How to remember strength of different intermolecular forces

Moderators: Chem_Mod, Chem_Admin

Jocelyn Thorp 1A
Posts: 103
Joined: Wed Sep 18, 2019 12:20 am

How to remember strength of different intermolecular forces

Postby Jocelyn Thorp 1A » Thu Nov 14, 2019 2:29 pm

If I remember correctly, the types of intermolecular forces directly impact the strength of the bonds and thus influence properties such as melting points. Is there any way to remember which are stronger and which are weaker, other than just memorizing? If any part of my question is incorrect, please let me know.

Daria Azizad 1K
Posts: 116
Joined: Thu Jul 25, 2019 12:15 am

Re: How to remember strength of different intermolecular forces

Postby Daria Azizad 1K » Thu Nov 14, 2019 2:33 pm

You can make a visual model of the intermolecular attractions by drawing the Lewis structures. Based on how close the electrons or nucleus of one molecule/atom can get to another nucleus or electron pair, you can distinguish what will be stronger. For instance, H forms Hydrogen bonds with lone pairs on O, N, and F, so it's proximity is very close, since they are small atoms. Therefore, hydrogen bonding will always be stronger.

Bryce Ramirez 1J
Posts: 120
Joined: Sat Aug 24, 2019 12:16 am

Re: How to remember strength of different intermolecular forces

Postby Bryce Ramirez 1J » Thu Nov 14, 2019 2:58 pm

All molecules have LDF, but these are relatively weak compared to the rest of the intermolecular forces. But this is an easy one to remember because nearly all will have it. In terms of remembering the others, knowing why each bond is formed can give insight into how strong or weak it is.

JasonLiu_2J
Posts: 109
Joined: Sat Aug 24, 2019 12:17 am

Re: How to remember strength of different intermolecular forces

Postby JasonLiu_2J » Thu Nov 14, 2019 3:03 pm

Another way to remember the order of intermolecular force strength is to think about the strength of the charges on ions, dipoles, and induced dipoles respectively. Ions are charged atoms of elements, and thus, already have a full charge associated with them (+1, +3, -2 etc.) Thus, the interactions between these charged particles will be very strong. Dipoles on the other hand are due to partial charges. Dipoles arise in a molecule due to a different in electronegativity, and the charges on the elements in the molecules are not complete charges. These partial charges are less charged than the full charges associated with ions and so form IMFs that are weaker than those between ions. Finally, induced dipoles are simply moments of unequal charge distribution. They are fleeting moments in which different parts of the molecule have either a slightly positive or slightly negative charge. These are the weakest in terms of full charges and so form the weakest interactions. After you understand these three, you can make combinations that will give you the order of IMF strength. Ion-Ion is strongest since it has two of the full charge objects, whereas Ion-dipole is second strongest because it has one full charge and one partial charge "object" in the interaction. This extend down to dipole-dipole, dipole-induced dipole, and finally, induced dipole-induced dipole. Hydrogen bonding a specific case that you would just memorize as being an especially strong dipole-dipole interaction, and so would be between ion-dipole and dipole-dipole. Hope this helps!

Jocelyn Thorp 1A
Posts: 103
Joined: Wed Sep 18, 2019 12:20 am

Re: How to remember strength of different intermolecular forces

Postby Jocelyn Thorp 1A » Thu Nov 14, 2019 3:22 pm

JasonLiu_3E wrote:Another way to remember the order of intermolecular force strength is to think about the strength of the charges on ions, dipoles, and induced dipoles respectively. Ions are charged atoms of elements, and thus, already have a full charge associated with them (+1, +3, -2 etc.) Thus, the interactions between these charged particles will be very strong. Dipoles on the other hand are due to partial charges. Dipoles arise in a molecule due to a different in electronegativity, and the charges on the elements in the molecules are not complete charges. These partial charges are less charged than the full charges associated with ions and so form IMFs that are weaker than those between ions. Finally, induced dipoles are simply moments of unequal charge distribution. They are fleeting moments in which different parts of the molecule have either a slightly positive or slightly negative charge. These are the weakest in terms of full charges and so form the weakest interactions. After you understand these three, you can make combinations that will give you the order of IMF strength. Ion-Ion is strongest since it has two of the full charge objects, whereas Ion-dipole is second strongest because it has one full charge and one partial charge "object" in the interaction. This extend down to dipole-dipole, dipole-induced dipole, and finally, induced dipole-induced dipole. Hydrogen bonding a specific case that you would just memorize as being an especially strong dipole-dipole interaction, and so would be between ion-dipole and dipole-dipole. Hope this helps!


this is absolutely very helpful thank you so much!! I was struggling with the concept but I think now I understand!

Jocelyn Thorp 1A
Posts: 103
Joined: Wed Sep 18, 2019 12:20 am

Re: How to remember strength of different intermolecular forces

Postby Jocelyn Thorp 1A » Thu Nov 14, 2019 3:24 pm

Daria Azizad 1K wrote:You can make a visual model of the intermolecular attractions by drawing the Lewis structures. Based on how close the electrons or nucleus of one molecule/atom can get to another nucleus or electron pair, you can distinguish what will be stronger. For instance, H forms Hydrogen bonds with lone pairs on O, N, and F, so it's proximity is very close, since they are small atoms. Therefore, hydrogen bonding will always be stronger.


I hadn't realized that atom size correlated to the strength of the bond itself, though looking back it makes sense. Thank you!


Return to “Interionic and Intermolecular Forces (Ion-Ion, Ion-Dipole, Dipole-Dipole, Dipole-Induced Dipole, Dispersion/Induced Dipole-Induced Dipole/London Forces, Hydrogen Bonding)”

Who is online

Users browsing this forum: No registered users and 1 guest