Bonding and Potential Energy

[Tyler Angtuaco 1G]

Can someone explain the trends between IMF forces in bonding and potential energy?

[Ariel Davydov 1C]

Sure! Listed from strongest to weakest, the intermolecular forces responsible for interaction between molecules are ion-ion, hydrogen bonding, ion-dipole, dipole-dipole, dipole-induced dipole, and London/dispersion forces. Overall, the major theme in strength of intermolecular forces is that the stronger the intermolecular forces within a molecule are, the more energy is required to break those bonds and separate like-molecules. All molecules have dispersion forces as well, since all molecules have electrons and thus can create temporary interactions from temporary dipoles.

Here is one example showing the difference in strength of intermolecular forces: it will take more energy to break the attraction between two ions (where the strongest intermolecular forces are ion-ion) than two nonpolar molecules (where the strongest intermolecular forces are dispersion forces). This tells us that the boiling point of the two ions will be higher than that of the two nonpolar molecules.

Similarly, stronger intermolecular forces will have lower potential energies, as they will be more stable with stronger attractions to one another. For example, the chlorine ion Cl- and H2O display primarily ion-dipole intermolecular forces and have a potential energy of -15 kJ/mol, while HCl and N2, which display dipole-dipole attractions, have a potential energy of -2 kJ/mol.

Knowing the magnitude of strengths of intermolecular forces can help you predict which molecules will have higher or lower boiling points and potential energies. General trend is that stronger intermolecular forces will have lower potential energy and higher boiling points. If you are asked to compare two different molecules or sets of molecules that display the same prominent intermolecular forces, use the strength of the dispersion forces to determine which molecule has stronger attractive forces. For dispersion forces, the more polarizable a molecule is (greater molar mass, more electron rich), the stronger the forces will be.

Hope this helps!