Understanding Bond Length and Strength

Consider these two molecules, HF and HBr. Which of them has a greater or longer bond length? Is it HF or HBr? Which bond is longer? Now hydrogen is a very small molecule. Now what about fluorine and bromine? What are the sizes of those two atoms? Bromine is a lot bigger than fluorine. So as a result, HBr has a longer bond length than HF. When you're dealing with the halogens, you have fluorine, chlorine, bromine, and iodine. And as you go down the periodic table, the atomic size increases. And so as the atom gets bigger... the bond length increases. So therefore, HBr is longer than HF. So if HBr is longer, what do we know about HF other than the fact that it's shorter? What about the bond strength between these two bonds? When you move two atoms away from each other, the bond length increases, but the strength of the bond weakens. It becomes easier to break a bond. When the nuclei of two atoms are brought closer, the bond strength increases. So because HBr is longer, the bond strength is weaker. And because HF is shorter, the bond strength is greater, it's stronger. Now, for those of you who like to deal with numbers, for HF, the bond length is about 0.92 angstroms. And for HBR, it's about 1.4 angstroms. Now, in terms of the bond strength, HF has a bond strength of about 571 kJ per mole. And HBR, it's about 366. kilojoules per mole. That's the bond strength. So as you can see, as the bond length decreases, the bond strength increases. So these two are inversely related. As that goes down, the bond strength goes up. So here's a table that can summarize this information. So let's look at all of the halogens attached to a hydrogen atom. So in the first column, I'm going to write the bond length. And in the second column, the bond strength in kilojoules per mole. So as we said before, the bond length for HF was about 0.92 angstroms. And the bond strength, if you round it, it's about 570 kilojoules per mole. And for HBR we said it was about 1.4 angstroms and the bond strength 366 kilojoules per mole. HDI is about 1.28 angstroms and the bond strength is 432 and for HI is about 1.6 angstroms and Bond strength is 298 So notice that iodine is the largest atom compared to the halogens. It's bigger than bromine, bromine is bigger than chlorine, and chlorine is bigger than fluorine. So as you go down the group, the bond length increases, but the bond strength, you can see, it's decreasing. And so we have that inverse relationship. So knowing that, which bond would you expect to be stronger? The carbon-bromine bond or the carbon-iodine bond? Well, we know that iodine is a lot bigger than bromine. So therefore, the CI bond will be longer and the CBR bond will be shorter. So the shorter bond is always going to be the stronger bond, and the one that's longer is going to be the weaker bond. So the carbon-bromine bond is stronger than the carbon-iodine bond. Now let's analyze the carbon-carbon bond in ethane, ethene, and ethane. Which one would you say is longer? And which one would you expect to be shorter? Which one is the strongest and which one is the weakest? Now, what you need to know is that single bonds are longer than double bonds and triple bonds. Triple bonds are the shortest, single bonds are the longest. Now, because single bonds are longer, they're also weaker. So this is the weakest bond. and triple bonds are the strongest bond compared to the other two. Now, the reason why triple bonds are stronger is because three bonds are stronger than one. Here, you only have a sigma bond. A double bond... contains a sigma and a pi bond, but a triple bond contains a sigma and two pi bonds. So the triple bond is stronger because there's simply three bonds compared to one or two bonds. However, on a one-to-one basis, if you compare a sigma bond and a pi bond, the sigma bond is stronger than the pi bond. But if you have a sigma and a pi bond versus just a sigma bond, the sigma and the pi bond will win. So now let's put some numbers to this. So let's talk about the bond length and the bond strength. So the carbon-carbon bond length in ethane is about 1.5 angstrom. And for ethane, the carbon-carbon double bond, it's 1.3 angstrom units long. And for the triple bond, it's about 1.2. So as we can see, the triple bond is the shortest of the three. So we should expect that it's going to be the strongest. Now for ethene, the bond strength is approximately 380 kilojoules per mole. And for ethene, the bond strength is about 730 kilojoules per mole. As you can see, it's a lot stronger because we have two bonds as opposed to one. And for the triple bond, it's going to be about 960 kilojoules per mole. So even though the triple bond is the shortest, it's going to be the strongest bond. And the single bond is the longest. However, it's going to be the weakest of the three bonds. Now, let's change direction. Let's talk about the CH bonds rather than the CC bonds. So let's analyze the CH bond in ethane, ethene, and in ethine. So they're all single bonds. So do you expect them to be the same or different? So which of those CH bonds is longer and which one is stronger? Feel free to pause the video and go ahead and try that problem. So let's start with ethane. This single bond, it's about 1.1 angstrom units long. For ethene, it's 1.08. And for ethine, it's about 1.06. So notice that they're close to each other, but they're not the same. A CH bond next to a carbon-carbon triple bond is shorter than a CH bond next to a carbon-carbon single bond. And for one reason, I mean, triple bonds tend to be shorter than single bonds, so it's reasonable to expect that this is going to be shorter. Now let's talk about the bond shrifts. So the bond strength for a CH bond in ethane is 423 kJ per mole. And in ethene, it's 460 kJ per mole. And for ethane, it's about 560 kJ per mole. So this one is going to be shorter and it's going to be stronger compared to the other two. And in ethane, it's the longest of the three, and it's also the weakest of the three. Now let's talk about why, other than the fact that this is longer because it's next to a longer bond. What is the hybridization of the carbon atom in ethane? And also, what are the hybridization of the carbon atoms in ethene and in ethine? In ethane, the carbon has an sp3 hybridization. In ethene, it's sp2 hybridized. And in ethine or acetylene, it's sp hybridized. And hydrogen is just an s-orbital. So let's focus on the carbon because for hydrogen it's the same. An sp3 situation means that we have 25% s, 75% p. And for the sp2 carbon, its orbitals are 33% s, 67% p. And for the sp carbon in acetylene, it's 50% s. and 50% p. So the CH bond in ethane is longer because it has more p character. In acetylene or ethane, the CH bond is shorter because it has more s character. And if we think about it, the s orbital on average tends to be shorter than the p orbital. The s orbital is closer to the nucleus, and the p orbital is further away. And so that's one reason why, where if a bond has more p character, it's going to be longer. And if a bond has more s character, it's going to be shorter. Consider these three molecules. Which carbon-carbon bond highlighted in red is the longest, and which one is the shortest? So they're all single bonds. So how can we tell which one's going to be the shortest, and which one is going to be the longest? A quick and simple technique is to look at the bonds next to it. So for the first one, we have two triple bonds. So that tells us that this single bond is going to be the shortest. Here we have a triple and a double, and here we have a double and a single. So therefore, we know this is going to be the shortest bond, and this is going to be the longest bond, because it's around longer bonds. Now, on a test, if you get a question like this... you need to look at the hybridization of the carbons that form that bond. So what are the hybridizations of these two carbon atoms? Those two carbon atoms are sp-hybridized. Now for these two, the one on the left is sp-hybridized, and the one on the right has an sp2 hybridization. And then for the last two, the CH2 carbon is sp3 hybridized, and the CH carbon is sp2 hybridized. So the one that has more S character is going to be the shorter bond. And sp has more S character than sp2 or sp3. So we could say that the C-C bond in the first example is the shortest because it has more S character. due to the fact that it's around shorter bonds. The last one is the longest because it has more p-character. sp3 has more p-character than sp2 or sp. But if you see a bond surrounded by longer bonds, then this bond is going to be slightly longer than the other ones.

