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:
The ethyne produced in the third reaction will then react with any lithium and beryllium remaining in the vessel as follows:
The hydrogen gas produced by the above two reactions will then react with lithium and carbon (if there’s any left) as follows:
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.
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.
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.
James Kennedy 