Intermolecular interactions and physical properties

Question 1

the trend in boiling temperature to the relative molecular mass of the first ten unbranched alkanes is

Answer 1

an increase in boiling temperature

Question 2

the only significant intermolecular interaction between alkane molecules is

Answer 2

the London force

instantaneous dipole-induced dipole

Question 3

the two reasons for the increase in boiling temperature of a series of unbranched alkanes are:

Answer 3

• as the molecular mass increases, the number of electrons per molecule increases so the London forces increase
• as the length of the carbon chain increases, the number of points of contact between adjacent molecules increases, and as London forces exist at each point of contact between molecules, the overall intermolecular London force of attraction increases

Question 4

branched alkanes have…………….boiling points than their unbranched isomers

Answer 4

lower

Question 5

the more branching in the molecule, the………………..points of contact between adjacent molecules, so the overall London forces………………..

Answer 5

fewer points of contact between molecules, so overall London forces decrease

Question 6

the two significant intermolecular interactions between alcohol molecules are

Answer 6

• instantaneous dipole-induced dipole

- hydrogen bonding with the O-H group

Question 7

alcohols have…………boiling points than alkanes because:

Answer 7

higher boiling points because of the O-H group which allows for the formation of intermolecular hydrogen bonds

Question 8

in alcohols, the predominant bonding (intermolecular) is……………..for the first 3 members, then…………………..for the others

Answer 8

hydrogen bonding for the first 3 members, then London forces

Question 9

the trend in boiling points of the hydrogen halides is ……………..from HCl to HI

Answer 9

an increase

HF is higher than HI

Question 10

water’s two anomalous properties are:

Answer 10

• relatively high boiling point for a molecule with so few electrons
• density of solid water being less than that of liquid water at 0 degrees C

Question 11

explain the trend in the table below:

Boiling temp.| No. electrons|Strength of hydrogen bond
H2O: 100 10 22
NH3: -33 10 17
HF: -84 10 29

Answer 11

• though HF has stronger hydrogen bonds than H2O, it forms an average of 1 hydrogen bond per molecule whereas water forms 2 on average, so the hydrogen bonding is much more extensive in H2O
• not all the hydrogen bonds in HF are broken, due to the polymerisation of HF even in the gas phase, so fewer bonds have to be broken to make HF boil
• ammonia falls between the two, with weaker hydrogen bonds, but a higher boiling temperature than HF as it forms an average of 1 hydrogen bond pr molecule, but almost all of its molecules are broken on vaporisation

Question 12

water molecules in ice are arranged in rings of……….held together by ………………

Answer 12

rings of six, held together by hydrogen bonds

Question 13

ice is less dense than liquid water at 0 degrees C because

Answer 13

• the structure of the molecules in the ice means there are large areas of open space inside the rings
• when the ice melts, this structure is destroyed and the average space between the molecules decreases, causing an increase in density

Question 14

in order for a substance to dissolve, the conditions to be met are:

Answer 14

-the solute particles must be separated from each other and then become surrounded by solvent particles
-the forces of attraction between the solute and solvent particles must be strong enough to overcome the solvent-solvent forces and the solute-solute forces
in other words, the forces of attraction, F, must be:
F solvent-solute > F solute-solute
F solvent-solute > F solvent-solvent

Question 15

many ionic solids will……………..in water

Answer 15

dissolve

Question 16

ionic solids, such as sodium chloride, NaCl, dissolve in water because:

Answer 16

• the ionic lattice structure means the outer edges can be influence by polar molecules such as water because of the charges on the individual ions which make up the compound
• in NaCl, the Na+ ions are attracted by the δ- end of the water molecule (the O)
• the Na+ then has an ion-dipole interaction with the water
• in NaCl, the Cl- ions are attracted by the δ+ end of the water molecule (the H2)
• the Cl- then hydrogen bonds with the water molecule

Question 17

the process of dissolving an ionic solid is known as………………

Answer 17

hydration

Question 18

the energy released when an ionic solid dissolves in water is called

Answer 18

the hydration energy

Question 19

draw a diagram showing how ethanol hydrogen bonds with water

Answer 19

CH3CH2 H δ+
\ /
O δ- : ||||||||||||||| H δ+ —–O δ-
/
H δ+

: lone pair of electrons
- / \ bond
|||||| hydrogen bonding
δ partial charge

Question 20

the solubility of alcohols …………………with increasing hydrocarbon chain length because……………..

Answer 20

decreases

because the London forces predominate between the alcohol molecules

Question 21

non-polar molecules such as…………………….cannot dissolve in water

Answer 21

alkanes

Question 22

non-polar molecules cannot dissolve in water because

Answer 22

the attraction between the alkane and water molecules is not sufficiently strong to disrupt the hydrogen bonded system of the water molecules

Question 23

many polar molecules have limited solubility in water because

Answer 23

they either form weak hydrogen bonds with water compared to the hydrogen bonds between the water molecules, or they do not form any hydrogen bonds with the water

Question 24

the ‘rule of thumb’ for non-aqueous solvent is

Answer 24

like dissolves like