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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

6. Thermodynamics and Kinetics


If a reaction is exothermic, shouldn’t that be enough to determine favorability? Actually, no!

Even if a reaction is highly exothermic, the level of order it requires may make it statistically improbable.


Explaining what entropy is.

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Alright, guys. And I want to talk about what I would consider the most confusing term of the Gibbs Free Energy equation. And that is entropy. Alright. So entropy, uh, generally stated we said that it was a measure of disorder in the system. Okay, but that's a really confusing definition. I would rather go with an easier definition, Okay. And what that is is that entropy is the tendency of a sense system to take its most probable form. Okay, so that means that if we have two different states, one is highly ordered and one is not as ordered. Statistically, it's more likely to be in the disordered state. Okay? And that's what entropy has to do with it Has to do with probability. Really? Okay, So what that means is that even if a reaction is highly eggs a thermic like if you talk about bond dissociation, energies and entropy, that's do with eggs A thermic but the level of order that it requires might make it statistically improbable. Okay, so basically, what we do in with entropy is we have to analyze. Is this gonna be statistically more probable or statistically less probable? And that is what entropy is. Okay. Remember that we define that Entropy says that, um, a positive. I mean, I'm sorry. A negative value is gonna b'more ordered. Okay, because remember that basically a positive value means that it's more disordered. That means that your entries getting bigger. So if you have a negative value, that means you're entropy is getting smaller, or it's getting more ordered. Okay? And it turns out that we're never gonna have to calculate the entropy in terms of calculating the entropy of the surrounding environment or the entropy of a system, because the fact that we don't have really the tools to analyze that in Oracle One. Okay, but what you are going to be asked to do, okay, is you're gonna be asked to analyze if something is going to have a positive delta s or negative Delta s on. That's what you wanna do. We don't want to figure out the exact number we just want to figure out. Is this gonna be higher entropy or lower entropy? Okay, now there is one situation where you might calculate Delta s, and that's if you're given every other variable. If you're given the tea and adult h and adult Aggie, Then sure, you could just use algebra to figure out Delta s. But I'm just saying that in the absence of this being just a simple algebra problem, you're not gonna be asked just to calculate with the Delta s is of oven environment. Okay, so let's talk about these three phenomenon that make reactions more probable or make Delta Esco up, and it turns out that all of them are gonna be favored. Bye. Hi, Temperature. Okay, so that will be That should be really clear in a little bit when I go back to the equation.

Entropy (ΔS) is the tendency of a system to take its most probable form.

  • Negative values (-) indicate less probable = Unfavored
  • Positive values (+) indicate more probable = Favored

There are 3 common ways to make reactions more probable (increase ΔS)

  • They all become more likely as we add heat to the reaction (increase T)

3 ways to increase entropy.

