Intermolecular interactions

Question 1

What are intermolecular forces?

Answer 1

Forces that act BETWEEN simple molecular substances
Not within the chemical bonding

Question 2

3 types of intermolecular forces

Answer 2

Induced dipole-dipole interactions (London/ dispersion forces)
Hydrogen bonds
Permanent dipole-dipole interactions

Question 3

Where do London forces act between?

Answer 3

Between ALL atoms and molecules, even noble gases

Question 4

What causes London forces?

Answer 4

Attraction between instantaneous dipole and inducing a temporary dipole

Question 5

How instantaneous forces are formed

Answer 5

Electrons move around within orbitals quickly
At any point, the electrons are more likely to be on one side than the other = instantaneous dipole

Question 6

How instantaneous dipoles induce a temporary dipole

Answer 6

The difference in charge across the atom in an instantaneous dipole can cause electrons in adjacent atom to move and induce a dipole
This attraction = a London force

Question 7

What is the overall effect of London forces?

Answer 7

Atoms are attracted to each other despite dipoles being created and destroyed all the time

Question 8

What do London forces hold molecules in?

Answer 8

A lattice arrangement which is a simple molecule structure

Question 9

Strengths of London forces depends upon?

Answer 9

How large the molecule is because having larger electron clouds means stronger induced dipole thus London forces
Larger surface area = more molecular surfaces of contact = stronger London forces

Question 10

How does the strength of London forces effect melting and boiling points?

Answer 10

Stronger London forces means more energy to overcome London forces thus higher mp/bp
Dependent upon surface area of molecules and having larger electron clouds

Question 11

How do alkanes having a longer carbon chain effect it’s mp/bp

Answer 11

Between alkane molecules are London forces
Longer carbon chain means larger molecular surface so more electrons to interact
Thus stronger London forces which require more energy to overcome and melt/boil

Question 12

How do alkanes being branched affect its mp/bp

Answer 12

Branching means alkanes pack less close together so less points of contact
Fewer London forces form = less energy to overcome and melt /boil alkane

Question 13

Permanent dipole-permanent dipole interactions

Answer 13

Occurs when a molecule with permanent dipole (because it’s a polar molecule) interacts with another permanent dipole molecule/ induces a dipole and thus electrostatic attraction between

Question 14

Do molecules have permanent dipole-permanent dipole interactions as well as London forces?

Answer 14

Yes they occur at the same time in a molecule with a permanent dipole

Question 15

Mp/bp with molecules with permanent dipole-permanent dipole interactions between them

Answer 15

Genuinely higher than molecules with only London forces because they are stronger

Question 16

Hydrogen bonding

Answer 16

When hydrogen covalently bonds with N, O, F
A polar bond is formed because they are very electronegative
And because hydrogen has high charge density due to its small size it forms bonds to the LONE PAIRS on other N, O, F atoms

Question 17

What has to be present for hydrogen bonds to form?

Answer 17

Hydrogen bonded to nitrogen, oxygen or fluorine

Question 18

Why does hydrogen bonding only occur with N, O, F?

Answer 18

Because they are so electronegative
Polarises the covalent bond to hydrogen and because H is small but has a large dipole, it can form hydrogen bond to lone pairs

Question 19

What does the hydrogen form bonds with to the other molecules in hydrogen bonds

Answer 19

LONE PAIR OF ELECTRONS in N, O, F

Question 20

When drawing hydrogen bonding diagram

Answer 20

Bond is to the lone pair of electrons
The bond is from covalent bond to hydrogen bond
=180°

Question 21

What substances have hydrogen bonds?

Answer 21

Water
Ammonia
Hydrogen fluoride
Any organic substance with alcohol group/amine group

Question 22

Do hydrogen bonds effect properties?

Answer 22

Yes largely because they are the strongest

Question 23

If a substance can form hydrogen bonds this means compared to other substances…

Answer 23

It will have a much higher melting/boiling point because it’s the strongest type of intermolecular force so more energy required to break tgese

Question 24

Boiling points of group 7 hydrides

Answer 24

High in HF
Massive decrease down to HCl
Slowly increases to HBr then HI

Question 25

Why does hydrogen fluoride have a much higher bp than other group 7 hydrides?

Answer 25

Because it can form hydrogen bonds between each molecule, as well as other intermolecular forces
Which is the strongest intermolecular force thus requires most energy to overcome

Question 26

Why does the bp increase from HCl to HBr to HI?

Answer 26

Because although there is an decrease in induced dipole interactions (due to smaller dipoles) there are more electrons per molecule thus stronger London forces which require a lot of energy to overcome

Question 27

Bp in group6 hydrides

Answer 27

Very high in H2O
Sudden decrease down to H2S
Then increases slightly to H2Se then H2Te

Question 28

Why does H2O have highest bp?

Answer 28

Because it can form hydrogen bonds

Question 29

Why does the bp increase from H2S to H2Se to H2Te

Answer 29

Because although there is an decrease in induced dipole interactions (due to smaller dipoles) there are more electrons per molecule thus stronger London forces which require a lot of energy to overcome