Handheld chemistry

There was a time when chemists regularly reported the taste of newly synthesized compounds as well as other physical data (density, color, etc.). There was also a time when chemistry kits suggested doing chemistry in your hand, for fun. For a piece I wrote for Nature Chemistry (Homemade chemists) I found these instructions in a 1937 manual for a Chemcraft chemistry kit:

I'm a little cautious about using calcium oxide (CaO) as the reaction when it comes in contact with water is famously exothermic (you can cook an egg with it, see the video, and back in the day transporting CaO, or quicklime, by wooden ship, was hazardous duty). I wondered how exothermic was this reaction, and how much ammonia did it make relative to what you might encounter in a barn (the breakdown of urine yield ammonia) or your cat's litter box.

I'll admit to using Hess' law for fun. For those who have not enjoyed (endured?) an introductory chemistry class, Hess' law makes use of the fact that the energy content (heat of formation) of a molecule is a state function. Like altitude, it doesn't matter how you get to the top of the mountain from the valley, climbing straight up the side or meandering up a series of switchbacks, the change in altitude remains the same. So if I know where I am starting (the reactants, in this case calcium oxide (CaO) and ammonium chloride (NH₄Cl)) and where I end (the products, calcium chloride (CaCl₂, ammonia and water), I can figure out how much energy is used up (endothermic) or given off (exothermic).

The handheld reaction is 2 NH₄Cl(s) + CaO(s) → 2 NH₃(g) + H₂O + CaCl₂(s). I looked up the heats of formation in a handy table. To get a sense of magnitude, for 60 grams of CaO, which is about a tablespoon of material, the heat of formation is -635 kJ...or about the same amount of energy you can get from eating 3 Oreos. Overall, this reaction needs about 100 kJ to use up those 60 grams of CaO, in this case the energy comes from your warm hand. [Ed. note: While handheld chemical synthesis is an interesting way to "burn" calories, this is not a recommended weight loss technique!]

So your hand won't melt. Good to know. But if it were me, I'd do this in a test tube and warm it with my hand!

What the reaction does produce a surprising amount of ammonia. If you let the reaction go to completion (and since I don't know how fast the reaction proceeds, I can't tell you how long that will take), using about a 1.5 grams of ammonium chloride, and all the ammonia stays in a 1 cubic meter area around your hand, the concentration would be about 450 ppm. Since the CDC considers the IDLH (immediate danger to life and help) for ammonia to be 300 ppm, this would not be a great experiment to try in the tiny basement bathroom I used as a lab when I was a kid. Still, if you did this just until you could smell the ammonia, for most people that is about 50 ppm, a level considered reasonable for a brief (less than 5 minute) exposure. Levels inside a barn might be around 120 ppm.

Wash those hands.

Chemistry by accident

I just finished another Thesis column for Nature Chemistry, this one on the notion that chemistry sets are an essential part of turning kids into chemists — more particularly, what I called the Uncle Tungsten trope: risky chemistry is more fun and makes better chemists. As part of the article, I wondered how many accidents there are in home labs (not counting home meth labs). It turns out that in the US, the Agency for Toxic Substances and Disease Registry (ATSDR) keeps track of hazardous substance events. The data suggests there are around 1000 chemical incidents in private homes each year, and the vast majority involve carbon monoxide (nearly all the fatalities are caused by CO) or inappropriate mixing of common household chemicals (usually of bleach and something else: ammonia, pool acid, pesticides). As far as I can tell, none of the accidents were part of amateur chemistry gone awry.

There are no narratives linked to the data, but a chemist can read between the lines. When the primary chemical listed in a chemical accident is sucrose — table sugar — (a) what is the secondary chemical likely to be? (b) What was the intended goal of the experiment?

Answers: (a) potassium nitrate (or potassium chlorate) and (b) solid rocket fuel (or sparklers or smoke bombs or...). Sucrose oxidizes readily (toasted marshmallows, anyone?), and potassium salts (KNO₃, KClO₃) are good oxidizing agents.

It should go without saying, but do not try this at home. Especially do not try mixing bleach with anything. It will not make a stronger cleaner, bug killer, or weed killer. But it might kill you.

Grapes of Wrath

My youngest came home from a father-son event with a new interest in healthy foods. I put grapes on the table with dinner. "There are grapes for dinner," he exclaimed. Who are you and what have you done with son? ran through my mind.

At the end of dinner he puts two grapes on his plate and carefully cuts them nearly in half. Then he ducks into the kitchen. "Come on, Mom!" Warm grapes? He'd eaten all the chicken, there was nothing left on his plate to veronique.

