Intermolecular Interactions and Physical Properties

Question 1

what is the relationship between the molecular masses of alkanes and their boiling points

Answer 1

an increase in their molecular forces results in an increase in heir boiling points

Question 2

is the relationship proportional

Answer 2

• no

- the graph for it curves until its nearly horizontal

Question 3

what is the most significant intermolecular interaction between the alkane molecules

Answer 3

london forces

Question 4

what are the two main factors that result in this increased boiling temperature as their chain length increases

Answer 4

• the increase in the electrons per molecule

- the increase in the number of points of contact between adjacent molecules

Question 5

why does an increase to the electrons per molecule lead to an increased boiling temp

Answer 5

• the number of instantaneous and therefore induced dipoles would increase
• as there are more carbon atoms and therefore more bonds
• leading to more london forces of attraction between the molecules

Question 6

why does an increase in the number of points of contact between adjacent molecules increase the boiling temp

Answer 6

• instantaneous dipole-induced dipole forces would exist at each point of contact between the molecules
• more point of contact means more intermolecular (london) forces of attraction

Question 7

why do branch chain alkanes have lower boiling temps than their unbranched isomers

Answer 7

• because branching means fewer points of contact between adjacent molecules
• this leads to a decrease in the number of london forces they can produce between them
• leading to an overall decrease in the intermolecular force of attraction between them

Question 8

what two main intermolecular forces do alcohols form and why

Answer 8

• london forces for the same reason anyone can

- hydrogen bonds because they have the OH group

Question 9

between CH3OH and CH3CH3 which both have 18 electrons, which one has the higher boiling temp and why

Answer 9

• the CH3OH (methanol)
• methanol can form hydrogen bonds between adjacent alcohol molecules which ethane cant
• this additional force of attraction needs more energy to break and therefore has a higher boiling temp

Question 10

what intermolecular force’;s strength should remain similar between them anyway

Answer 10

• the london forces strength
• because they have the same number of electrons
• therefore they can create similar numbers of instantaneous-induced dipole forces

Question 11

what is the enthalpy change of vaporisation a measure of

Answer 11

• the amount of energy required to completely separate the molecules of a liquid
• converting it into a gas at the same temperature

Question 12

what can the enthalpy change of vaporisation therefore dictate

Answer 12

the strength of intermolecular interactions between molecules

Question 13

what are two if waters most important anomalies

Answer 13

• it has a relatively high melting and boiling temp for a molecule with so few electrons
• the density of ice at 0C is less than that of water at 0C

Question 14

why does water have a high boiling temp compared to other molecules with similar electron counts

Answer 14

• a lot of waters intermolecular bonds are hydrogen bonds which are strong
• due to it just being hydrogen and oxygen
• this unusually high number of strong forces of attraction are what give it its high boiling temp

Question 15

despite HF and H2O having 10 electrons (so london forces should be similar) and HF having slightly stronger hydrogen bonds, what is the main reason why H2O’s boiling temp still higher

Answer 15

• HF can only form an average of one h bond per molecule
• whereas H2O can from an average of 2
• so hydrogen bonds are much more extensive in water
• leading to higher boiling temps than it despite each h bond of HF being slightly stronger

Question 16

what is a characteristic of HF itself that also prevents its boiling temp from being higher than waters

Answer 16

• not all of the h bonds in HF are broken on vaporisation

- because HF is substantially polymerised even in the gas phase

Question 17

what is the structure of the water molecules in ice

Answer 17

• they are arranged in rings of 6

- held together by hydrogen bonds

Question 18

why is the density of ice therefore less than waters

Answer 18

• the structure creates large areas of open space inside the rings
• when it melts the ring structure is destroyed
• causing the average distance between the molecules to decrease
• therefore leading to an increase in density

Question 19

what two conditions need to be met in order for a substance to dissolve

Answer 19

• the solute particles must be separated from each other
• then become surrounded by solvent particles
• secondly the forces of attraction between the solute and solvent particles must be strong enough to overcome the solvent-solvent and solute-solute forces

Question 20

where does (some or all of) the energy to dissolve an ionic solid by separating the ions come from

Answer 20

the hydration of the ions by water molecules

Question 21

how are cations separated from the crystal lattice of an ionic solid

Answer 21

• the partially negatively charged end of the water molecules (the oxygen atom) attract the cations strongly enough to remove them from the lattice
• the cations become surrounded by water molecules
• with the oxygen end of the H2O molecules facing it

Question 22

what is this interaction between cations and water molecules called

Answer 22

ion-dipole interaction

Question 23

how are anions separated from the crystal lattice of an ionic solid

Answer 23

• the o+ end of the water molecules (hydrogen atoms) attract the anions strongly enough to remove them
• the anions become surrounded bu water molecules
• with the hydrogen end of the H2O molecules facing it

Question 24

what is the energy released from this manner of hydration called

Answer 24

hydration energy

Question 25

what new bonds can alcohols make in water and why

Answer 25

• hydrogen bonds between them and water molecules
• because they both have oxygen atoms and the H2O has hydrogen atoms
• allowing a bond between the O of the alcohol and the H of the water

Question 26

why does the solubility of alcohols in water decrease with increasing hydrocarbon length

Answer 26

• because london forces predominate between the alcohol molecules
• making it harder for them to be separated by water molecules

Question 27

are alkanes soluble in water

Answer 27

no

Question 28

why

Answer 28

• because theyre non polar
• the attraction between them and H2O molecules isnt strong enough to overcome the hydrogen bonding between H2O molecules
• therefore the bonds between water cant be broken to make new ones with alkanes

Question 29

what are the two reasons polar molecules would have a limited solubility in water

Answer 29

• they dont form hydrogen bonds with water

- the hydrogen bonds are weak compared to H2Os original h bonds

Question 30

what is the general rule of thumb with non-aqueous solvents

Answer 30

• like dissolves like

- molecules in the same homologous series usually dissolve within each other