1.9 Intermolecular forces

What are intermolecular forces

Intermolecular forces are forces that exist between different compounds

What are the three main types of intermolecular forces (IMFs)

Dipole-dipole

Dispersion (london) forces

Hydrogen bonding

What is the order of strength between them and why is it important

1. Hydrogen bonding

2. dipole-dipole

3. dispersion forces

The greater the IMF, the more energy is required to break the bonds between them raising the melting/boiling point

• note: Ionic bonds are much stronger than all of these

what is dipole-dipole

Dipole-dipole is the IMF between a partially positive atom and a partially negative atom in a different compound.

• This occurs in polar-covalent compounds

What are london forces

Because electrons are constantly moving, partial positive and negative dipoles can be instantaneously and briefly created causing a short bond to occur between the partially + atom of one molecule and the partially - atom of another. These bonds exist momentarily yet still impact the substance it takes place in.

These always exist for every type of covalent bond yet are most essential for non-polar covalent molecules

What is hydrogen bonding

When hydrogen binds to a highly electronegative atom (either O, N, or F), its electron density is drawn away causing a partial + charge on the hydrogen.

Hydrogen bonding is the electrostatic attraction between this H+ atom and the lone pairs of an electronegative atom of a different molecule

How does increasing the # of carbons in a parent chain of a substance affect its intermolecular force.

When you increase the # of carbons in a parent chain of a substance, you increase the intermolecular forces within it.

This can mean that for the same temperature, a substance can exist in different phases of matter simply by increasing or decreasing the number of carbons present within its parent chain.

How does the molecular arrangement of the same molecule affect the intermolecular force

Linear structures create a greater IMF as it allows for more bonding between molecules due to increased surface area, while branched structures limit bonding by reducing surface area and creating more distance between molecules