Bond Length vs Size of Molecule

[jessicaosuna_1F]

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?

[Breanna Ouyang 1I]

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.

[Liam Bertrand 3]

Larger atoms do mean weaker bonds. However, there are other factors that influence a boiling point, so those are probably the reason why the boiling points are what they are.

[Hazelle Gunawan 3G]

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.

[Ashley Kim 3H]

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.

[Andrew Yoon 3L]

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.

[Alessia Renna 1D]

longer bonds are weaker but type of bonding (like london dispersion forces) can influence boiling point as well!

[Janna Shakiba]

A good trick for figuring out which molecule has the higher boiling point is to look for which one has an OH- group (or the most OH- groups).

[Ryan Agcaoili 2E]

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.

[305572629]

bond length generally increases as atomic radius increases (down the periodic table and towards the left)

[Tatyana Bonnet 2H]

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.

[Simi Kapila_3E]

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.

[Sean Phen]

Longer bonds do have a weaker bond, but you also have to consider other factors to determine which boiling point is higher.

[reyvalui_3g]

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.

[Kiara Phillips 3L]

You are right larger atoms do have weaker bonds, it's just that boiling also weakens bonds. Sometimes the two relate to each other sometimes they do not it depends from situation to situation.