Intermolecular Forces

Question 1

Molecular shape affects

Answer 1

vapour pressure, melting point, boiling point, solubility

Question 2

What does VSEPR theory stand for

Answer 2

valence shell electron pair repulsion

Question 3

What does VSEPR theory do

Answer 3

uses our knowledge of the valence electrons in the atoms of a molecule to predict the shape of a molecule

Question 4

What is the VSEPR theory based on

Answer 4

the principle that negatively charged electron pairs in the valence shell repel each other, and are arranged as far away from each other as possible (the angle of the bonds is as large as possible)

Question 5

What are lone pairs of electrons

Answer 5

non-bonding pairs of electrons, influence the molecule’s shape but are not considered part of the shape

Question 6

What are the molecule shapes

Answer 6

linear, bent, trigonal planar, tetrahedral, trigonal pyramidal, trigonal bipyramidal, octahedral

Question 7

What exceptions to the octet rule are there

Answer 7

Boron -> 3 electron pairs (eg. BH3)
Beryllium -> 2 electron pairs (eg. BeH2)

Question 8

How are the physical properties of covalent molecular substances determined?

Answer 8

the strength of the forces between the molecules (intermolecular forces)

Question 9

How strong are intermolecular forces compared to intramolecular forces?

Answer 9

10-100 times weaker

Question 10

Why do covalent molecular substances usually have much lower melting and boiling points than ionic, metallic and covalent network substances?

Answer 10

The forces between the molecules are much weaker

Question 11

What happens when a covalent molecular substance is converted from a solid to a liquid or gas

Answer 11

the weak intermolecular forces are broken

Question 12

vapour pressure

Answer 12

the pressure that the gaseous molecules exert on the closed container walls when the rates of evaporation and condensation become equal

Question 13

Strong intermolecular forces mean what vapour pressure and why?

Answer 13

low vapour pressure, because the molecules are held tighter, making it harder for them to escape from the surface of the liquid

Question 14

What energy allows molecules to overcome intermolecular forces

Answer 14

kinetic energy

Question 15

boiling point of a liquid

Answer 15

the temperature at which the liquid’s vapour pressure reaches the atmospheric pressure of the surroundings

Question 16

electronegativity

Answer 16

the tendency of an atom in a covalent bond to attract electrons

Question 17

What is the trend for electronegativity on the periodic table?

Answer 17

increases left to right across periods, decreases down the groups

Question 18

electron density

Answer 18

the measure of the probability of an electron being present at a particular location within an atom

Question 19

non-polar diatomic molecules

Answer 19

bonds with an equal distribution of valence electrons (no charge on either end of the molecule)

Question 20

polar diatomic molecules

Answer 20

molecules with an imbalanced electron distribution

Question 21

Why are some molecules polar

Answer 21

in covalent bonds between two different atoms, the electrons will stay closer to the more electronegative atom because it has a stronger pull on the electrons

Question 22

what does the imbalance in electron distribution mean

Answer 22

the more electronegative atom is negatively charged and the less electronegative atom is positively charged -> two oppositely charged poles

Question 23

electric dipole

Answer 23

the separation of the positive and negative charges

Question 24

Why are intermolecular bonds much weaker than ionic bonds?

Answer 24

the partial charges on a polar molecule are a lot smaller than the charges on ions

Question 25

Symmetrical molecules are:

Answer 25

non-polar

Question 26

Asymmetrical molecules are:

Answer 26

polar

Question 27

how does polarity affect solubility

Answer 27

polar substances dissolve in polar solvents and vice versa 'like dissolves like'

Question 28

What are the types of intermolecular forces:

Answer 28

dipole-dipole, hydrogen bonding, dispersion forces

Question 29

what molecules do dipole-dipole force occur in

Answer 29

only in polar molecules

Question 30

How strong are dipole-dipole forces

Answer 30

Relatively weak because the partial charges on the molecules are small

Question 31

How is the strength of dipole-dipole forces related to melting and boiling points of the substance

Answer 31

stronger forces -> higher melting and boiling points, because they bond the molecules together in the solid or liquid

Question 32

more polar molecules have:

Answer 32

stronger dipole-dipole forces between them

Question 33

where do dipole-dipole forces result from

Answer 33

the attraction between the positive and negative ends of the polar molecules

Question 34

hydrogen bonding occurs how

Answer 34

only between molecules in which a hydrogen atom is covalently bonded to an oxygen, nitrogen or fluorine atom

Question 35

why does hydrogen bonding only occur with oxygen, nitrogen or fluorine?

Answer 35

they are small and highly electronegative, strongly attract electrons in a covalent bond

Question 36

What charge does the hydrogen atom have in hydrogen bonding

Answer 36

a significant partial positive charge, which is then attracted to lone pairs of electrons in the nitrogen, oxygen and fluorine in neighbouring molecules

Question 37

How strong are hydrogen bonds compared to dipole-dipole and ionic/covalent bonds

Answer 37

relatively strong, approx 10 times stronger than dipole-dipole, 1/10 the strength of an ionic/covalent bond

Question 38

How do hydrogen bonds affect melting and boiling points

Answer 38

higher melting and boiling points

Question 39

What are the two key requirements for hydrogen bonding

Answer 39

1. a hydrogen atom covalently bonded to an oxygen, nitrogen or fluorine atom 2. a lone pair of electrons on the nitrogen, oxygen or fluorine atom of neighbouring molecules

Question 40

Why do chlorine atoms not form hydrogen bonds

Answer 40

they have high electronegativity, but they have a large atomic radius which reduces the concentration of negative charge around the atom

Question 41

Why is ice less dense than water

Answer 41

water molecules can form hydrogen bonds with four other water molecules in a regular crystal lattice, where the molecules are held further apart than in liquid water -> less dense and floats on water

Question 42

Dispersion forces

Answer 42

the forces of attraction between non-polar molecules

Question 43

What are dispersion forces caused by

Answer 43

by temporary dipoles in the molecules as a result of random movement of the electrons surrounding the molecule (instantaneous dipoles)

Question 44

Explain the process of dispersion forces

Answer 44

Sometimes electrons gather more closely at one end, causing one end to be more negative than the other, which can induce dipoles in meighbouring molecules too. The temporary dipoles attract each other to create dispersion forces

Question 45

How do dispersion forces get stronger

Answer 45

increases as the size of the molecule increases (larger number of electrons, easier to produce temporary dipoles), therefore have higher melting and boiling points

Question 46

Halogens form what kind of molecules

Answer 46

diatomic non-polar, only have dispersion forces

Question 47

What shape molecules usually have stronger dispersion forces

Answer 47

longer chains stronger than more compact molecules with similar numbers of electrons

Question 48

Why do longer chains have more dispersion force

Answer 48

more contact area to interact with its neighbouring molecules