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the first one is increasing the number of molecules in reaction. So, um, any reaction that's gonna make more moles of equipment, more moles of molecules, arm or equivalents of molecules after the reaction is over is gonna be entropic. Lee favored Why? Because what it does is it allows us to arrange these molecules in more ways. All right, so here's a really good example. Thermal cracking. Okay, This is used by the petroleum industry to take large hydrocarbons, ones that don't really do anything. You can't put them in your car or anything. Then they put them with hydrogen and high heat. Okay. And what happens is that thes long hydrocarbons spontaneously start breaking into smaller pieces. Okay, Why would this be a favorite reaction? Okay, it actually takes energy to break those bonds. So why would these long hydrocarbons break? The reason is because at high enough temperature, what starts to happen is that if we could break one molecule into molecules, right, that's gonna be in tropically favored, because now I can arrange those molecules in different orders. Okay, So what that means is, the chances are that if I have to systems that of equal energy. One is that I have all of the molecules perfectly arranged, that they're all in a line or all of the molecules scattered all over the place. Which one's gonna be more probable? The one where it's scattered. Okay. And that's why thermal cracking is favored. You take large hydrocarbons and you turn them into smaller ones by using high heat, high heat then makes the entropy part of the equation very, very favored. So now the more pieces I could break it into the Maurin tropically favored. That is all right. This is just one example. But there's lots of examples in organic chemistry of reactions that make more molecules than they start off with. Meaning that they're in tropically favored. Okay, um, so let's go into the second one. The second one is a phase transition. Okay, if I can transfer form something from a solid to a liquid or from a liquid toe a gas, that's going to be a more probable arrangement. And the reason is because the molecules are now gonna have greater vibrational freedom. Okay, remember, the only difference between a solid liquid and gas is how much energy these molecules have okay. And how much motion they're allowed Toe display in a solid. There's very little vibration that can happen. They can only move in a very constrained space. Okay, whereas in a gas, they're bouncing all over the place. For example, of the gas in this room right now, it's bouncing off the walls. It's hitting me. That has a lot more freedom of motion than, ah, solid does. So if I have two systems of equal energy one where my molecules constrained till one tiny little place next to another one and and then another system where the molecules can scatter us faras they want which one is more favored, the one that they can scatter it. Okay, so what that means is that we have reactions that, as you turn liquids into gasses or solids into liquids there also in tropically favored. Ah, really good example is a reaction that we're gonna learn in or go to called Dhi Qar. Box elation. Dhi Qar box elation takes a carb oxalic acid at high heat. Okay, and it turns it into just a key tone and co two gas. Okay, why is that favorite? Because at high heat now the entropy part of my equation becomes more powerful. It gets ah, higher. Wait. Okay. And if I'm turning something into a gas, that's gonna be more favored because the gas is more favorite than a liquid. Okay. And by the way, this this molecule here would be a liquid at room temperature. Okay, Now, another thing to keep in mind is that this is also favored because of another thing, which is this is also favored by the making more molecules rule. Remember that. Here I have one molecule total at the end of the equation. I have one molecule here and one molecule here. So I'm going from one mole to two moles of molecules. Okay, So what that means is that this is also gonna be in tropically favored, not just because the phase transition, but also because I'm making a greater number of molecules. All right, so let's look at the last one. The last one is increasing. Um, freedom of motion of a molecule. Okay. And this specifically has to do a sigh. Click molecules. If I can convert a sigh click molecule into an ace I click molecule. That's gonna be more probable. Probable because now it's gonna increase the freedom of rotation off thes carbon chains. Okay, so this is also reaction that you're gonna see a lot in or go to lack tone ring opening. Okay, A lack tone is just a cyclic Esther. Okay, that's the name of a lack tone. We're gonna talk about that a lot more or go to what you see here is that in the presence of heat, what I can do is I can take my lack tone, and it could break it apart into I could break the chain, the ring apart into two parts of the chain where this oxygen and this oxygen were linked together before and basically were the same thing. Okay, Now what I do is I put one oxygen on each and now I have my carbon chain that is, has more freedom of motion. Now, why is the chain better than the ring? Because remember when we talked about If you remember, we talked about conformers, we talked about single bonds being able to rotate. Okay. What would happen is that these single bonds can all rotate a lot more than a ring in a ring. You actually can't rotate it all. So what that means is that this one is gonna have a whole lot more conformers possible than the first one. In fact, the first one doesn't have any. Okay, that's also more probable. Because if you have to, um, molecules of equal energy and you have one where all of the bonds or eclipsed that's what this would be. This would be all eclipsed. Okay, Because I would have all the h is coming off the same side. Okay, if you have one where all the h is our eclipse, and then you have another one where they're Frito to rotate and make anti and make KAOS and make a bunch of different confirmations. Which one is going to be more favored? The one on this side. So this one is also going to be in tropically favored toe. Open that ring as I increase the heat. Okay. And as I jack up the heat mawr, that's going to favor all of these reactions. And like I said, the reason has to do with Gibbs free energy. And remember that gives free energy just basically stated that Delta G is equal to Delta H minus t Delta s. So what that means is that as I increase T, what I'm gonna do is I'm gonna increase the amount that entropy matters. Okay, so if I raise all these reactions to 1000 degrees, the entropy isn't even gonna matter anymore. I'm not going to care about bond association energies, because instead, what I'm gonna care about is how many molecules am I getting at the end, or what's the freedom of rotation of these molecules? All right. Interesting. Right. And that's going to tell us which ones more statistically favored. And that's gonna be which one is more spontaneous, alright?

1. Increasing the Number of Molecules

Reactions that create extra molecules are more probable since there are more ways to arrange them.


2. Phase Transition

Transformation of solid to liquid or liquid to gas is more probable since the molecules will have a greater vibrational freedom.


3. Increasing Molecular Freedom of Motion

Converting cyclic molecules to acyclic molecules are more probable since it increases freedom of rotation.