Intermolecular Forces
Chemistry: AQA A Level, topic 1.3.8
Types of Intermolecular Forces
Intermolecular forces operate between all types of molecules and are much weaker than intramolecular forces. We shall look at three types of intermolecular forces:
Induced dipole-dipole interactions
- These are the weakest intermolecular interactions we shall consider.
Permanent dipole-dipole interactions
- These are stronger than induced dipole-dipole interactions but weaker than hydrogen bonds.
Hydrogen bonds
- These are the strongest form of intermolecular forces we shall consider.
- They are still much weaker than intramolecular forces (e.g. covalent bonds).
Induced Dipole-Dipole Interactions
Induced dipole-dipole interactions are also known as van der Waals or London forces. They are formed from temporary dipoles.
Temporary dipoles
- The electron clouds around molecules are constantly in motion.
- One moment, all the electron density can be on one side of the molecule. The next, it can be somewhere totally different.
- This is called a temporary dipole - there are partial charges, but they change very rapidly.
Induced dipoles
- If one molecule has a temporary dipole, its partial charges will exert a force on nearby molecules.
- The partial charge of one molecule can push away the electrons in another, or attract them towards it.
- This means that temporary dipoles will induce dipoles in nearby molecules.
- Once a dipole has been induced, it will be attracted to the initial dipole.
- This is called an induced dipole interaction, or a van der Waals force.
Strength
- Van der Waals forces are not all the same strength.
- The strength depends on the number of electrons in a molecule.
- Molecules with lots of electrons will have much stronger van der Waals forces.
- This is because they will have larger fluctuations in electron density.
- This leads to larger temporary dipoles and stronger dipole-dipole interactions.
Test yourself on Intermolecular Forces
Intermolecular Forces
Which of the following is NOT a type of intermolecular force?
Which of the following forces and bonds is strongest?
What are 'van der Waals forces' also known as?
What is the strength of van der Waals interactions dependant on?