Part 14: The Chem in SpaceChem - Round 5
The Chem in SpaceChem, Round 5(Assume STP conditions unless stated otherwise.)
10: Gas to Liquid and Solid
Input molecules: Methane, Carbon Dioxide
Output molecules: Water, Carbon
Net reaction: CH4 + CO2 --> 2H2O + 2C
This one is a nice way to show that there’s a lot to say, even about simple, common molecules.
Methane, CH4
Melting point: -182 °C, -296 °F
Boiling point: -161 °C, -258 °F
Molecular mass: 16.04u It’s not important to understand this unit. If it becomes relevant I will explain it. You can use the numbers to see how heavy a molecule is compared to another molecule.)
Density: 0.72 g/L
Methane is best known as the main component of natural gas. It is colorless and odorless. The smell in natural gas for home use comes from an added odorant. The main use of methane is of course for combustion. When burning, it uses oxygen to form water and carbon dioxide.
The second big use of methane is for conversion to syngas, a mixture of carbon monoxide and hydrogen gas. Syngas has many uses in the chemical industry.
Methane is also known as a strong greenhouse gas. It can trap 25 times more heat per mass unit than carbon dioxide. Other than releases caused by direct human use, the main contributors to methane in the atmosphere are digestion of organic materials in wetlands and livestock flatulence and burping. There is actually a theory that a long time ago, dinosaur flatulence caused a period of global warming.
Carbon dioxide, CO2
Sublimation point: -78 °C, -109 °F
At atmospheric pressure, carbon dioxide goes directly from solid to gas, without passing through the liquid phase.
Molecular mass: 44.01u
Density: 1.99 g/L
Carbon dioxide. Best known for being my nickname.
Other than that, it is an important product of many combustion reactions, and the best known greenhouse gas. Most organisms, including plants, release CO2 when they 'burn' nutrients. However, plants and cyanobacteria can use the energy from sunlight to use CO2 and water to create oxygen and sugar molecules. Organisms that can do photosynthesis are a net consumer of carbon dioxide.
When carbon dioxide is dissolved in water, it forms a weak acid called carbonic acid, as seen in fizzy drinks. Carbonic acid can react to form carbonate ions, which are a part of minerals such as soda ash and limestone. In Africa, there are some lakes which occasionally have so-called 'limnic eruptions'. Lake Kivu, Lake Monoun and Lake Nyos slowly collect carbon dioxide from underground chambers, until the deep, pressurized layer of water becomes saturated. When the water is stirred in any way (land slide, storm, ...) it's like someone shakes a can of coke and all of the CO2 will suddenly erupt out of the water. People have actually been asphyxiated by the CO2 'cloud'. Carbon dioxide is denser than air, so a concentrated ‘cloud’ tends to stay near the ground.
Below the sublimation point, CO2 forms a solid known as dry ice. To get an idea how cold that is: from personal experience I know it's relatively safe to juggle dry ice, but if you hold it for more than half a second or so you get frostbite. Dry ice is commonly used as a refrigerant.
When carbon dioxide is pressurized, it can become liquid. In that form it is stored in fire extinguishers. And a fourth phase of CO2 is also commonly used. Beyond a certain temperature and pressure (the critical point), the gas and liquid phases of any substance become indistinguishable. Supercritical CO2 is a fantastic solvent for caffeine, and it’s used to extract caffeine from coffee and make decaf. It’s a safe alternative to the toxic solvents that were used in the past.
Water, H2O
Melting point: 0 °C, 32 °F
Boiling point: 100 °C, 212 °F (you knew that, right?)
Molecular mass: 18.02u
Density: 1.00 kg/L Note this is kg, the gases were in grams
Water. Boring. Or is it?
My opinion is that water is actually one of the most interesting compounds around. Water is a polar molecule, which means that one side of the molecule has a slight positive charge, the other side is slightly negatively charged. These charged molecules attract each other, making the intermolecular forces (forces among molecules, as opposed to forces within molecules) way stronger than those of compounds that are otherwise the same. In case of water and some other molecules, this polar force is somewhat confusingly called a ‘hydrogen bond’ (it is not a true bond). Because of hydrogen bonding, water melts and boils at a relatively high temperature for such a small molecule (it’s barely heavier than methane!).
