

250px-hydroboration-oxidation_of_1-methyl-cyclohex-1-ene

At the bottom of the vessel, there’s a sludge, which contains the following liquids and solids:

Molten lithium, Li(l): 0.22 mol

Lithium is used in the production of ceramics, batteries, grease, pharmaceuticals and many other applications. We’ve got 0.22 moles of lithium, which is about 1.5 grams.

Beryllium powder, Be(s): 1.00 mol

Beryllium is used as an alloying agent in producing beryllium copper, which is used in springs, electrical contacts, spot-welding electrodes, and non-sparking tools.

Lithium hydride, LiH(s): 0.60 mol

Lithium hydride is used in shielding nuclear reactors and also has the potential to store hydrogen gas in vehicles. Lithium hydride is highly reactive with water.

Boron-lithium system, B3Li(s): 0.178 mol

We made this today… but what is it? Not much is known about this compound – in fact, it doesn’t even have a name other than “boron-lithium system, B3Li”. It’ll probably decompose eventually in our experiment – maybe when we alter the pressure or temperature of the vessel at some later stage. We’ll need to keep an eye on this one.

The original H2(g) and B(s) have been reacted completely in our experiment.

What’s the pressure in our vessel now?

At the end of our reaction, the temperature of our vessel is still set at 350 °C and the pressure of the gaseous phase inside the vessel can be calculated to be a moderate 638 kPa as follows:

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*It should also be noted that some evidence exists for a reaction between LiH and BH3, forming Li(BH4). The reaction seems to take place stepwise with increasing temperature. A quick read of this paper suggests that in our vessel, which is at 350 °C, any Li(BH4) formed would actually break back down into boron powder and hydrogen gas, which would in turn react with each other and with lithium metal to form BH3 and LiH again. The net result would be a negligible net gain of LiH and a negligible net loss of boron powder. We will continue calculating this Periodic Table Smoothie under the assumption that if any Li(BH4) forms, it breaks down before we add the next element, and the overall effect on our system is negligible.

**Li(BH4) is an interesting compound: it’s been touted as a potential means of storing hydrogen gas in vehicles – it’s safer and releases hydrogen more readily than LiH, which was mentioned above.[5]

Next week, we’ll add element number 6, carbon, and see what happens.

References

  1. “Borane”. Wikipedia. N.p., 2016. Web. 14 Apr. 2016.
  2. Okamoto, H. “The B-Li (Boron-Lithium) System”. Bulletin of Alloy Phase Diagrams 10.3 (1989): 230-232.
  3. Matkovich, V. I. Boron And Refractory Borides. Berlin: Springer-Verlag, 1977. Print.
  4. Gaulé, G. K. Boron, Volume 2: Preparation, Properties And Applications. New York: Plenum Press, 1965. Print.
  5. Saldan, Ivan. “A Prospect For Libh4 As On-Board Hydrogen Storage”. Open Chemistry 9.5 (2011): n. pag. Web.

Let’s add helium gas

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‘Helium’ page from Theodore Gray’s amazing book, The Elements

Last week, we put 1.00 mole of hydrogen gas into a cylinder. The resulting pressure was 243 kPa and the temperature was maintained steady at 20°C. This week, we’ll add 1.00 mol of helium gas, He(g), to the vessel and see what happens.

Will the helium react with the hydrogen?

No. Helium is completely inert. Hydrogen and helium will co-exist without undergoing any chemical reactions.

What will the resulting pressure be?

This is a very simple calculation. With 2.00 moles of gas in the vessel, the pressure would be double what it was before. This is known as Dalton’s law of partial pressures: the total pressure in a vessel is equal to the sum of all the pressure of the individual gases in the vessel.

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Let’s convert that into pounds per square inch (psi) for easy comparison with everyday objects.

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That’s about the same as a hard bicycle tyre.

How fast are the molecules moving?

Remember from last week that when our vessel contained only hydrogen gas, the molecules were moving around randomly with an average speed of 1760 metres per second.

Kinetic molecular theory states that the kinetic energy of a gas is directly proportional to the temperature of that gas. The formula for kinetic energy is shown below:

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At constant temperature, heavier particles move more slowly than lighter ones. Even though they have the same kinetic energy, helium atoms at 20 °C move slower than hydrogen molecules at 20 °C because they have almost exactly double the mass. How much slower does the helium move? Let’s find out.

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The molecules are moving at 1245 metres per second, or 4482 km/h. This is slower than the hydrogen gas by a factor of exactly root 2.

The molecules in our vessel could orbit the Earth in just 6 hours if they were to move in a single direction at this speed. Because the motion of particles in our gas mixture is random – they jiggle about rather than move in a single direction – they stay securely in the vessel.

Conclusion after adding helium

No chemistry’s happening in the vessel – not yet. Molecules of hydrogen and helium are simply co-existing in our vessel, bouncing off each other at different speeds and not interacting in any other way.

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

For some chemistry to happen, we’ll need to add the next element, lithium. We’ll do that next week.

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.