chemistry 2.6 - intermolecular forces

What are intermolecular forces?

Forces between molecules that are very weak

How are London forces / induced dipole bonds formed?

In charge clouds the electrons are always moving quickly and at some point there will be more electrons on one side than the other meaning the atom has instantaneous / temporary dipole. This then induces a dipole in neighbouring atoms as electrons repel so the partially positive end of one cell is next to a partially negative end on another meaning the attract. The dipoles are constantly created and destroyed but the overall effect is that the atoms are attracted to each other

Which molecules are London forces present in.

All

What forces are in a simple molecular structure (e.g. iodine)?

Atoms held together by covalent bonds to form I2 but these molecules are joined by London forces = low melting points

How does side of molecules effect the strength of London forces?

Large molecules = larger electron clouds = stronger London forces

How does surface effect the strength of London forces?

Bigger surface area = more exposed electron clouds = stronger London forces

How are alkanes joined and what effects the strength of the bonds?

Covalent bonds between atoms and London forces between molecules and the longer the carbon chain the more molecular surface contact = more electrons can interact = stronger London forces. Less branched = more closely packed = molecular surface contact bigger = stronger London forces

What are permanent dipole - permanent dipole bonds?

The partially positive and partially negative charges on polar molecules are attracted to the oppositely charged ends of other polar molecules

Why do permanent dipole - permanent dipole bonded molecules have higher melting points?

Because they have London forces and permanent dipole - permanent dipole bonds