Water is one of the molecules of which the three phases gas, liquid and solid all naturally occur on earth. Water vapor is actually a greenhouse gas too. But its greenhouse effect is very strongly dependent on the weather, so it can’t be quantitatively compared to the others.
Solid water is quite special. Most solids are denser than their liquid form, but as we all know, ice floats. This is because ice is less dense than liquid water. Water molecules in ice are ordered in a hexagonal shape, which actually has more empty space between the molecules than the unordered liquid.
Life as we know it exists on Earth because ice floats. Earth has undergone a number of ice ages, and at least one time it was probably completely covered in ice. However, because ice floats and is rather good insulator, the oceans never completely froze. This allowed sea life to thrive despite of the horribly cold surface. Even if the sun were to suddenly go out, sea life would be able to go on for at least hundreds of thousands of years (most life on the surface would be gone within the year). If ice sank, the oceans would’ve frozen completely (except maybe close to volcanoes), killing most if not all life during the worst ice ages. Even if it were able to recover, evolution would have to start over again.
There is a lot of different life on earth. Some of it doesn’t need oxygen. Some don’t need light. Some can survive at incredible temperatures. But all of them need water. It’s no surprise that astronomers who are looking for life on other worlds are mainly searching for water.
Carbon, C
Sublimation point: About 3800 °C, 6800 °F
Atomic mass: 12.01u
Density: 2 – 3 kg/L
First of all, ‘carbon chemistry’ is a large field within chemistry. Carbon chemistry is basically the chemistry of carbon compounds. If you’d like to know just how large carbon chemistry is, I’d advise you start with some light reading, a few 1000-page textbooks will do. I’ll just focus on elemental carbon.
As you can see, this time the sublimation point and density are rough estimates. One reason for that is that for some reason common sources seem to disagree on the sublimation point. The second, more important reason is because of the many allotropes (forms) of carbon. Different allotropes have very different properties.
Click here to see some allotropes of carbon
One of the most common allotropes is ‘amorphous’ carbon (picture g in the previous link), which is basically coal. Not much to say about that: it’s rather soft, black, and it burns.
Let’s move on to a more interesting one: diamond (picture a). Carbon turns into its diamond form under high pressure. Each carbon atom in diamond is bonded to four others. Diamond is the hardest material known to man, it’s an electrical insulator, and it’s transparent, with interesting optical properties (that’s physics talk for ‘ooh, shiny’). Diamonds are chemically unstable at atmospheric pressure and will very, very slowly turn into the graphite form. So in fact, diamonds are not forever.
Graphite (picture b), for instance used in pencils, consists of flat sheets. Each sheet is made of carbon atoms that are each bonded to three others, in a hexagonal pattern. The attractive forces among different sheets are weak, making graphite a soft material. Graphite conducts electricity, but only along the sheets. It is chemically stable, so it can be used as an electrode for electrochemistry purposes.
A single sheet of graphite is called graphene. Graphene is basically a 2D form of carbon. Graphene was actually first isolated in 2004, by using sticky tape to pull off a single layer from graphite.
Other allotropes of carbon include the fullerenes (pictures d, e, f) which are basically ‘graphene rolled into a ball’ and nanotubes (picture h), ‘graphene rolled into a tube’. These allotropes were discovered since the 1990’s and have interesting properties. There’s much we don’t know yet, but nanotubes have enormous tensile strength, and unusual electrical and thermal conduction properties. If we can possibly build a space elevator, it will probably use nanotubes in some way.