He hits the start button and suddenly the grapes start arcing, and one actually bursts momentarily into flame. I'm stunned. No metal, but the arcing is clear. We try various experiments - do you have to leave the grapes connected (no), does it work with other things (carrots), can you char a grape (yes).

What's going on? Hang on, we were producing plasmas in the kitchen. Not the kind that circulates in your veins, but the kind that stars are made out of. Plasma is often called the 4th phase of matter - the iconic triad being solid, liquid and gas. (There are many other phases in which substances can exist, in fact - such as liquid crystals and supercritical fluids.)

Plasmas are gases in which a large number of electron are "free", rather than associated with a molecule or atom.

I'm still trying to come to grips with the idea that I can create a (very tiny) ball of plasma in my kitchen.

(Read more in the paper : "Microwave Mischief and Madness" by H. Hosack, N. Marler, D. MacIsaac of Northern Arizona University, The Physics Teacher 40, 14 (2002).

Random Facts about Ludwig B.

Not that Ludwig B. - the other Ludwig B: Ludwig Boltzmann, an Austrian physicist.

Boltzmann's name is familiar to many science students through the eponymous constant: 1.381 x 10^-23 Joules/mole-Kelvin, which appears in many equations. The constant (usually written as k) arises from the proportionality between the absolute entropy of a system (S) and the number of possible arrangements of that system (W). Boltzmann's expression of the entropy, S=k ln W, is inscribed on Boltzmann’s tombstone in Vienna, Austria. Boltzmann did not write it in this form, however, Planck did.

Boltzmann also has two other equations named for him, the first is a diffusion equation used in neutron transport theory and the second describes particles in a gravitational field. In 1904, Boltzmann gave lectures on mathematics at the World’s Fair in St. Louis. He was also a popular lecturer in philosophy at the University of Vienna. Boltzmann is considered the founder of statistical mechanics, and a strong proponent of the “atomistic” view that underscored the importance of understanding the behavior of atoms and molecules in order to understand the bulk.

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Loosely, the entropy is a measure of the "randomness" in a system.

Hydrazine: Hype or Hypergol?

Last week the US government announced that it believes it has successfully breached the fuel tank on a dead satellite, effectively destroying the toxic fuel stored on board: 1000 pounds of hydrazine. Hydrazine is a simple nitrogen compound, two NH₂ groups joined by a NN single bond. How does such a simple compound power a rocket?

Hydrazine is a hypergolic propellant - one that ignites as soon as it comes into contact with an oxidant (something that will react with it to effectively strip away some electrons from the reactant and force the molecule to bond differently, the changes in the bonds between atoms are what release the energy). Hypergolic is apparently a term coined by the German rocket program from hyper (very) + ergon (Greek for work) + ol (from oleum, the Latin for oil). Hydrazine is that, a liquid (if not particularly oily one) that can be used to push satellites around in orbit - to do work.

Hydrazine is a solid in the satellite's tanks, and once thawed can be catalytically and rapidly decomposed. Almost any metal will do, though iridium is the usual choice. The reactions produce lots of very hot gases, which you can direct through a thruster:

3 N₂H₄ → 4 NH₃ + N₂
N₂H₄ → N₂ + 2 H₂
NH₃ + N₂H₄ → 3 N₂ + 8 H₂

A little thermochemistry can quickly tell you just how much energy you might produce from 1000 pounds of hydrazine. The overall reaction is:

5 N₂H₄ → 5 N₂ + 10 H₂

which releases 50,000 Joules of energy per mole of hydrazine. A mole of hydrazine weighs about 32 grams, so you get enough energy to make a cold cup of coffee hot from just over an ounce of hydrazine (do NOT try this at home!). If all the hydrazine in that satellite went up at once, it would release about 8 billion Joules (enough to keep the average US citizen in energy for more than a week).

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A photo of a standard satellite thruster.

Maxwell's demon

James Clerk Maxwell was a Scottish mathematician and physicist. He is perhaps most famous for his extension and refinement of Faraday's equations describing magnetic and electric fields. He reduced the necessary set of equations to four simple partial differential equations, the eponymous Maxwell equations, publishing the work in 1873.

He also worked in thermodynamics, lending his name to another set of four key differential relationships (the Maxwell relations). He also independently derived the Boltzmann distribution of the kinetic energies of gas molecules. Maxwell's demon, a "finite being" who opened a door between to a box of molecules, letting them in and out depending on the amount of energy they had was a rhetorical device Maxwell used to show that entropy and heat flow were, at their core, statistical phenomena.

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Lord Kelvin (aka William Thomson) applied the demon tag, Maxwell used the term "finite being"!