Month: February 2025

Xanax For Molecules

On By rolcottIn Physics: Entropy and Thermodynamics, UncategorizedLeave a comment

Vinnie plops down by our table at Cal’s Coffee. “Hi, guys. Glad you’re both here. Susan, Sy here says you’re an RDX expert so I got a question.”

“Not an expert, Vinnie, it’s just one of a series of compounds in one of my projects. What’s your question?”

“How come the stuff is so touchy but it’s not touchy? You can shoot a bullet into a lump of it, nothing happens, but set off a detonator next to it and WHAMO! Why do we need a detonator, and what’s in one anyway?”

“Sy, what sets off an H‑bomb?”

“An A‑bomb. You need a lot of energy in a confined region to crowd those protons enough that they fuse.”

“And what sets off an A‑bomb?”

“Hey I know that one, Susan, I saw the Oppenheimer movie. You need some kind of explosives going off just right to cram two chunks of plutonium together real fast so they do the BANG! thing instead of just melting. Wait! I see where you’re going — little explosions trigger big explosions, right?”

“Bravo! You’ve got the idea behind activation energy.”

“Geez, another kind of energy?”

“Yes and no, Vinnie. Enthalpy and its cousins are about the net change when something happens. We can use them to predict how a complex reaction will settle down, but they don’t tell us much about the kinetics, how fast things will happen. Think for a minute about those H‑bomb hydrogen atoms. What prevents them from fusing?”

“I guess under normal conditions they’re too far apart and even when they get close their electron clouds push against each other.”

<Sketching on a paper napkin> “Fair enough. Okay, here’s what the potential energy curve looks like, sorta. There’s hydrogen atom A over there at the right-hand end of the curve. B‘s a second hydrogen on the left and heading inwards. With me?”

“So far.”

“Right. Now, B comes roaring in with some amount of kinetic energy and hits the potential energy bump where those electron clouds overlap. If it has enough kinetic energy to overcome that barrier, it keeps on going. Otherwise it bounces back with the kinetic energy it had maybe minus some that A picked up in the recoil.”

“So the first barrier is the electron‑electron repulsion, but the potential dips in the middle where the clouds merge and that’s where molecules happen.”

“Right, Sy. But then there’s the second barrier as B‘s positive charge encounters A‘s. Inverse‑square law and all that, it’s an enormous hurdle. Visualize lots of Bs with different kinetic energies running up against that wall again and again until finally, if the pressure’s high enough, one gets past and the fusion releases more energy than the winning B had originally. The higher the wall, the fewer Bs hit As per unit time and the slower the reaction.”

“Looking at the before‑and‑afters, the reaction only happens if energy’s released, but how fast it goes is that barrier’s fault.”

“Perfect, Vinnie. Take RDX, for example. You’re right, it’s touchy. If you’ve got the pure stuff, never look at it cross‑eyed unless you’re behind a blast shield. Lots of energy released, very low energy of activation.”

“But like I said, you can shoot a gun at it, no effect.”

“That wasn’t pure RDX, it was probably some version of C‑4.”

“Yeah, C‑4, don’t know any of the details.”

“C‑4’s explosive is RDX, but it’s also got some plasticizer for that putty consistency, and a phlegmatizer. I love that word.”

“Phlegmatizer? That’s a new one for me.”

“It’s an additive to keep the explosive calm — phlegmatic, get it? — until it gets excited on purpose, which is the detonator’s job.” <scribbling on a stack of paper napkins> “Okay, here’s that same activation energy curve, an RDX particle on the right, and an incoming shock wave. The gray region is the phlegmatizer, usually paraffin or a heavy oil. Think of it as a shock absorber, absorbing or deflecting the shockwave before it can activate the explosive. A detonator’s designed to activate and erupt so quickly that its shock peak arrives before the phlegmatizer can spread it out.”

“Like they say, timing is everything.”