Feasibility of the reaction
CH4 + CO2 --> 2H2O + 2C
Feasibility: low
CH4 is actually a rather stable molecule. It needs a strong oxidizer such as O2 to react. CO2 is also very stable, and it doesn’t need to be oxidized, but to be reduced (the opposite). If you want to do just this reaction, you’d need to heat the molecules to the enormous temperature at which they decompose. Then you need to separate the elemental carbon, and when you cool down the rest, it should form water. This would cost lots of energy, and it would be hard to do without getting unwanted side-products.
It would be easier to do this in several steps. First, reduce the CO2. The reduction to CO isn’t too hard with some acid and some electricity, but the last O won’t come off easy. Internet suggests a few ways using specific catalysts and using either a lot of heating or a lot of electricity. That way C and O2 can be formed. Alternatively, we could look at nature. Plants ‘fix’ CO2 in carbohydrates using solar energy. Deep in the ground, the carbohydrates are very slowly converted into pure carbon. It’s probably possible to speed this process up.
The second step would be to simply burn CH4 in order to get H2O.
If this reaction could be done without wasting lots of energy, it would help to solve the global warming problem, because you are basically storing two greenhouse gases as solid carbon.
But that’s not the case, and I really can’t think of any practical use of this reaction.
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11: Reppe Chemistry
Input molecules: Acetylene, Carbon Monoxide, Water
Output molecules: Acrylic acid
Net reaction: C2H2 + CO + H2O --> C3H4O2
Acetylene, ethyne, C2H2
Sublimation point: -85 °C, -121 °F
Molecular mass: 26.04u
Density: 1.18 g/L
Acetylene is a simple hydrocarbon with a triple bond between the carbon atoms. When kept in its pure form, it decomposes easily and explosively. It also forms an explosive mixture with oxygen. Acetylene is often stored dissolved in acetone. Acetylene combustion causes a very hot flame, over 3300 °C or 6000 °F. For that reason, it is used for welding.
Another important use of acetylene is as a reactant for making a wide variety of organic compounds. It always needs a metal catalyst for these reactions. This type of reaction was invented by German chemist Walther Reppe in the first half of the 20th century. Acetylene reactions like this are commonly known as Reppe Chemistry.
Carbon Monoxide, CO
Melting point: -200 °C, -326 °F
Boiling point: -191 °C, -312 °F
Molecular mass: 28.01u
Density: 1.25 g/L
Carbon monoxide is formed when there’s not enough oxygen to completely burn up a carbon-containing compound. It is dangerous, because it binds to hemoglobin in blood and won’t let go. The resulting carboxy-hemoglobin can’t transport oxygen. It’s also a cause of photochemical smog, but it is unstable and after a while it turns into carbon dioxide. Carbon monoxide is an important reactant for a lot of industrial chemistry.
Acrylic acid, prop-2-enoic acid, C3H4O2
Melting point: 14 °C, 57 °F
Boiling point: 141 °C, 286 °F
Molecular mass: 72.06u
Density: 1.05 kg/L
Another organic molecule, this one consists of a ‘vinyl’ (ethenyl) group connected to a carboxylic acid. Acrylic acid and its derivatives easily combine themselves to form polymers. It’s used to make plastics (such as acrylic glass, also known as plexiglas), adhesives, paints, etc. It is also used to make superabsorbent polymers which are used in diapers and sanitary pads.
Feasibility of the reaction
Net reaction: C2H2 + CO + H2O --> C3H4O2
Feasibility: high
This very reaction was once the main way to produce acrylic acid. It requires a metal catalyst. There’s also some safety precautions required due to acetylene’s explosive nature.
Nowadays, acrylic acid is synthesized from propene, which is a cheap byproduct of gasoline production from oil. Oil wasn’t yet common when Reppe invented his technique.
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Challenge 5: Cyanide Reassembly
Input molecules: Hydrogen Cyanide
Output molecules: ‘1-Azo 1,2 Propadiene’
Net reaction: 3 HCN --> C3H3N3
Prussic acid, Hydrogen Cyanide, HCN
Melting point: -13 °C, 8 °F
Boiling point: 26 °C, 79 °F
Molecular mass: 27.03u
Density: 0.687 kg/L (liquid phase)
Hydrogen cyanide is best known as a poison. It has a faint, bitter, almond-like odor that some people cannot smell because of a genetic trait. If a chemistry lab smells like almonds you might want to run. It is used as a pesticide and a chemical weapon, and was an important component of Zyklon B, the gas used by the Nazis to execute Jews. The cyanide ion inhibits an enzyme required for cellular respiration.