Consider these two molecules, HF and HBr. Which of them has a greater or longer bond length? Is it HF or HBr? Which bond is longer?

Now hydrogen is a very small molecule. Now what about fluorine and bromine? What are the sizes of those two atoms?

Bromine is a lot bigger than fluorine. So as a result, HBr has a longer bond length than HF. When you're dealing with the halogens, you have fluorine, chlorine, bromine, and iodine. And as you go down the periodic table, the atomic size increases.

And so as the atom gets bigger... the bond length increases. So therefore, HBr is longer than HF.

So if HBr is longer, what do we know about HF other than the fact that it's shorter? What about the bond strength between these two bonds? When you move two atoms away from each other, the bond length increases, but the strength of the bond weakens. It becomes easier to break a bond. When the nuclei of two atoms are brought closer, the bond strength increases.

So because HBr is longer, the bond strength is weaker. And because HF is shorter, the bond strength is greater, it's stronger. Now, for those of you who like to deal with numbers, for HF, the bond length is about 0.92 angstroms. And for HBR, it's about 1.4 angstroms.

Now, in terms of the bond strength, HF has a bond strength of about 571 kJ per mole. And HBR, it's about 366. kilojoules per mole. That's the bond strength. So as you can see, as the bond length decreases, the bond strength increases. So these two are inversely related.

As that goes down, the bond strength goes up. So here's a table that can summarize this information. So let's look at all of the halogens attached to a hydrogen atom. So in the first column, I'm going to write the bond length.

And in the second column, the bond strength in kilojoules per mole. So as we said before, the bond length for HF was about 0.92 angstroms. And the bond strength, if you round it, it's about 570 kilojoules per mole. And for HBR we said it was about 1.4 angstroms and the bond strength 366 kilojoules per mole.

HDI is about 1.28 angstroms and the bond strength is 432 and for HI is about 1.6 angstroms and Bond strength is 298 So notice that iodine is the largest atom compared to the halogens. It's bigger than bromine, bromine is bigger than chlorine, and chlorine is bigger than fluorine. So as you go down the group, the bond length increases, but the bond strength, you can see, it's decreasing.

And so we have that inverse relationship. So knowing that, which bond would you expect to be stronger? The carbon-bromine bond or the carbon-iodine bond? Well, we know that iodine is a lot bigger than bromine.

So therefore, the CI bond will be longer and the CBR bond will be shorter. So the shorter bond is always going to be the stronger bond, and the one that's longer is going to be the weaker bond. So the carbon-bromine bond is stronger than the carbon-iodine bond.

