Here’s An Easy Way to Learn Dynamic Equilibrium

Enjoy this video and transcript Karen the Chemistry Rockstar explain dynamic equilibrium the easy way on Twitch.

Dynamic equilibrium is a chemical equilibrium between a forward and the reverse reaction in which the reactions are equal. The result is the ratio between reactants and products stays the same.

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Video Transcription

I have a topic for today. And my favorite thing about this topic is that I’ve got a pretty fun analogy. It brings back good memories of when I lived in San Francisco and, uh, there’s a dance club there. And so I have my dance club analogy whenever I’m talking about equilibrium, but specifically when I’m talking about dynamic equilibrium.

What do we mean when we’re even talking about equilibrium? What does that mean? And so then we can take it a step further and talk about what does dynamic equilibrium mean.

All right. So I think, since we’re talking about chemistry, why not use a chemistry example? And I like to start out using really simple molecules so that we don’t get too overwhelmed.

So let’s put up a hydrogen molecule and hydrogen naturally at room temperature. It is a gas. Okay. So there is not a lot of hydrogen in our air right now. It’s fairly stable, but it’s also flammable. All right. So, uh, if you’ve heard about that horrible disaster of the Hindenburg, uh, that big Zeppelin, that caught fire because it was filled with hydrogen and now we use helium, which makes way more sense.

It’s light and it’s non reactive, but in any case, that was a disaster. But my point here is that hydrogen age two is a gas at room temperature, and it can react with another non metal that can also exist in the gas phase.

Now, iodine, it does exist as this beautiful, solid crystal that is this deep violet color. It’s quite lovely. So it is a solid at room temperature, but this is the thing is that it’s also volatile. Meaning just like water, when you have a pot of water and above that liquid water, even though water is liquid a room temperature, there are gas molecules of water that are vaporizing above it. And that’s the same thing with iodine.

When you have a solid crystal of iodine, there’s some of it that is released as gas molecules that are coming off of there. And if that happens in the presence of hydrogen gas, they can react together and they can give hydrogen iodide also in the gas phase. And so I’m gonna draw no. So I’m drawing this arrow up, uh, given, leaving me a little bit of space, cuz we’re gonna show how this is an equilibrium and it show it forms arms, hydrogen iodide, also gas phase, but we need to make sure this chemical reaction is balanced, right?

We need to have mass balance. If we have two hydrogens on this side and we have two iodides and iodine iodine atoms on this side, then we need to make sure that that’s true on this side as well.

So that’s why we put in that two psychometric coefficient. All right. So now we’ve got this balanced chemical reaction and at first glance, this is great, nothing wrong with it looks good. But is this really what is happening on the molecular level? If we were able to go in with some kind of microscope, which we can’t do with molecules too well, there’s tricks there.

But anyway, if we were able to look in and see these molecules in what they were doing, we would see that yes, indeed. We’ve got hydrogen iodine that are reacting together and forming hi, but we would also notice that there was something else happening.

What is happening. We would see that those I molecules are breaking apart and they’re going essentially backwards and they’re reforming the original reactance, the hydrogen, the iodine. And so to really show what’s going on, we would want to show a backwards arrow as well to convey what is happening with this chemical reaction. And so we call this a chemical equilibrium.

Now, when we use the word chemical reaction in a generic sense, we’re usually talking about reactants and products and it’s often convenient to just ignore the fact that it is actually an equilibrium, but everything is actually qui. It is <laugh>, it’s just convenient to not have to say it all the time. So this is this whole idea here is what I wanna get into in terms of talking about dynamic equilibrium. So we’ve got these molecules, H two I two, some of them are colliding and just bouncing off each other and not reacting, but some are colliding in a certain way and they’re creating the products.

And then we’ve got these breaking apart and then reforming these reactants. This is constantly happening constantly. And so the molecules that are over here are not always going to be the same atoms that they were two minutes ago.