~ Rich Olcott

Tightening Up Fast And Loose

On By rolcottIn Physics: Entropy and Thermodynamics, UncategorizedLeave a comment

Cal brings out a fresh batch of scones. He’s tonging them onto the racks when I suddenly get a whiff of mocha latte. I glance back and there’s Susan Kim, grinning at me. “Hi, Sy. Grab your scone and a table. I have a bone to pick with you.”

A few moments later we’re seated. Cal’s coffee’s especially smooth today. “Okay, what’s the bone?”

“You’re playing fast and loose with your enthalpy definition. Yes, there’s change in temperature times entropy, enthalpy’s thermal component, and an expansion‑contraction component you called pressure‑volume. But it’s just sloppy to call what’s left ‘the chemical portion.’ What it is, really, is the combination of every other kind of energy something has that some process could extract. Chemical reactions are just one piece.”

“Strong words, coming from a chemist. What else should be packed in there?”

“Radioactivity, for one. It’s a heat source that doesn’t depend on chemical reactions. Atom for atom, a nuclear disintegration can yield millions of times more energy than a chemical reaction does. Trouble is, radioactive atoms only break down when they feel like it so the energy’s all random heat. I’m sure there’s a bunch of other non‑chemical ways to increase something’s apparent enthalpy.”

“Hmm. Challenge accepted. … It’s all about which process will extract some kind of energy from your something. How about the something’s a tightly‑wound spring? No, wait, that’s chemical, because the energy’s stored in stretched metal‑metal bonds.”

“No, I’ll accept spring tension because there’s no change in chemical composition during the unwind process. What’s another one?”

“Ah. Easy. Kinetic energy if the something’s flying through the air to hit something else.”

“Now you’re cooking. Gravitational potential energy if it’s falling down. Oh, suppose it’s magnetized and goes through a conductive loop on the way down?”

“Nope, doesn’t count. The object’s kinetic energy would produce a jolt of electrical potential in the loop, but it’s own magnetization wouldn’t change. Nice, that distinction sharpens the point — what you count as enthalpy’s third component depends on which change process you’re talking about. If there’s no chemical change, then the chemical part of the internal component of the enthalpy change is zero. In the early days of thermodynamics, for instance, everyone was working on steam. Water may corrode your equipment over the long term, but otherwise it’s just hot water molecules becoming not‑as‑hot water molecules and there’s no change in internal energy. The only energy terms you have to think about are pressure‑volume and temperature‑entropy. That’s why they defined it that way.”

“Which one wins?”

“Hmm?”

“You’ve pared enthalpy changes down to just two kinds of energy. I’ve got to wonder, which one has the bigger contribution?”

<pulls up a display on Old Reliable> “This is just for the water‑steam system, mind you. Vinnie was surprised. It’s all based on specific heat measurements so visualize one kilogram of liquid water.”

“A liter, right.”

“The line labeled ‘Mechanical’ is the amount of energy you’d get by expanding that kilogram from 0°C up to the temperatures laid out on the x‑axis. No significant expansion up near boiling temperature, then it follows the Ideal Gas Law, PV=nRT. At atmospheric pressure and in this temperature range the expansion relative to 0°C runs about 200 kilojoules per kilogram.”

“And the ‘Thermal’ line?”

“That’s lab‑measured heat capacity values I pulled from the CRC Handbook, each multiplied by the corresponding temperature in kelvins. That’s the amount of energy our kilogram of water molecules holds just by being at the temperature it’s at. The gas makes a nice straight line, at least in the range before heat shatters the molecules.”

“That’s what, fifteen or sixteen times more energy than the mechanical part? Wow! You know, back in Physical Chemistry class they just threw around lots of confusing thermodynamics formulas but never put numbers to them. I had no idea the entropy effect could just swamp whatever else.”

“Numbers do make a difference.”

“This clarifies something I didn’t understand back then. Entropy’s about randomness, right, and a gas molecule can be in more locations in a large volume than in a small one. V=nRT/P says volume rises linearly with temperature and that’s the linear rise in your chart.”

