Total Energy and Thermodynamic Systems (PLA 4)

Ben Lear

Hey everyone, welcome back to The Honors Element—thanks for joining us for another deep dive into the chemistry that shapes, well, basically everything around us. I’m Ben Lear, and as always I’m here with Morgan Vincent. And today, Morgan, we’re getting into the nitty-gritty: what do we mean by a “system” in thermodynamics? Because honestly, it’s one of those things that you think you get, until you realize there’s a lot of nuance there.

Morgan Vincent

Right! I mean, it’s that classic, “let’s be precise or else everything falls apart later.” So, in thermodynamics, a “system” is just the piece of the universe you’re looking at. It could be the gas inside a metal box, or, weirdly, just a tiny cubic centimeter of air, floating in the middle of a classroom. The whole point is, you’re choosing where your focus is, and what matters for your question, but you have to spell that out. Otherwise, you know, chaos—science chaos.

Ben Lear

Yeah, and people new to chemistry—or even old hats—sometimes get tripped up by the boundaries part. Sometimes it’s literal walls, like a sealed flask in an experiment. But sometimes that boundary is just an imaginary line in space. Either way, the math treats them the same… but if you pick the wrong boundaries, your conclusions can go off the rails really fast.

Morgan Vincent

And this is big, because once you’ve drawn your system, the next question is: what’s allowed to cross that boundary? If nothing at all gets in or out—no matter, no energy—that’s what we call an isolated system. The classic thought experiment is the perfectly sealed box: completely cut off from the rest of the universe.

Ben Lear

Right. But most of the time, something can cross. If only energy can move in or out—like heat or light—but matter stays put, that’s a closed system. Think of a sealed soda can warming up in the sun: the liquid can absorb heat, but the mass inside doesn’t change.

Morgan Vincent

And finally, the one we bump into constantly: an open system. That’s when both matter and energy can move across the boundary. Boil a pot of water on the stove—steam escapes, heat flows in. That’s an open system. Honestly, most living things fall into this category too.

Ben Lear

Exactly. You set those boundaries depending on what you want to measure, control, or really what we ask you for in the question!

Morgan Vincent

You know Ben, students usually come to me worried about “getting the type of system wrong.” But my advice is always the same: be clear about the system, and the rest will unfold.

Ben Lear

Yeah, so once you define your system, everything outside is the surroundings. Together, they’re the “thermodynamic universe” for that process. And the fun thing about this is that the system and the surroundings are always balancing each other out.

Morgan Vincent

So let’s talk energy—the thing that keeps systems and surroundings balanced. Total energy, usually written as U, is the sum of kinetic and potential. U = KE + PE. Simple formula, huge implications!

Ben Lear

Kinetic energy is motion, one-half m v². Potential comes from position or interactions. In physics, that’s often gravity, but in chemistry it’s mostly charges—Coulomb’s law. That’s the real connection to atoms and molecules!

Morgan Vincent

And here’s the key: when U changes, it’s because either the particles are moving differently—that’s kinetic—or they’re arranged differently—that’s potential. Think of a pendulum: top is all potential, bottom is all kinetic, then it flips back again.

Ben Lear

But it almost never stays that simple in the real world! Often energy is not kept just in the system, but an be transferred to the surroundings. Have you ever biked down a hill? Height turns into speed, but brakes and friction make some of that energy transferred to the surroundings as heat. That’s why being clear and precise in defining your systems is important—you’ve got to know what’s changing and where the energy actually goes.”

Morgan Vincent

In chemistry you’re always trying to figure out, did my molecule just gain kinetic, or did it get “rearranged” into a lower potential state? You really have to step back and ask what’s going on in the system and surroundings. And this idea of transferring energy, brings us to our next topic: the first law of thermodynamics

Ben Lear

The universe, at least for any process you’re looking at, is just the system and its surroundings, nothing else. And the total energy? It’s always the same. But—energy can flow back and forth: the system gains it, surroundings lose it, or vice versa.

Morgan Vincent

And what’s super cool is the idea of state functions here. So, internal energy—U—is a state function, which just means it depends on where you start, and where you finish, not how you get there. Like, drive from Philly to Pittsburgh, whether you go straight or detour along every back road, the altitude change is the same. The path? That affects other stuff, but not the total change in elevation. The total energy behaves the same way.

Ben Lear

Exactly. So, let's put this into an example—a closed system, like water evaporating at constant pressure. Think about a pot with a lid, water inside, heat source underneath. You add energy—heat comes in and manifests as kinetic energy in the system. The water molecules jiggle more and some escape as vapor. The system (the water) gained energy, but only through heat and no mass was lost. The surroundings (everything else) lost that energy. And just like we said, if you look only at the start and finish, internal energy change is just the difference between those two states, not how fast you heated the water, not the exact steps.

Ben Lear

That’s right. And again: whether it’s a real-world process like evaporation or something more abstract from last class’s electrolysis episode, this principle holds. Energy is never destroyed or created out of thin air, it just moves from system to surroundings. As long as we define our boundaries clearly and label our states, the math—and the chemistry—work out every time. I might say that’s reassuring.

Morgan Vincent

So, when you pull it all together: define your system, set the boundaries, and then keep track of how energy moves in or out. Internal energy is a state function, so all that matters is where you start and where you finish. The details in between? That’s path-dependent stuff, but is important. Values of heat flows---heat and work---depend on the path taken. They are not state functions.

Ben Lear

Exactly. And that’s the first law of thermodynamics in action—energy conserved, never created or destroyed, just shifting between system and surroundings. If you keep those rules straight, the rest of thermo starts to click.

Morgan Vincent

Alright, we’ll stop there for today. Next time, we’re going to zoom in on how heat and work actually show up in real processes, and why keeping them straight can make or break your understanding. Should be a fun one.

Ben Lear

Bye everyone, and see you next time on The Honors Element.