Chemistry - intermolecular forces

evidence for intermolecular forces

noble gases can exist as solid and liquid so there must be forces between the atoms, ideal gas law

three types of intermolecular force

induced dipole dipole interactions, permanent dipole dipole interactions, hydrogen bonding

induced dipole dipole interactions

forces of attraction between two non polar atoms or non polar molecules

electrons move at very high speeds in orbits

weakest force, short range, cumulative so significant

instantaneous dipole

when more electrons lie on one side of the atom/molecule than another, this produces an induced dipole in a neighbouring atom/molecule which is therefore attracted

what are LDFs proportional to

number of electrons

how does the shape affect LDFs

straight chain alkanes have greater LDFs between them (higher mp and bp) than branched chain alkanes of similar Mr as they pack more tightly so have a greater area of contact between the molecules

permanent dipole dipole interactions

polar molecules attracted towards one another by them, negative end of one molecule is attracted towards the positive end of another, interactions decrease as distance between the molecules increases

permanent vs LDf

stronger than LDF so have a significant effect on the boiling point of a substance

HCl vs Argon

HCl is permanent, differences in electronegativity create a dipole in HCL so on boiling the PDD forces between the HCL molecules must be overcome, argon atoms are non polar, only have LDFs, HCl has a higher boiling point due to the PDD forces requiring more energy to overcome

hydrogen bonding

strongest permanent dipole dipole intermolecular force, between F, O or N (small highly electronegative element with a lone pair)

H atom in a covalent bond

much smaller than other atoms so the + charge is spread over a smaller volume so has a high polarising power (can induce a dipole in other things), the + attracts e- density from the F/O/N on the other molecule, H is sandwiched between its covalent bond to one atom and its H bond to the other

water

2 lone pairs on oxygen, 2 O-H bonds, so average of 2 H-bonds per molecule

NH3

1 lone pair of N, 3 N-H bonds so average of 1 H-bond per molecule

HF

3 lone pairs on F, 1 F-H bond so average of 1 H-bond per molecule

boiling point of noble gases

increase down the group due to the increase in number of electrons and therefore the strength of LDF

group 4 hydrides

similar to noble gases, as the number of electrons increase, so does the strength of the LDF

group 5, 6 and 7 hydrides

different pattern, lightest in each group has a significantly higher mp or bp

some covalently bonded molecules which exist as solids

ice (hydrogen bonding) and iodine (large number of electrons), are held as simple molecular lattice structures attracted by intermolecular forces

solubility

when one liquid dissolves in another, the molecules of one become surrounded by the molecules of the other, the ability of liquids to dissolve depends on the relative strengths of these three sets of intermolecular forces

when will dissolving occur

if the intermolecular forces between molecules in the mixture are similar to those between the molecules in separate liquids

water and ethanol

both have hydrogen bonds, when mixed together, water forms hydrogen bonds with ethanol almost as readily as it does with itself

water and tetrachloromethane

CCl4 is a liquid organic solvent and has LDFs, they do not dissolve as the only forces of attraction between them would be LDFs, which are much weaker than the hydrogen bonds between the water molecules so the water molecules remain firmly hydrogen bonded to each other

cyclohexane and tetrachloromethane

both non-polar solvents, LDFs are the only intermolecular forces, when mixed together, the only forces of attraction would be LDFs which are similar to those between molecules in separate liquids so they are completely soluble in each other

electrical conductivity

covalent compounds with simple molecules or with simple molecular lattice structures do not conduct electricity as they have no charged particles

what does the presence of hydrogen bonds between water molecules account for

the unexpectedly high values for the boiling point and freezing point of water, on average a liquid water molecule will have 3 out of 4 possible hydrogen bonds

in ice

there is an open lattice as the 2 hydrogen atoms and 2 lone pairs of electrons results in a tetrahedral

ice occupies a

greater volume than water, so its density is less than water at 0

when ice melts

regular lattice breaks up so water molecules can then pack more closely and the liquid has a higher density, in liquid water there are strong hydrogen bonds between water molecules but there is no ordered packing as hydrogen bonds are continually broken and formed

where do intermolecular forces happen

only in simple molecular