Are dipole-dipole forces or london dispersion forces stronger?
Dipole-dipole forces. (But it kind of depends on the compound.)
Both dipole-dipole forces and London dispersion forces are intermolecular forces, which means that they're both forces between different molecules.
Dipole-dipole forces occur when the molecules are polar, and the positive side of one molecule is slightly attracted to the negative side of another.
Because they need dipoles to exist, they're only present in polar molecules.
London dispersion forces occur when one molecule, which is typically non-polar at the beginning, has a "temporary dipole." That's when electrons just so happen to be more on one side of the molecule. This creates an "induced dipole" in another molecule.
London dispersion forces are the attractions between the temporary dipole and the induced dipole.
They don't require dipoles, so London dispersion forces can be present in both polar and non-polar molecules.
Because London dispersion forces are temporary, they're weaker than the permanent dipole-dipole attractions.
However, London dispersion forces can be quite strong sometimes!
In large molecules which also have a large number of electrons (which means that the electron cloud can be distorted to a greater degree), London dispersion forces can be so strong that the molecules are liquids or solids at room temperature!
For example, n-pentane is a non-polar molecule that is a liquid at room temperature.
Its non-polarity tells us that the only intermolecular forces it has are London dispersion forces.
This is the structure of n-pentane:
The molecule is very large, which means that not only are there more London dispersion forces present, but also stronger London dispersion forces.
This allows n-pentane to be liquid at room temperature.