~ Rich Olcott

The Little Engine That Cooled

On By rolcottIn Physics: Entropy and Thermodynamics, UncategorizedLeave a comment

“Chemical potential energy is something else, Sy. You’ve got like this lump of putty just sitting there and suddenly WHAMO! Kinetic energy all over the place.”

“Sounds like you’ve been playing with explosives, Vinnie.”

“Sorta. Some of the Specials down at the base let me watch a couple of their C-4 practice shots. You know anything about C-4?”

“A little, like what it’s made of. Susan Kim’s interested in the main ingredient for some chemical reason. She calls it RDX, drew me a picture of it once. Nice symmetrical molecule loaded with nitrogen and carbon atoms just itching to fly away as a dozen separate gas molecules. Funny, how such violent stuff can be so relaxed until just the right thing sets it off. Like some people I know.”

Combustion of an RDX molecule

“Ouch. Yeah, it happens, but I’m mellowing, okay? A dozen fragments per molecule, got it. Hey, what chemical is ‘NOx‘?”

“Could be nitrous oxide N2O, or nitric oxide NO, or some combination depending, which is why there’s no number in the equation in front of oxygen’s O2. Combustion is messy.”

“Yeah, enthalpy all over the place! Those separate gas molecules spread out to a way bigger volume than the solid molecule used up. Lotsa pressure‑volume work there, right?”

“True, but gas expansion’s only one factor in an RDX discharge. Did the guys at the base mention that if you detonate that putty when it’s spread thin it can burn through an I‑beam?”

“So there’s heat, too. Can’t be much stacked up against the expansion.”

“Don’t be so sure. I’m not up on RDX thermochemistry. I never asked Susan and I don’t know whether she or anyone knows the breakout. It’s hard to do a precision measurement on an explosion, even if you do it in milligram quantities. I’ve got a good substitute for that, though. Water’s way simpler and much more thoroughly studied.”

“How is water a substitute? It doesn’t explode.”

“True, but it boils. No changes in molecular bonding, so enthalpy’s chemical part isn’t a factor. Carnot taught us to figure the pressure‑volume and thermal parts separately. Suppose you load a liter of water into a cylinder‑piston arrangement that stays at one atmosphere pressure. Get it up to boiling temperature then measure the energy input while the water boils away. The water absorbs energy while it turns to steam, right, even though there’s no change in temperature.”

“It stays at 212°?”

“212°F is 100°C or 373 K, stays steady provided the pressure stays at one atmosphere, 14.7 psi or 101325 pascals, whichever units you want to use. Pressure and temperature work together when it comes to phase changes. Anyway, the only way your rig can maintain that exact pressure is to do some kind of work, lifting a weight or something, until the cylinder’s final volume above the piston is 1705 liters. That’ll be 172 kilojoules of useful work.”

“Big cylinder.”

“Granted, but we supposed a liter of water. Scale the equipment to handle just a milliliter of water and the swept volume’s down to 1.7 liters. Neat how the metric system works. But now you’ve got a design decision to make. You can release the steam with a loud CHUFF that carries away 92% of the energy you put into it—”

“That’s no good.”

“— or you can run it through a condenser that preheats the feed water for the next cycle. Saves a lot of fuel that way.”

“That’d be my choice.”

“Mm-hm. That was Watt’s crucial improvement on Newcomen’s design. Funny thing, though. Both guys are credited with ‘inventing the steam engine’ but neither one built a device like the engines we’re used to, ones that develop power by pushing on a piston. The big demand in their day was pumping water out of mine shafts. Newcomen and Watt built vacuum gadgets.”

Watt’s steam pumping engine

“I had a well once. You can’t pull water up more than about 35 feet.”

“Right. Vacuum pumping is limited. Unfortunately, so was manufacturing technology in Watt’s time. Making a piston and cylinder that would fit together efficiently over a wide temperature range was a big challenge.”