Now let's analyze the carbon-carbon bond in ethane, ethene, and ethane. Which one would you say is longer? And which one would you expect to be shorter? Which one is the strongest and which one is the weakest?

Now, what you need to know is that single bonds are longer than double bonds and triple bonds. Triple bonds are the shortest, single bonds are the longest. Now, because single bonds are longer, they're also weaker.

So this is the weakest bond. and triple bonds are the strongest bond compared to the other two. Now, the reason why triple bonds are stronger is because three bonds are stronger than one.

Here, you only have a sigma bond. A double bond... contains a sigma and a pi bond, but a triple bond contains a sigma and two pi bonds. So the triple bond is stronger because there's simply three bonds compared to one or two bonds. However, on a one-to-one basis, if you compare a sigma bond and a pi bond, the sigma bond is stronger than the pi bond.

But if you have a sigma and a pi bond versus just a sigma bond, the sigma and the pi bond will win. So now let's put some numbers to this. So let's talk about the bond length and the bond strength.

So the carbon-carbon bond length in ethane is about 1.5 angstrom. And for ethane, the carbon-carbon double bond, it's 1.3 angstrom units long. And for the triple bond, it's about 1.2.

So as we can see, the triple bond is the shortest of the three. So we should expect that it's going to be the strongest. Now for ethene, the bond strength is approximately 380 kilojoules per mole. And for ethene, the bond strength is about 730 kilojoules per mole.

As you can see, it's a lot stronger because we have two bonds as opposed to one. And for the triple bond, it's going to be about 960 kilojoules per mole. So even though the triple bond is the shortest, it's going to be the strongest bond. And the single bond is the longest.

However, it's going to be the weakest of the three bonds. Now, let's change direction. Let's talk about the CH bonds rather than the CC bonds.

So let's analyze the CH bond in ethane, ethene, and in ethine. So they're all single bonds. So do you expect them to be the same or different? So which of those CH bonds is longer and which one is stronger? Feel free to pause the video and go ahead and try that problem.

So let's start with ethane. This single bond, it's about 1.1 angstrom units long. For ethene, it's 1.08. And for ethine, it's about 1.06. So notice that they're close to each other, but they're not the same.

A CH bond next to a carbon-carbon triple bond is shorter than a CH bond next to a carbon-carbon single bond. And for one reason, I mean, triple bonds tend to be shorter than single bonds, so it's reasonable to expect that this is going to be shorter. Now let's talk about the bond shrifts.

So the bond strength for a CH bond in ethane is 423 kJ per mole. And in ethene, it's 460 kJ per mole. And for ethane, it's about 560 kJ per mole.

So this one is going to be shorter and it's going to be stronger compared to the other two. And in ethane, it's the longest of the three, and it's also the weakest of the three. Now let's talk about why, other than the fact that this is longer because it's next to a longer bond. What is the hybridization of the carbon atom in ethane? And also, what are the hybridization of the carbon atoms in ethene and in ethine?

In ethane, the carbon has an sp3 hybridization. In ethene, it's sp2 hybridized. And in ethine or acetylene, it's sp hybridized.

And hydrogen is just an s-orbital. So let's focus on the carbon because for hydrogen it's the same. An sp3 situation means that we have 25% s, 75% p.

And for the sp2 carbon, its orbitals are 33% s, 67% p. And for the sp carbon in acetylene, it's 50% s. and 50% p. So the CH bond in ethane is longer because it has more p character.

In acetylene or ethane, the CH bond is shorter because it has more s character. And if we think about it, the s orbital on average tends to be shorter than the p orbital. The s orbital is closer to the nucleus, and the p orbital is further away.

And so that's one reason why, where if a bond has more p character, it's going to be longer. And if a bond has more s character, it's going to be shorter. Consider these three molecules. Which carbon-carbon bond highlighted in red is the longest, and which one is the shortest? So they're all single bonds.

So how can we tell which one's going to be the shortest, and which one is going to be the longest? A quick and simple technique is to look at the bonds next to it. So for the first one, we have two triple bonds.

So that tells us that this single bond is going to be the shortest. Here we have a triple and a double, and here we have a double and a single. So therefore, we know this is going to be the shortest bond, and this is going to be the longest bond, because it's around longer bonds. Now, on a test, if you get a question like this...

you need to look at the hybridization of the carbons that form that bond. So what are the hybridizations of these two carbon atoms? Those two carbon atoms are sp-hybridized.

Now for these two, the one on the left is sp-hybridized, and the one on the right has an sp2 hybridization. And then for the last two, the CH2 carbon is sp3 hybridized, and the CH carbon is sp2 hybridized. So the one that has more S character is going to be the shorter bond. And sp has more S character than sp2 or sp3. So we could say that the C-C bond in the first example is the shortest because it has more S character.

due to the fact that it's around shorter bonds. The last one is the longest because it has more p-character. sp3 has more p-character than sp2 or sp.

But if you see a bond surrounded by longer bonds, then this bond is going to be slightly longer than the other ones.

Transcript for:Understanding Bond Length and Strength

Transcript for:
Understanding Bond Length and Strength