Part 11: The Chem in SpaceChem - Round 4
The Chem in SpaceChem, Round 4Note: As The Chem in Rounds 1 – 4 were written after the tournament, they may refer to later rounds.
8: PVC V
Inputs: Vinyl Chloride, Chlorine Radìcal
Outputs: Chlorinated PVC
I discussed the Chlorine radical in Round 8. I’ll talk about the other two compounds here.
Vinyl Chloride, Chloroethene, C2H3Cl
Melting point: -154 °C, -245 °F
Boiling point: -13 °C, 7 °F
Molecular mass: 62.5 u
Density: 0.911 kg/L (as a liquid)
Vinyl chloride is a toxic gas with a sweet smell. It is usually stored as a liquid under pressure. There are two common ways to prepare it. Acetylene can be reacted with HCl gas to form vinyl chloride. Alternatively, with the right catalyst, ethylene will react with chlorine gas. As ethylene is commonly produced from ethane, engineers are looking for ways to produce vinyl chloride directly from ethane in order to reduce costs. While this reaction is possible, it takes a lot more effort to keep things under control and not get side-products, so it is not as common.
In the past, vinyl chloride was used as an aerosol spray propellant, as a refrigerant and according to Wikipedia, even as an inhalational anaesthetic. Of course, they stopped using the stuff because of its toxicity. Nowadays, it’s (almost) exclusively used as an intermediate to make polyvinyl chloride (PVC).
Chlorinated PVC
Melting point: 395 °C, 743 °F (But the glass temperature, at which it becomes soft and rubbery, is around 110 °C).
Boiling point: Decomposes
Molecular mass: Varies, but around 100 000 u or more would be a good guess.
Density: 1.56 kg/L
Regular PVC is a polymer, a plastic used for water and sewer pipes, among other things. Like most polymers, its exact properties depend on the average chain length of each molecule. It is made by polymerizing vinyl chloride, which is done by a so called radical chain reaction. A molecule called the initiator produces a radical, a compound that has an unpaired, reactive electron. The radical reacts with the vinyl chloride, and using one of the electrons making up the double C=C bond, it binds to a vinyl chloride molecule. At that moment, the other ‘end’ of the double bond has a spare electron, and we have a new radical. It will react with another vinyl chloride monomer and so on, until the reaction is terminated by pairing up two radicals. By that time, the chain is many thousands of monomers long.
Chlorinated PVC or CPVC is an alternative to PVC. It is made by adding chlorine to PVC and exposing it to UV light. The Cl2 molecules decompose into Cl radicals, which will replace hydrogens in the PVC. The increased chlorine content gives CPVC some advantages over regular PVC. It is more ductile and it has better heat resistance. With the glass temperature over 100 °C, CPVC can be used to transport nearly boiling water. However, as it takes an extra production step to make this stuff, it’s a bit more expensive.
Feasibility of the reaction
Feasibility: High
As I explained above, this reaction is certainly possible. However, in reality it would take two separate steps, first the polymerization of vinyl chloride into PVC and second the actual chlorination. A small amount of initiator is required to get the polymerization going. Note that the initiator is not actually a catalyst, as it is used up.
Reaction energy: Exothermic
The polymerization of vinyl chloride into PVC releases 105.6 kJ/mol (that’s moles of vinyl chloride), enough to need constant cooling. I haven’t found the energy of the chlorination step, but I am guessing it is slightly exothermic, as the resulting molecule seems more stable. Even if I’m wrong about that, I don’t think the energy of that reaction is nearly as much as that of the first step, so the total procedure is still exothermic.
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9: Better than Graphene
I talked about Nitrogen in Closed Tournament Round 2. Let’s look at Boron and Boron Nitride.
Boron, B
Melting point: 2076 °C, 3769 °F
Boiling point: 3927 °C, 7101 °F
Atomic mass: 10.8 u
Density: Between 2.35 kg/L and 2.52 kg/L, depending on the allotrope.
Boron is one of the lighter elements, with atomic number 5. It is relatively rare, as it is not formed by nuclear fusion in stars, but by cosmic ray spallation. When heavier elements are hit by cosmic rays, they can lose a bunch of nuclear particles, forming lighter elements such as B.
The best known boron compound occurring naturally is probably borax, which has the formula of Na2B4O7, with usually a bunch of water molecules attached. Borax is used, among other things, in household cleaning products and in certain brands of ‘slime’ or ‘flubber’ or ‘putty’.
Another compound that is often used is boric acid, H3BO3. It is used as an antibacterial compound and an insecticide, but mainly to make fiberglass. Boron is also a part of borosilicate glass, such as Duran and Pyrex. These kinds of glass can survive fast, big changes in temperature and are commonly used in cookware and laboratory glassware.
Boron is an essential plant nutrient (used in plant cell walls). Researchers found that boron is an ‘ultratrace element’ in rats. If you give rats purified air and ultrapurified foods from which all traces of boron are gone, they get less healthy. Apparently, the incredibly tiny amount of boron in dust particles in the air is enough to prevent this. It is likely that a similar tiny amount is needed in other mammals such as humans, but we have no idea what it’s needed for (animals don’t have cell walls, just cell membranes).
Boron nitride, BN
Decomposition temperature: 2327 °C, 4220 °F (h-BN) 2973 °C, 5383 °F (c-BN). It turns into nitrogen gas and boron. It might be possible to melt the stuff at a higher pressure.
