LDF vs dipole-dipole

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Kelvin Chung 1C
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Joined: Sat Aug 24, 2019 12:15 am

LDF vs dipole-dipole

Postby Kelvin Chung 1C » Fri Nov 22, 2019 9:53 am

Can LDF sometimes have more effect on the boiling point of a compound than its dipole-dipole forces?
For example, when the structures of two different compounds are identical except for its central atom, are there times when the difference in the molar masses of the central atoms (and hence the difference in LDF strength) affects boiling point more than the difference in the atoms' electronegativity (and hence the difference in dipole-dipole strength)?

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Joined: Sat Aug 24, 2019 12:17 am

Re: LDF vs dipole-dipole

Postby JasonLiu_2J » Fri Nov 22, 2019 10:04 am

Yes, the strength of the LDF forces can have a greater influence on the relative boiling points of two molecules than the difference in dipole dipole forces. Consider the example of HCl vs HI. Since Cl is more electronegative than I, the HCl molecule would be more polar and thus, have a stronger dipole dipole force. However, HI is actually the one with the higher boiling/melting point, as it has much strong LDF interactions due to the size of the iodine atom. The HI molecule is more polarizable, which gives it a higher boiling/melting point. Thus, the relative strengths of the LDF interactions are often more important than the relative dipole-dipole interaction strengths when it comes to finding boiling/melting points. Note that this only occurs if both molecules in question have both LDF and dipole-dipole interactions. If one only has LDF while the other has LDF and dipole-dipole, then the dipole-dipole interacting molecule would have a higher boiling/melting point

Justin Quan 4I
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Joined: Sat Sep 14, 2019 12:17 am

Re: LDF vs dipole-dipole

Postby Justin Quan 4I » Fri Nov 22, 2019 12:13 pm

To add on, if two molecules have similar or identical dipole-dipole forces, then LDF will have a significant difference on relative boiling points of two molecules. If, for example, the only difference between two molecules is their central atom, just the difference in size between the central atom will affect how much LDF there is, thus the boiling points will be different.

Kylie Lim 4G
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Joined: Sat Aug 17, 2019 12:15 am

Re: LDF vs dipole-dipole

Postby Kylie Lim 4G » Fri Nov 22, 2019 3:53 pm

Unless the dipole-dipole IMF is hydrogen bonding, LDF can have a significant effect on the boiling/melting point of a molecule.

Minh Ngo 4G
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Joined: Thu Jul 25, 2019 12:17 am

Re: LDF vs dipole-dipole

Postby Minh Ngo 4G » Fri Nov 22, 2019 4:26 pm

Yes since different atoms have different strengths or forces

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