CH2 - Intermolecular Forces

What is the difference between intermolecular and intramolecular forces?

Intramolecular:

• Forces or bonds that act within the molecules

  • E.g. Between H-Cl, there is a covalent bond

Intermolecular:

• Forces or bonds that act between separate molecules

  • E.g. Between H-Cl - - - - - H-Cl, there is a dipole-dipole bond

What are the different types of intermolecular bonds?

• Dispersion forces

• Dipole-dipole bonds

• Hydrogen bonds

What are dispersion forces?

Dispersion forces:

• Forces that act between dipoles of separate molecules. As electrons flow around an atom or molecule, instantaneous dipoles will form, making a partially positive and negative side. The partially positive side of one will form a dispersion force with the negative side of the other.

• Weakest of the intermolecular forces

• Molecules which have higher masses (more electrons) will have higher dispersion forces as there is more potential for greater and more repulsive/attractive sides of dipoles

• Molecules with longer chains will have greater dispersion forces, as longer molecules have more surface area for contact between molecules

What are dipole-dipole bonds?

Dipole-dipole bonds:

• Dipole-dipole bonds are formed between polar molecules

• Polar molecules have permanent dipoles, as their structure is created from one atom having a higher electronegativity than the other (higher electron densities), meaning one side will always be negative and the other always positive

• Because of this permanent dipole, similarly to that of a dispersion force, the positive end of one molecule will be attracted to the negative of another, creating a bond

• Dipole-dipole bonds are stronger than dispersion forces as the molecules are permanently dipoles, therefore there is no chance of the electrons moving away from the bonds

What is a hydrogen bond?

Hydrogen bond:

• A hydrogen bond is a bond that can only occur between Hydrogen and Oxygen, Fluorine, or Nitrogen

• Hydrogen bonds happen with these atoms as hydrogen has a very low electronegativity, while O, F, and N have very high electronegativities

• Being small, the hydrogen is able to get very close to the nearby molecules, making the bonds even stronger

• There must be a lone pair on the O, F, or N of neighbouring molecules in order for the hydrogen to bond to it

• Hydrogen bonds are about 10x stronger than dipole-dipole bonds

• Hydrogen bonds have a very high melting and boiling point as they are the strongest of the intermolecular bonds