Intermolecular interactions and physical properties Flashcards
the trend in boiling temperature to the relative molecular mass of the first ten unbranched alkanes is
an increase in boiling temperature
the only significant intermolecular interaction between alkane molecules is
the London force
instantaneous dipole-induced dipole
the two reasons for the increase in boiling temperature of a series of unbranched alkanes are:
- 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
branched alkanes have…………….boiling points than their unbranched isomers
lower
the more branching in the molecule, the………………..points of contact between adjacent molecules, so the overall London forces………………..
fewer points of contact between molecules, so overall London forces decrease
the two significant intermolecular interactions between alcohol molecules are
- instantaneous dipole-induced dipole
- hydrogen bonding with the O-H group
alcohols have…………boiling points than alkanes because:
higher boiling points because of the O-H group which allows for the formation of intermolecular hydrogen bonds
in alcohols, the predominant bonding (intermolecular) is……………..for the first 3 members, then…………………..for the others
hydrogen bonding for the first 3 members, then London forces
the trend in boiling points of the hydrogen halides is ……………..from HCl to HI
an increase
HF is higher than HI
water’s two anomalous properties are:
- 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
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
- 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
water molecules in ice are arranged in rings of……….held together by ………………
rings of six, held together by hydrogen bonds
ice is less dense than liquid water at 0 degrees C because
- 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
in order for a substance to dissolve, the conditions to be met are:
-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
many ionic solids will……………..in water
dissolve
ionic solids, such as sodium chloride, NaCl, dissolve in water because:
- 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
the process of dissolving an ionic solid is known as………………
hydration
the energy released when an ionic solid dissolves in water is called
the hydration energy
draw a diagram showing how ethanol hydrogen bonds with water
CH3CH2 H δ+
\ /
O δ- : ||||||||||||||| H δ+ —–O δ-
/
H δ+
: lone pair of electrons
- / \ bond
|||||| hydrogen bonding
δ partial charge
the solubility of alcohols …………………with increasing hydrocarbon chain length because……………..
decreases
because the London forces predominate between the alcohol molecules
non-polar molecules such as…………………….cannot dissolve in water
alkanes
non-polar molecules cannot dissolve in water because
the attraction between the alkane and water molecules is not sufficiently strong to disrupt the hydrogen bonded system of the water molecules
many polar molecules have limited solubility in water because
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
the ‘rule of thumb’ for non-aqueous solvent is
like dissolves like
