Sapling W 5/6 #17

[arisawaters2D]

Could someone explain this question. I know Br2 must have only London forces, but with the molecules that don't have the same elements, how do I tell when it involves only a London/dispersion force??

[TrishaP_3E]

For me, it helps to draw out the Lewis structure. It's tricky because we haven't really gotten into VSPER yet, but generally, molecules with identical bonds around the central atom, and no lone pair around the central atom will be nonpolar and exhibit only LDF.

In this case, C2H6 is also an answer because C and H have such close electronegativities, that a C-H bond can be considered as not having a Dipole.

Hope this helps!

[Isabella Chou 1A]

To add on, BCl3 also only has dispersion forces. For BCl3, Br2, and C2H6, the dipoles cancel out in each molecule, so there is no net dipole moment in each molecule, which tells us that there are no dipole-dipole interactions, only dispersion forces.

CH3Cl has dipole-dipole interactions because the dipole moment of a CH3Cl molecule would point towards Cl (δ-) since Cl is more electronegative than C. NF3 is seemingly a symmetrical molecule; however, when we draw the Lewis structure, there is a lone pair on the N atom. The lone pair on N and the bonding pairs on each F atom will repel each other, but there is an unequal charge distribution in the molecule. This results in a dipole moment, so NF3 will have dipole-dipole interactions.

[Alan Huang 1E]

In general, all molecules will have London forces. The tricky part is figuring out which ones have dipoles.

Dipole-dipole forces occur when electronegativities between the elements tend to be between 0.4 and 1.9, and also when such bonds aren't cancelled out. It's often beneficial to compare electronegativities of various elements.

[arisawaters2D]

Thanks all! This helps a lot.

[Michelle Magana 2B]

Im stuck on this problem too and I'm not really understanding the concept. Would it be better to draw the lewis structure or label which of the atoms is more electronegative

[Chanel Mao 3D]

Im stuck on this problem too and I'm not really understanding the concept. Would it be better to draw the lewis structure or label which of the atoms is more electronegative

Hi! It would be better to draw the lewis structure and it would help if you knew the molecular geometry of each molecule. Since London Dispersion Forces only exist between non-polar atoms or non-polar molecules, we need to figure out which molecules are non-polar. Even though there is a difference in electronegativity between B and Cl (and therefore the bond between B and Cl are non-polar), BCl3 as an entire molecule has a trigonal planar geometric shape, and so therefore the molecule as a whole is non-polar. Hope this helped!

[David Liu 1E]

I think it's really helpful to draw out the structure because LD forces can only be used with nonpolar molecules, so drawing out the structures generally gives you an idea of how they should work, besides a few exceptions

[Jack Kettering 3D]

The answers are BCl3, Br2, and C2H6. This is due to the fact that the dipoles cancel out in each molecule, so there is no net dipole moment in each, which means that there are no dipole-dipole interactions, only dispersion forces.

[rhettfarmer-3H]

I think a big part of the question has to do with polarity. Therefore, it has to be shown that if you its polar it can make the dipole connections. SO there no polar molecules that are likely to have only London. And London happens in all molecules so the answers are BCL Br2 and c2h6

[905579227]

You basically just have to know which ones have Dipole-dipole becuase all of them have london forces