Intermolecular Forces 2
Chemistry: AQA A Level, topic 1.3.9
Permanent Dipole-Dipole Interactions
Permanent dipole-dipole interactions exist between two permanently polar molecules.
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Permanent dipoles
- Permanent dipoles have a δ- on one side and a δ+ on the other.
- The δ+ of one molecule will attract the δ- of another molecule.
- This means that the molecules will line up throughout a liquid or solid in this fashion.
- Permanent dipole-dipole forces are the second strongest molecular interaction.
Polar molecules
- Permanent dipole-dipole interactions are only possible in molecules that contain a permanent dipole.
- So they are only found between polar molecules.
Hydrogen Bonds
Hydrogen bonds only exist between some specific types of molecules. These intermolecular forces are stronger than induced and permanent dipole-dipole interactions.
Hydrogen bonds
- A hydrogen bond is the name given to a very strong permanent dipole-permanent dipole force.
- With some elements, hydrogen can form extremely strong dipole-dipole interactions.
- When these are strong enough to be considered a bond, it is called a hydrogen bond.
Where are hydrogen bonds found?
- Hydrogen bonds are found when you have a hydrogen atom bonded to either oxygen, nitrogen, or fluorine.
- That hydrogen atom will form a strong permanent dipole-dipole interaction with another oxygen, nitrogen, or fluorine atom.
- Examples of liquids with hydrogen bonds are:
- Water.
- Hydrogen fluoride.
- Ammonia.
Why do hydrogen bonds form?
- When hydrogen is bonded to an extremely electronegative element, it develops a strong δ+ charge.
- Hydrogen is a very small atom, so it has a high charge density in this situation.
- This allows it to form a strong bond with any highly δ- charged atom.
Test yourself on Intermolecular Forces 2
A permanent dipole is labelled by which of the following?
A hydrogen bond is a very strong type of what?
Which of the following could form hydrogen bonds?