How does London dispersion force differ from dipole-dipole interactions?

London dispersion forces are temporary, weak intermolecular forces, while dipole-dipole interactions are stronger and permanent.

London dispersion forces and dipole-dipole interactions are both types of intermolecular forces, which are the forces of attraction between molecules. However, they differ in their strength, permanence, and the conditions under which they occur.

London dispersion forces, also known as Van der Waals forces, are the weakest type of intermolecular force. They are temporary and occur between all molecules, whether polar or nonpolar. These forces arise due to the temporary fluctuations in the electron cloud of a molecule, which can induce a temporary dipole in a neighbouring molecule. This results in a weak, temporary attraction between the two molecules. The strength of London dispersion forces increases with the size and shape of the molecules involved; larger, more complex molecules have a greater number of electrons and a larger electron cloud, leading to stronger dispersion forces.

On the other hand, dipole-dipole interactions are stronger and more permanent than London dispersion forces. They occur between polar molecules, which have a permanent dipole due to a difference in electronegativity between the atoms in the molecule. This results in one end of the molecule having a slight positive charge and the other end having a slight negative charge. The positive end of one molecule is attracted to the negative end of another molecule, resulting in a dipole-dipole interaction. The strength of dipole-dipole interactions depends on the polarity of the molecules; the greater the difference in electronegativity, the stronger the interaction.

In summary, while both London dispersion forces and dipole-dipole interactions are important in determining the properties of substances, they differ significantly in their strength and the conditions under which they occur. Understanding these differences is crucial in predicting and explaining the behaviour of different substances in various conditions.