“Their engines sucked, huh?”

~ Rich Olcott

Deep Dive

On By rolcottIn Chemistry, Physics: Entropy and Thermodynamics, UncategorizedLeave a comment

“Sy, I’m trying to get my head wrapped around how the potential‑kinetic energy thing connects with your enthalpy thing.”

“Alright, Vinnie, what’s your cut so far?”

“It has to do with scale. Big things, like us and planets, we can see things moving and so we know they got kinetic energy. If they’re not moving steady in a straight line we know they’re swapping kinetic energy, give and take, with some kind of potential energy, probably gravity or electromagnetic. Gravity pulls things into a circle unless angular momentum gets in the way. How’m I doing so far?”

“I’d tweak that a little, but nothing to argue with. Keep at it.”

“Yeah, I know the moving is relative to whether we’re in the same reference frame and all that. Beside the point, gimme a break. So anyway, down to the quantum level. Here you say heat makes the molecules waggle so that’s kinetic energy. What’s potential energy like down there?”

<grabs another paper napkin> “Here’s a quick sketch of the major patterns.”

“Hmm. You give up potential energy when you fall and gravity’s graph goes down from zero to more negative forever, I guess, so gravity’s always attracting.”

“Pretty much, but at this level we don’t have to bother with gravity at all. It’s about a factor of 1038 weaker than electric interactions. Molecular motions are dominated by electromagnetic fields. Some are from a molecule’s other internal components, some from whatever’s around that brandishes a charge. We’ve got two basic patterns. One of them, I’m labeling it ‘Waggle,’ works like a pendulum, sweeping up and down that U‑shape around some minimum position, high kinetic energy where the potential energy’s lowest and vice‑versa. You know how water’s H‑O‑H molecules have that the V‑shape?”

“Yeah, me you and Eddie talked about that once.”

“Mm‑hm. Well, the V‑shape gives that molecule three different ways to waggle. One’s like breathing, both sides out then both sides in. If the hydrogens move too far from the oxygen, that stretches their chemical bonds and increases their potential energy so they turn around and go back. If they get too close, same thing. Bond strength is about the depth of the U. The poor hydrogens just stretch in and out eternally, swinging up and down that symmetric curve.”

“Awww.”

“That’s a chemist’s picture. The physics picture is cloudier. In the quantum version, over here’s a trio of fuzzy quarks whirling around each other to make a proton. Over there’s a slightly different fuzzy trio pirouetting as a neutron. Sixteen of those roiling about make up the oxygen nucleus plus two more for the hydrogens plus all their electrons — imagine a swarm of gnats. On the average the oxygen cloud and the two hydrogen clouds configure near the minimum of that U‑shaped potential curve but there’s a lot of drifting that looks like symmetrical breathing.”

“What about the other two waggles?”

“I knew you’d ask. One’s like the two sides of a teeterboard, oscillating in and out asymmetrically. The other’s a twist, one side coming toward you and then the other side. Each waggle has its own distinct set of resistance forces that define its own version of waggle curve. Each kind interacts with different wavelengths of infrared light which is how we even know about them. Waggle’s official name is ‘harmonic oscillator.’ More complicated molecules have lots of them.”

“What’s that ‘bounce’ curve about?”

“Officially that’s a Lennard-Jones potential, the simplest version of a whole family of curves for modeling how molecules bounce off each other. Little or no interaction at large distances, serious repulson if two clouds get too close, and a little stickiness at some sweet-spot distance. If it weren’t for the stickiness, the Ideal Gas Law would work even better than it does. So has your head wrapped better?”

“Sorta. From what I’ve seen, enthalpy’s PV part doesn’t apply in quantum. The heat capacity part comes from your waggles which is kinetic energy even if it’s clouds moving. Coming the other way, quantum potential energy becomes enthalpy’s chemical part with breaking and making chemical bonds. Did I bridge the gap?”

“Mostly, if you insist on avoiding equations.”

~ Rich Olcott