Intermolecular Forces

Intermolecular Forces

the attractions that occur between molecules

Dipole Dipole

Ion-Dipole

An intermolecular force that occurs between charged ions and polar molecuels with a dipole

An intermoleculr force that occurs between polar molecules

Hydrogen Bonding

London Dispersion Forces

An intermolecular force that is present in all molecules resulting from unequal distribution of electrons

A specialized type of dipole-diple interaction that involves molecules with O-H, N-H, or F-H bonds.

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In the animation, notice that the partially negative oxygen of water is attracted to the positive ion Na+. The partially negative hydrogens on the water are attracted to the negative ion, Cl-. Points to Remember:

• Ion-dipole IMF are the strongest IMF
• Cations are attracted to the partially positive part of a dipole
• Anions are attracted to the partially negative dipole
• Ion-dipole interactions are responsible for ionic compounds dissolving in water

Ion-Dipole Attractions

IIon-Dipoleattractions are the strongest type of IMF that involve attractions that occur between an ion and the oppositely charged dipole of a polar molecule.

London Dispersion Forces (LDF)

In the animation, notice that as the electrons move, there comes a point where they become concentrated in one side of the molecule, causing a shortlived positive and negative portion in the molecule. This happens very quickly and goes away quickly as the electrons continue to move. Larger molecules have more opportunities for the electrons to concentrate in an area and so larger molecules have larger LDF forces. Points to Remember:

• LDF are weak
• LDF occur in all molecules
• LDF only matter in non-polar molecules that don't have other IMF
• LDF strength increases with size of the molecule
• Responsible for higher boiling points in larger molecules

Source: http://www.dynamicscience.com.au/tester/solutions1/chemistry/bonding/bonding8a.htm

London dispersion forces are the weakest IMF. They occur when an instantaneous dipole is formed from electrons moving around a molcule and that induces a short-lived attraction with another molecule. This can happen in all molecules, but they aren't considered important unless there are no other IMF present, like in nonpolar molecules.

Dipole-Dipole Interactions

Dipole-dipole attractions occur between polar molecules. In this case, the partially positive portion of a molecule is attracted to the partially negative portion of another molecule. The image shows a dipole-dipole interaction between the partially negative chlorine in HCl and the partially positive hydrogen of another HCl molecule.

Points to Remember:

• Dipole-dipole occur between 2 polar molecules
• Dipole-dipole are stronger than LDF but weaker than hydrogen bonds and ion-dipole
• Dipole-dipole interactions allow polar molecules to dissolve in water
• Dipole-dipole interactions account for higher boiling points in polar compounds

Hydrogen Bonding

Hydrogen bonding is a specialized type of dipole-dipole attraction that occurs between molecules that have F-H, O-H, or N-H bonds. The large electronegativity difference in these bonds results in a stronger attraction than typical dipole-dipole interactions. In the animation you see the partially positive hydrogen from one water molecule forming an attractive interaction with the partially negative oxygen of another water molecule. Hydrogen bonding is the key to a small molecule like water having such a high boiling point compared to other molecules.

Points to Remember:

• occurs in molecules with F-H, O-H, and N-H bonds only
• stronger than typical dipole-dipole attractions but weaker than ion-dipole
• accounts for high boiling point in water and other hydrogen bonding molecules