M2, S2 Polarity and intermolecular forces Part 2 Flashcards
Wat do you need to remember about intermolecular forces?
They’re much weaker than covalent, ionic or metallic bonds
What are the different types of intermolecular forces?
What is the strongest type?
What are the forst 2 collectively known as?
- Induced dipole-diploe interactions or lodon (dispersion) forces
- Permanent dipole-dipole interactions
- Hydrogen bonding (this is the stongest type)
First 2 collectively known a s ‘van de Waals forces’
Where can induced dipole-diploe forces be found?
Between all atoms and molecules.
Induced dipole-dipole forces cause all atoms and molecules to be attracted to each other. Even noble gas atoms are affected despite not being at all inclined to form any other type of bond.
Why are induced dipole-dipole forces found between all atoms and molecules?
- Electrons in charge clouds are always moving really quickly.
At any particular moment, electrons in an atom are more likely to be to one side than another.
At this moment, an atom would have a temporary dipole.
- This dipole can cause another temporary (induced) dipole in the opposite direction on a neighbouring atom. The two dipoles are then attracted to each other.
- The second dipole can cause yet another dipole in a third atom.
- Because the electrons are constantly moving the dipoles are being created and destroyed all the time. Even though the dipoles keep changing, the overall effect is for the atoms to be attracted to each other.
Why don’t all induced dipole-dipole forces have the same strength?
Because larger molecules have larger electron clouds meaning stronger induced dipole-dipole forces.
Molecules with with greater surface areas also have stonger induced diploe-diploe forces because they have a bigger exposed electron cloud.
What effect do induced dipole-dipole forces have on the boiling point of a liquid compound?
Liquids with stonger induced dipole-dipole forces have will have higher boiling points.
This is because when you boil a liquid you need to overcome the intermolecular forces, so that the particles can escape from the liquid surface.
The stronger the intermolecular forces, the more energy you need, so you have a higher boiling point.
Explain the trend in the graph.
As you go down the group, the induced dipole-dipole forces (and the boiling points) increase because the number of shells of electrons increases, and so the atomic/molecular increases.
What physical properties do induced diplole-dipole forces affect?
boiling point/melting point
viscosity
How can induced dipole-dipole forces influence the structure of a molecule?
Induced dipole dipole foces can hold molecules together in a lattice. e.g. iodine
- Iodine atoms are held together by strong covalent bonds to form iodine molecules.
- But the molecules are then held together in a molecular lattice arrangenment by weak induced dipole-dipole attractions.
What are permanent dipole-dipole interactions?
- give an example and a diagram
Whe do permant dipole-dipole interactions happen?
Permanent dipole-dipole ineteractions happen in addition to (not instaed of) induced dipole-dipole interactions.
When can hydrogen bonding occur/
Hydrogen bonding can only coccur when hydrogen is covalently bonded to flourine, nitrigen or oxygen.
Why does hydrogen have a high charge denity?
Hydrogen has a high charge density because it’s so small and flourine, nitrogen and oxygen are very electronegative.
The bond is so polarised that a weak bond forms between the hydrogen of one molecule and and a lone pair of electrons on the flourine, nitrogen or oxygen in another molecule.
Wghat do molecules that have hydrogen bonding usually contain?
What effect does hydrogen bonding have on the properties of substances?
