1. 2.5.2 Van der Waals’ Forces

2. Intra and Intermolecular Forces
Intramolecular forces are the forces within a molecule
They are essentially
Ionic Bonds and Covalent Bonds.
They are powerful forces

3. Intermolecular Forces
Forces of attraction between molecules
They are generally weak
Three types
Dipole – Dipole [permanent]
Hydrogen Bonding [special type of permanent dipole – dipole]
Van der Waals’ Forces [transient or temporary dipole]

4. What is dipole?
Charge caused by the separation of the centres of + and - charge within an atom or molecule
Dipole-Dipole [Polar Bonding] caused by difference in electronegativity. Dipole permanent e.g. δ+ C=Oδ-
Hydrogen Bonding special dipole-dipole where H bonded to N, O or F. e.g. δ- O-H δ+ in water [1/10th]
Van der Waals’ Forces occurs in atoms or molecules where the difference in electronegativity is 0 and there should be no polarity. They are caused by temporary dipoles produced by random movement of electrons . E.g. H-H or He [about 1/1000 of covalent]

5. Why is NH3 soluble in water but PH3 is not?
NH3 is polar, PH3 is not. NH3 forms hydrogen bonds with the water, PH3 does not.
Metallic Bonds a lattice of +vely charged nuclei in a sea of electrons

6. Let us look at a Helium atom+
1. Centre of + and – charges are in the same place [centre] so there is an even distribution of charge

7. 2. Random movement of electrons results in two electrons being on one side of the atom. This side is now slightly negative δ- and the other side is therefore δ + + δ+
δ -

8. 3. Random movement of electrons results in two electrons being on another side of the atom. This side is now slightly negative δ- and the other side is therefore δ + + δ+
δ - + +
4. It is unlikely that the two electrons will be exactly opposite each other so the atom spends most of it time with one part slightly negative δ-and the other side slightly positive δ +
This is called a Temporary Dipole

9. Let us now look at a Neon atom
The bigger the atom the more electrons it has so there is more scope for a bigger charge differential between the two sides.
This is due to the possibility of more electrons being at one side.

10. +
1. Centre of + and – charges are in the same place [centre] so there is an even distribution of charge

11. +
δ +
δ -
2. Random movement of electrons results in more electrons being on one side of the atom. This side is now more slightly negative δ- and the other side is more δ +
This is a bigger Temporary Dipole

12. What is the effect of these Temporary Dipoles?
These atoms haves NO temporary dipoles at the moment.
They have a limited attraction between them

13. What is the effect of these Temporary Dipoles?
1. This atom has developed a temporary dipole
This atom still has NO temporary dipole
δ -
δ +

14. What is the effect of these Temporary Dipoles?
This atom now NOW an induced temporary dipole
δ -
δ +
δ -
δ +
The attractive force between the two atoms is now greater due to the two dipoles

15. What is the effect of these Temporary Dipoles?
Since the force of attraction is greater it is more difficult to remove one of the atoms from the other.
The heat needed to give one enough energy to evaporate is greater
Thus the boiling point is greater than if there were no dipoles
Thus neon has a higher boiling point than helium.
Melting points are also greater for the same reason

16. What is the effect of these Temporary Dipoles?
neon has a higher boiling point than helium because the atoms are bigger and therefore can produce greater temporary dipoles.
The boiling point of O2 is higher than H2 for the same reason i.e. because the atoms are bigger and therefore can produce greater temporary dipoles.
Molecules with dipoles tend to be soluble in water – like dissolves like

17. Everday experience of van der Walls’ Forces

18. Graphite is a form of carbon [allotrope] Graphite is pencil lead
Made up of sheets of carbon atoms in hexagons
Carbons are joined to esch other by covalent bonds
Sheets are held to each other by van der Walls’ forces.
Gif made by Mike Thompson of Truro School

19. When you put the pencil on the paper the force of attraction between the paper and the graphite is greater than the attraction between the grapite sheets.
Thus when you move the pencil the bottom graphite sheet sticks to the paper
The pencils slides along and drops to the paper to repeat the process.
Pencils can write underwater
The american spent millions developing a pen that could write in space.
The russians were smart !!!!!!!!!!!!!!!
They brought pencils

20. Paper / graphite attraction
Pencil moves
Van der Waals’ Forces
Since the paper/grapite attraction is greater than the graphite/graphite attraction the bottom layer stays put as the upper layers move to the side and there is now a black mark on the paper
Paper / graphite attraction