Some of those atoms might be back over here. And then two minutes later, some of those atoms that were over here are now back over there. So when we reach equilibrium and that is what’s the definition of equilibrium, this is when the concentrations stop changing. So the beginning of a chemical reaction, we have a lot of the reactants and they start forming products.

And then eventually it reaches this equilibrium where concentrations aren’t changing anymore. And we’re tempted to think everything is stopped. All of these that have collided, deformed that they’re done, they can kick back. All right, nothing more to do here, but no, these molecules are continually moving, which makes perfect sense.

Speaker 1: Right? We are above absolute zero. What’s happening at absolute zero in theory, cuz we can’t reach absolute 0.00000. But we’ve gotten pretty darn close 0.001 or something Calvin, but right at absolute zero, essentially all molecular motion has slowed down and almost stopped.

Okay. That’s an absolute zero though. What are we at? We’re at oh two 90, depending on where you are right now. I think I’m about 298 Kelvin room temperature, 25 degrees Celsius. So right now these molecules, they’re gonna be moving. They’re gonna be moving and they’re going to be changing. So that’s where this idea of dynamic equilibrium comes in. Is that sure. The total concentration of the reactions and the total concentration of the products isn’t changing, but are those molecules moving? Yes they are. Are they changing their direction? Their emotion? Yes they are. They’re moving a lot. And this is where my dance club analogy comes in. 

All right. So back in the nineties, I lived in San Francisco, late nineties and there was a dance club called the end up, I think because you know like 3:00 AM on Saturday night, that’s where you ended up or at least they wanted you to all right. So the end up, it was this really tall building and uh, it was actually kind of fun.

I remember, I don’t know why, but I went shopping grocery shopping early on a Sunday morning and I drove past the end up. And uh, I saw people, uh, leaving at that point and I was like, oh, this is interesting. And this analogy emerged a couple years later when I was teaching equilibrium. And I was like, Hey, that’s like the end up. So this is the deal is that there is some point in for a club when people start to arrive.

Now before that, what do you have in the club? You’ve got the Bo bartenders. You’ve got the bouncers, you have other PE the DJs and all that. So there are a few people that are in here and I’ll just draw them. Um, little, I’m not even draw stick figures. I’m just gonna draw little lines. Okay.

So there’s a few people in here and the people who are outside in line, they start coming in, they start entering. Now what’s true about a crowded club or concert or something like that in terms of the fire marshal, what does the fire marshal have to say about that? We can only have a certain number of people in there, right? So there’s some capacity. And so what does eventually happen? These people start coming in. The bouncer’s letting them in and the end up starts getting full. All right.

So lots of people in there and then it reaches capacity and you know why? I like this, that it’s a dance club is cuz one of these people do <affirmative> is, cause what are these people doing? They are dancing. Okay. And I love that because what are the molecules doing? They are moving, dancing. Okay. This constant motion. But what happens? You’re like at the end up and it’s like, I don’t like this DJ. I wanna check out this other one. So you bail and people begin to leave. Right? All right.

So what happens when three people leave? The bounce will lets three more people in. So then they get to go in as they go. But because those three people left, those three, three different people came in, we still have that capacity. We had 250 people let’s say that are in there. They’re still moving different people in different people go.

And that keeps happening. Different people in different people go. But there’s still that 250 person capacity. That’s filling this dance club. All right.

So this is essentially what has happened here once it’s filled and when there’s that molecular motion, people dancing, but there are different people coming in, different people leaving that is that same idea of dynamic equilibrium, where we have these atoms that are co colliding. And they’re forming that these molecules over here they’re breaking apart, or I should say these are molecules colliding that we’ve got.

Once equilibrium is reached, we’ve got this steady concentration of the reactance and the products. So that’s not changing just like the number of people and the dance club is not changing, but there’s still motion. It’s not like everything’s stopped. There’s still motion. There’s still different atoms doing different things. All right. So that is our concept of dynamic equal every am. Lots of motion, but steady concentrations.