Intermolecular forces

Question 1

What molecules are Van der Waal forces present in?

Answer 1

• These forces are present in ALL molecules.

Question 2

How do Van der Waals’ forces occur between chlorine molecules (as an example?)

4 key points.

Answer 2

• Electrons constantly moving around in molecule (ie. Cl- Cl.) This will lead to uneven distribution of electrons.
• This causes temporary dipole within the molecule.
• Temporary dipole induced a temporary dipole in neighbouring molecule (ie. negative dipole causes electrons to repel, move to other side of molecule and induces positive dipole.)
• Attraction between these temporary dipoles = Van der Waals’

Question 3

How do the size of Van der Waals’ force change with the Mr of a molecule/ atom?

Answer 3

• The bigger the Mr, the bigger the Van der Waals’ force.

Question 4

Why are Van der Waals’ sometimes referred to as temporary induced dipole- dipole attraction?

Answer 4

• Temporary: because electrons are constantly moving so, dipole that is negative may become positive after moving of electrons (position of the Van der Waals’ forces will change.)

Question 5

What are the 3 different names for Van der Waals’ forces?

Answer 5

• Induced dipole- dipole forces/ attractions.
• Dispersion forces
• London forces

Question 6

What are permanent dipole- dipole forces? Give the example of HCl?

Answer 6

• These are forces between poles/ slight charges on polar molecules.
• Ie. for HCl: force between slightly positive hydrogen/ slightly negative chlorine.

Question 7

True or False

There are permanent dipole- dipole forces on ALL molecules that contain polar bonds.

Answer 7

• False.
• There are only permanent dipole- dipole forces between polar molecules (ie. a molecule may have polar bonds but not be a polar molecule ie. carbon dioxide.)

Question 8

What are the 3 types of intermolecular forces?

Answer 8

• Van der Waals’
• Permanent dipole- dipole forces.
• Hydrogen bonding.

Question 9

What is hydrogen bonding?

Answer 9

• Interraction between H and N,O,F only. (N, O, F = very electronegative atoms. Hydrogen nucleus = exposed, so, strong attraction between positive H nucleus/ lone pair on N, O, F.)
• N, O, F must have a lone pair in order for hydrogen bonding to occur.

() - extra info for clarification.

Question 10

Nuemonic to remember what atoms are involved in hydrogen bonding!

Answer 10

• That’s eNOF
  (nitrogen, oxygen, fluorine :))

Question 11

What 3 main components should you be including in hydrogen bonding diagrams?

Answer 11

1.) The charges on all dipoles.
2.) All lone pairs (draw as clouds.)
3.) Dotted line between lone pair/ the cloud (on H, O, F) and the slightly positive hydrogen pole.

Question 11

What are the melting and boiling points of molecular substances influenced by?

Answer 11

• Melting/ boiling point of moleculae substances influences by strength of these intermolecular forces.

Question 12

Give 3 common examples of where hydrogen bonding may occur.

Answer 12

• HF
• H₂O
• NH₃

Question 13

True or False

A molecule that has hydrogen bonding will ONLY have this interraction as its intermolecular force.

Answer 13

• False.
• A molecule that has hydrogen bonding will also have permanent dipole- dipole attraction AND Van der Waals’.

Question 13

True or False

A molecule that has permanent dipole- dipole interractions will ONLY have these intermolecular forces.

Answer 13

• Molecule that has permanent dipole- dipole interractions will also have Van der Waals’ forces.

Question 14

Put the 3 intermolecular forces in order of strength from weakest to strongest.

Answer 14

1.) Van der Waals’
2.) Permanent dipole- dipole attractions.
3.) Hydrogen bonding.

Question 15

Why does water have a much higher boiling point than methane (even though they have very similar Mr’s?)

Answer 15

• Water has hydrogen bonding/ permanent dipole- dipole attractions WHEREAS, methane only has Van der Waals’ forces.

Question 16

How does hydrogen bonding allow ice to float on water?

Answer 16

• As solid, the hydrogen bonds are FIXED (as molecules aren’t moving) and allow for molecules to arrange in tetrihedral structure. The separation of water molecules from each other is large due to hydrogen bonding so, there is a lot of space in the molecule (in middle of tetrihedral structure) - giving ice a low density, allowing it to float on water.

() - extra info for clarification