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I thought that longer bonds were weaker, but in Lavelle's Lecture #18 at time 23:30ish, he is explaining that CCl4 has a higher boiling temp that CH4, because the CCl4 molecule is bigger. I understand the logic for both of these statements, but also I feel they are somewhat contradictory. Can somebody explain better?
When CCl4 has a higher boiling point because of its size is Lavelle referring to its London dispersion forces? When you increase size (or molar mass) polarizability increases, and so the attractive LDFs increase in strength and thereby make the boiling point higher. Questions about boiling and melting points have to do with intermolecular forces and not intramolecular forces, and bond lengths refer to intramolecular forces.
You're correct that longer bonds are weaker! However, I think the boiling point is mostly in reference to intermolecular forces and not the bonds within individual molecules themselves. Since boiling isn't about breaking bonds, but instead exciting molecules to the point where they change phases, larger molecules will exert greater forces on each other and will be harder to excite.
Both CH4 and CCl4 are single bonds. But the reason why CCl4 has a higher boiling point is because it has more electron shells. This makes the polarizability increase, thus increasing the strength of LDFs, which then increases the boiling point.
Although bond length definitely affects the strength of the bond, we also have to consider the intermolecular forces. Although both compounds only have LDF's. CCl4 has a higher boiling point than CH4 because it has stronger LDF interacting. This is due to it having more electron shells, which increases its polarizability, which then increases the energy to break the LDF's.
When dealing with the boiling point, the bigger the size of the molecule, the more electron distortion that it can experience, thus resulting in more dipole moments. It is important to note the difference between IMFs and covalent bonds.
The atomic size would effect bond strength in covalent bonds as the bigger the atom the larger the distance between the two the weaker the bond. This is talking about the bond between individual atoms. The boiling point is referring to intermolecular forces between molecules, so CH4 interacting with other CH4s. In this case the London dispersion forces are playing the role of determining boilings/melting points. The stronger the force the higher the boiling point. London dispersion forces are strongest when the molecule is bigger because it is more polarizable and easily distorted. Hydrogen bonds are stronger than LDFs so they will have a higher boiling point than a molecule that only has LDFs.
In this example, the larger molecules have stronger london dispersion forces, making it more difficult to dissociate than the smaller molecule, the one with weaker london dispersion forces, thus the larger molecule has a higher boiling temperature.
Both molecules are nonpolar, so the attractive force that determines the boiling point of the molecules is the London Dispersion Force. Since CCL4 is heavier it has greater dispersion forces, raising its boiling point.
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