London dispersion forces (size vs molecular weight)

[plusalpha]

When there are two compounds that only have London dispersion forces, does the one with the greater SIZE have stronger London forces or does the one with a larger MOLECULAR WEIGHT have stronger London forces?

Also just to clarify, molecules that have individual polar bonds, but are non-polar as an entire molecule WON'T have dipole-dipole interactions (only London forces), correct?

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[omarski]

Could you give an example? I know that increased size = more london dispersion forces = higher boiling pt
I'm going to go out on a limb here and say that I think size wins due to more surface area enabling more london dispersion forces. Don't quote me on that though, I might be wrong. But in most scenarios, larger molecules with more surface area that can stack always have the higher boiling points as a result of more london dispersion forces. A good way to see this is to compare the boiling points of the noble gases, which demonstrates more electrons = more temporary dipoles = more london dispersion forces = higher boiling point.

As for your second question, the link I provided answers it really nicely towards the bottom. Individual polar bonds don't make a bond polar; rather, when the polar bonds are asymmetrical and don't cancel out, the molecule is polar. So Something like CCl4 has 4 dipoles going from the Carbon to each Chlorine, but the overall molecule is considered nonpolar due to the cancellation of dipoles. And you are correct, as a result, they will only have dipole-dipole interactions on the intermolecular level.
So intramolecular level = may have dipole-dipole interactions, but just because they do doesn't mean it'll translate into the intermolecular level if dipoles cancel out. A good comparison to CCl4 is HCCl3, where the dipoles don't cancel and as a result the molecule is polar.

Note: HCCl3 has dipole-dipole interactions on the intermolecular level; however, CCl4 is larger, making its london dispersion forces 'keep up' with the the dipole-dipole interactions of HCCl3 which can be seen when comparing their boiling points.
CHCl3

61.2°C
CCl4

76.8°C

This resource is really good at explaining both of your questions : http://www.chemguide.co.uk/atoms/bonding/vdw.html

 

[DentalLonghorn2014]

Highest BP, MP, Viscosity, and surface tension = Highest Intermolecular forces
Remember, LD forces is one of the Intermolecular forces.
The bigger the size of the molecule, the more IM forces it has.

All molecules have LD forces but NON-POLAR molecules will strictly have LD forces.

Can anyone verify this por favor? I may have had too much eggnog and I dont remember...lol

 

[omarski]

Highest BP, MP, Viscosity, and surface tension = Highest Intermolecular forces
Remember, LD forces is one of the Intermolecular forces.
The bigger the size of the molecule, the more IM forces it has.

All molecules have LD forces but NON-POLAR molecules will strictly have LD forces.

Can anyone verify this por favor? I may have had too much eggnog and I dont remember...lol

Yes and remember as those three variables increase (BP, MP, Viscosity), vapor pressure decreases (volatility decreases).
Also remember if a question is asking about these properties in a solution, they are known colligative properties that are a result of manipulating solute to solvent ratios.

And yes, all molecules have LD forces! Non-polar molecules will strictly have LD forces on the intermolecular level. Just in case, as I wrote above, that doesn't mean there isn't any polarity cancelling out in the intramolecular level. If that's too in depth and I'm confusing you, ignore it haha. But you're right with what you said!

 

[DentalLonghorn2014]

Yes and remember as those three variables increase (BP, MP, Viscosity), vapor pressure decreases (volatility decreases).
Also remember if a question is asking about these properties in a solution, they are known colligative properties that are a result of manipulating solute to solvent ratios.

And yes, all molecules have LD forces! Non-polar molecules will strictly have LD forces on the intermolecular level. Just in case, as I wrote above, that doesn't mean there isn't any polarity cancelling out in the intramolecular level. If that's too in depth and I'm confusing you, ignore it haha. But you're right with what you said!

Appreciate it buddy!