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

Bond dissociation energies describe the strength of chemical bonds. They can be determined experimentally. With that said, we’ll need some basic math for this section to determine the Enthalpy (∆H˚) for reactions. 



How to calculate enthalpy using bond dissociation energies.

Play a video:
Was this helpful?
Alright, guys. So let's go ahead and talk about one of the most important topics and thermal dynamics, which is using bond dissociation energies to calculate the entropy of a reaction. All right, so bond dissociation energies are values that are going to describe the strength of chemical bonds. All right, now, in chemist chemistry courses that are way beyond the level of this course organic chemistry, we actually can use math to figure out exactly why two atoms have the exact strength of bond that they do. Okay, But in this course, you're never going to be asked that you're never going to be asked to explain why the bond between H and H is 436 killed. Geothermal. Okay, that's not gonna happen. So these are gonna be basically experimental values that you're gonna be given that you're going to use as reference to just figure out one question. Is this going to be an extra thermic reaction or is this going to be an endo thermic reaction? Now, keep in mind just because we know the entropy doesn't know that we know doesn't mean that we actually know the spontaneity. Remember that the entropy is just one component of the spontaneity. We're also going to need to know the temperature and the Delta s. But right now we're worrying about is the Delta H. We're just saying, Is this exo thermic or endo Thermic. Alright, let's remind ourselves of what exo thermic means. It just means that I'm gonna get a negative energy at the end. And what that means is that I'm making Bonds remember that it's favorable to make bonds. You release energy as you save energy between those orbital's. Okay. And then if I get a positive value, that means I'm breaking bonds because, remember, it takes energy. I have to put energy into the system to make these atoms fly apart again. All right, so here I've given you guys my own chart of bond dissociation, energies. All these values should be exactly the same toe whatever chart you're gonna be given. The only thing I did is I just didn't want to take up a full page with every single value. So I just gave you guys some of the most important ones here. And as you can see, they're basically broken up into bonds of hydrogen. So I'm just gonna put hydrogen bonds, even though don't get that confused with hydrogen bonding. That's just something different. Okay, then I put here metals. Okay, So things bonded to metal and then finally, um, basically like things bonded to each other. I don't know how to say that better, but just, like method to metal, oxygen, oxygen, stuff like that. Okay, So you guys could see just the general trends. And what you can see is that there's a general trend that as my atoms get bigger, Okay, as my atoms get bigger, what's gonna happen is that these bonds are going to get weaker. Okay, so you can see how it's not perfect. It's not a perfect trend. And like I said, you have to use very complicated math to defend it. But as a general rule, what we can say is that as these atoms that are attached to each other get bigger, the bonds are a little bit weaker, and that has to do with the distance between them. Okay. These bigger atoms, we're gonna have a further distance than the smaller ones. All right, so one of the weakest bonds here is gonna be I I Why? Because those air to humongous atoms, they're trying to make a bond to each other. They're gonna be much further apart than hydrogen hydrogen will be. And that is part that can partially explain the huge difference in energy between hydrogen, hydrogen and iodine. Isaan. All right. So, as I just told you, if we make a bond, that's gonna be a negative value. So that means that I would assign a negative value to whatever bonds I'm making. Okay, So if I'm making hoh, that means I would put a negative value on that. If I'm breaking a bond, then I'm gonna sign a positive value to that. Okay, So what I want you guys to do for the next two problems is I want you guys to analyze the reaction that's taking place. You don't need to know the mechanism. You don't need to know exactly what's going on. What you do need to know is okay. Which bonds are being broken, which bonds are being made all right, and then assign values to those using the chart that I gave you above at the end. After you, some all that together, you're gonna know the entropy of the reaction. And you're gonna be able to tell me, Is this exo thermic or endo? Thermic. So go ahead and try it for yourself.

Enthalpy (ΔH°) is the sum of bond dissociation energies for the reaction.

  • Negative values (-) indicate the making of new bonds = Exothermic
  • Positive values (+) indicate the breaking of new bonds = Endothermic


Calculating Enthalpies

Play a video:
Was this helpful?
alright, guys. So, like I said, you don't need to know the mechanism of this reaction. What we do need to know is what bonds are being broken and what bonds are being made. All right, so on my re agent side. Okay, Re agents. What I have is the bonds that are breaking, because what I can see is that I used to have a c H bond here, but now that's turning into CBR. So that means that at some point I must have broken that bond. Okay, so that means that this is going to be the breaking side. Okay, if this is the breaking side, what sign of entropy should this have? Should it be positive or negative? It should be positive, right? Because it's taking energy to make these bonds come apart. So all of the values that I put over here are gonna be positive. All right, then all the values I put over here for the making side are gonna be negative, because either one these air ones that are going to release energy into the system. So now all I have to do is identify these bonds, Okay? One of them, I know already is going to be See h three h. I know that bond is getting broken because right now I see. Like I said, it used to be there. And now it isn't another one that has to get broken is BR too, because BR two is turning into HBR. So this is gonna be br br. That's another one that's gonna require energy to break. Now let's look at my negative charge, not charge my negative side, my exo thermic side. So basically the bonds that I'm making are equal to C H three br because that's a new bond that didn't exist before. And another one is HBR. That's a bond that didn't exist before. So now all I have to do is pull down the values for my chart and which everyone's are on my re agent side are going to be, or when braking side are gonna be positive, which everyone's air on my product side or my making side are gonna be negative. So let's go ahead and look up. So what are the values that I'm bringing down? Well, the CH three h is gonna be 4 36 so it's gonna be positive for 36 br br is gonna be It looks like Where is that? It's right here. So that's gonna be 1 92 positive 1 92. Okay, then on the negative side or on the on the making side ch three br I'm gonna just take myself out of the screen for a second so you guys can see So in the negative sign, um, ch three br is gonna be right here. That's gonna be 9 23. So I'm gonna give that a negative 9. 23 and then HPR is gonna be negative. 3. 68. Okay, so I'm pulling it back down. We have all the values we need. Hi, I'm back. So now all we need to do is plug all these values into a calculator and whatever and answer is that's gonna be the end answer. And I'm here. Sorry about that. Stop Auto focusing. Okay, so let's go ahead. I've got my calculator here. Let's go ahead and enter these values. I have 4 36 plus 1 minus 2 93 minus 68. And what that should give you at the end is that you're in therapy, your Delta H is equal to negative 33 Kill a jewel's Permal. All right, so it's gonna answer a really big question for us. Is this x a thermic or is this Endo thermic? And the answer is that this has to be eggs A thermic because at the end, the some of my bond association energies is actually going to be a negative number, which means that I'm getting more stable at the end. I'm making I'm making more bonds or I'm gaining more energy than I am losing by breaking those bonds. So it would be an extra thermic reaction. Do you know if this would be a spontaneous reaction? No, we have no clue yet. We still need the temperature in the Delta s to figure that out. But we know with certainty that this is an extra thermic reaction. Okay, Now I want you guys to do the same thing for the second question. Once again, all in your own. Try to figure it out, and then I'll give you guys the answer

I hope that made sense! Let's try another similar problem with I, instead of Br and see how that changes things. 


Predict the sign and magnitude of ∆Ho in kj/mol for the following reaction. Identify the reaction as either exothermic or endothermic.