Industrially, HCN is used to make a number of organic compounds with a variety of uses. One of them is used to make a certain kind of nylon. It is also used to produce sodium cyanide and potassium cyanide, which are used to extract gold from ore.
In nature, it is formed in pits/seeds of some fruits such as apples, apricots and bitter almonds. Do not eat those seeds. It’s also a component of wood smoke and tobacco smoke.
1-Azo 1,2 Propadiene
Melting point, Boiling point, Density: No clue.
Molecular mass: 81.08u
This one was a real puzzler. When I first looked at it, I thought it couldn’t possibly be stable. Turns out I was wrong! But information about this molecule is hard to find, and it took me a while to dig it up.
First of all, the name is wrong. We can start by leaving out the numbers. Those should only be included when the name would otherwise be ambiguous, which is not the case here. Next, ‘azo’ means a R – N = N – R’ group, where R and R’ stand for other functional groups. The – N = N = N group is actually called an azide. The name of this compound is azido-propadiene. It doesn’t surprise me that the person who designed this challenge didn’t know the proper name, as azides are quite exotic molecules.
Let’s take a look at the actual molecule. A relatively large part of it consists of nitrogen atoms. One rule of thumb in organic chemistry is that compounds with many nitrogens tend to be unstable, explosively so. The nitrogens can free a whole lot of energy by forming nitrogen gas N2. Examples of explosive nitrogen compounds are trinitrotoluene (TNT) and nitroglycerine.
Another thing that’s interesting is the number of bonds on each nitrogen. ‘Regular’ nitrogen can have three bonds, but SpaceChem allows five. This has to do with the fact that the valency (amount of bonds on an atom) changes if an atom gets charged. The maximum bond numbers in SpaceChem are a bit strange and inconsistent. It seems they just picked whatever allows for the most interesting real life molecules. Anyway, the azide nitrogens have 3, 4 and 2 bonds. This is chemically valid, because the middle nitrogen actually has a + charge and the final one has a – charge.
Now, azides have been known for a century and a half, and some are actually relatively stable. I mean, they don’t just explode from an angry stare, you actually have to shake them a bit or something. The most common one is sodium azide NaN3, which is used in car air bags. When disturbed by heat or electricity, it quickly decomposes, forming a lot of nitrogen gas. Another interesting azide is found in zidovudine/AZT/retrovir, the most common drug used in AIDS therapy. It is simply a thymidine (building block of DNA) with an azide attached. The DNA replication mechanism of the HIV-virus gets (partially) blocked when it encounters this thymidine analogue.
Our compound has C = C double bonds, which change its chemistry completely. Scientists were unable to synthesize compouns with an azide and 2 C = C bonds until two decades ago. But they finally found a way. They discovered that these compounds are rather unstable, as expected. They can only exist for a few seconds, but that’s long enough to quickly analyze and determine the structure. Azido-propadiene then reforms into a cyclic structure called a triazole:
It will react further into other compounds from there.
Azido-propadiene has no known uses except for chemical novelty and reminding me that we can make some crazy molecules if we try hard enough.
I learned something new and interesting today, and as far as I'm concerned that's always a good thing 
.Feasibility of the reaction
Net reaction: 3 HCN --> C3H3N3
Feasibility: low
See how many bonds you need to break and create at once? That’s just not going to happen.
According to the source I found, Azido-propadiene was prepared from iodo-propadiene. That’s propadiene with one H replaced by a iodine atom. In a substitution reaction, the iodine was replaced by N = N = N from a certain large azido compound. Nearly all azido compounds are made from sodium azide, which is synthesized from N2O and NaNH2.