‘Molecular’ mass: 24.82 u
Density: 2.2 kg/L (h-BN), 3.48 kg/L (c-BN)
This substance certainly isn’t a boring nitride. Boron has one less electron than carbon, while nitrogen has one more. If boron and nitrogen both make three normal bonds, and two of the leftover nitrogen electrons are used to make a fourth B–N bond (this type of bond where both electrons come from one atom is called a dative bond), you get a structure very similar to pure carbon.
Like carbon, boron nitride exists in various forms. All forms are large molecular crystals, except for the amorphous form, which can be described on the molecular level as ‘a mess’. This means there are no actual BN ‘molecules’ when it’s a solid crystal, but huge crystal-sized ‘molecules’. Such molecular crystals aren’t usually seen as a polymer, as their properties are rather different.
The most stable form of BN has a hexagonal structure, similar to graphite. Like graphite, h-BN is rather soft. It’s used as a lubricant and as an additive in cosmetic products. A form similar to diamond called c-BN is almost as hard as true diamond, but it is thermally and chemically more stable than the real thing. There is also a form similar to the carbon allotrope lonsdaleite, which might actually be harder than diamond.
The form similar to graphene (a single layer of hexagonal BN) is called ‘nanomesh’. It’s chemically very stable, and can be used to trap molecules. It could possibly be used in catalysis, quantum computing and storage media like hard drives. Useful and interesting stuff.
It is also possible to make carbon nanotube equivalents, by rolling up sheets of hexagonal-BN (similar to graphene). However, while carbon nanotubes can be conductors or semi-conductors, nanotube BN is an electrical insulator. Like c-BN, nanotube BN is chemically more stable than carbon, giving it great potential in nanotechnology.
Feasibility of the reaction
Feasibility: High
Although the stuff is usually made from boron oxide or boric acid and ammonia, it is possible to have boron directly react with nitrogen plasma at temperatures over 5500 °C. This process is used to get ultrafine particles of BN, used for lubricants and toners.
Reaction energy: Exothermic, -250.91 kJ/mol (per BN unit, starting with N2 molecules).
Well, we are making something very stable out of 2 elements. Even though the strong nitrogen-nitrogen bond has to be broken, the new B-N bonds make more than up for it.
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Challenge 4: Liquid glass
Input molecule: Silica
Output molecule: Sodium silicate
Silica, Silicon dioxide, quartz, sand, SiO2
Melting point: Around 1600 °C, 3000 °F
Boiling point: 2230 °C, 4046 °F
‘Molecular’ mass: 60.1 u
Density: 2.65 kg/L
Silica is best known in the crystalline quartz form, which makes up over 10% of Earth’s crust. It is also the most common constituent of sand. Silica is mostly (95%) used in building materials such as cement. Other than that, it is a main component of regular glass and of ceramics. To my surprise, it is also commonly used as a food additive (as a flow agent so powdered foods get mixed easier and to absorb water). It is also used as an abrasive in toothpaste. In the quartz form, each Si atom is surrounded by four oxygens, but as every oxygen is shared with another Si atom, the net formula is SiO2.
Amorphous silica or silica gel is another form. It’s used in tires and shoe soles, but you might know it from those little packs they put in boxes with new electronics, as the stuff absorbs water well. Silica does not dissolve well, but trace amounts do occur in the human body. It is probably important in the strength of connective tissues, both on the level of bones but also within the cell.
Sodium silicate, Liquid glass, waterglass, Na2SiO3
Melting point: 1088 °C, 1990 °F
Boiling point: Unknown
‘Molecular’ mass: 122.1 u
Density: 2.4 kg/L
The compound with formula Na2SiO3 is officially known as sodium metasilicate. All compounds with a formula Na2(SiO2)nO are known as liquid glass. As a dry solid, it consists of a covalently bonded –O-SiO22––O–SiO22–– polymer with two Na+ cations surrounding each SiO22– group. Structurally, this puts it somewhere between a molecular compound and a salt.
When hydrated, it reacts with water forming Na2SiO2(OH)22– (+ crystal water), which isn’t a covalent polymer but a salt. This behavior as well as the structure of silica show that silicon isn’t completely metal nor completely non-metal, it is somewhere in between, a metalloid. The solid looks like a white powder that dissolves in water, but I think the hydrate looks somewhat like glass, once it has hardened.
It is mainly used as an adhesive or ‘cement’ to hold paper surfaces together in cardboard and to fix cracks in car exhausts. It’s also used to close pores in concrete, plaster and the like, as it reacts with leftover ‘uncured’ concrete. And it’s used in waste water treatment, as it binds to colloidal particles, forming larger aggregates that sink to the bottom. In the past, liquid waterglass was commonly used to preserve eggs, as it kept the water in but bacteria out.
A rather spectacular use of waterglass is as the solution needed to make a chemical garden, one of the most beautiful chemistry experiments. Basically, if you drop colorful salts in the solution they will start growing upwards, not completely unlike plants.
Feasibility of the reaction
Feasibility: Low
Another fusion/fission puzzle. Won’t work. It is interesting to note that sodium silicate is usually made from silica, together with sodium carbonate. Carbon dioxide is a waste product of that reaction. If my calculations are correct, that reaction is somewhat endothermic at about 87 kJ/mol.