1.4 intermolecular forces Flashcards
intermolecular
Between neighbouring
molecules
van der waals
The attraction between
instantaneous and induced
dipoles on neighbouring
molecules
permanent dipole-dipole attraction
Attraction between the positive end, δ+, of the permanent dipole on a molecule with the negative end, δ-, of the permanent dipole of a neighbouring molecule.
hydrogen bond
The attraction between a lone pair of electrons on a very electronegative atom (i.e. N, O, F) in one molecule and a hydrogen atom in a neighbouring molecule, in which the hydrogen atom is covalently bonded to a very electronegative atom (N, O, F).
order of forces (weak to strongest)
- van der Waals’ forces
- Permanent Dipole-Dipole attractions
- Hydrogen Bonding
non-polar
Non-polar molecules ONLY contain van der Waals’ forces (eg between monoatomic
noble gases, hydrocarbon molecules, halogen molecules and other diatomic
molecules)
why do van der waals exist
Electrons in an atom are not in a fixed position but are continually moving.
At any one moment, there may be more electrons on one side of the molecule than on the other and this produces an instantaneous dipole (d+ d-) in the molecule.
Any other atom nearby will experience an electric field due to this dipole and will develop an induced dipole.
The atoms/molecules are attracted to each other by this weak induced dipole-induced dipole attraction (van der Waals’).
trend down group
As the size of a molecule increases, the RFM increases and the number of electrons
increases and so the van der Waals’ forces get stronger.
why are molecules polar
Many molecules are polar due to an unsymmetrical distribution of charge within each
molecule, ie the bonding electrons lie closer to one atom than the other, thus
creating a dipole. This is due to the fact that the two bonding atoms have different
electronegativity values, thus forming a polar bond.
The partial charges on a polar molecule (+ -) attract oppositely charged dipoles on
another polar molecule. This gives rise to an intermolecular force known as a
permanent dipole-dipole attraction.
Permanent dipole-dipole attractions exist in addition to van der Waals’ forces and are
stronger than van der Waals’ forces.
for hydrogen bonding to occur…
The hydrogen atom must be attached directly to an atom with high electronegativity
(N, O, F). This gives an electron deficient/polar hydrogen atom (H+) on one
molecule,
AND
A lone pair of electrons on a highly electronegative atom (N, O or F) on another
molecule
effects of H-Bonds
Solubility in water – a covalent molecule is soluble in water if it can form H-bonds
with a water molecule.
Higher than expected boiling points – if a covalent molecule exhibits intermolecular
H-bonding then its boiling point will be higher than expected because these strong H-
bonds require much energy to break
Boiling Points of Group IV, V, VI and VII Hydrides
An increase in boiling point of comparable compounds within a group would be
expected due to increasing number of electrons leading to increasing Van der
Waals forces.
*Group 4 hydrides (CH4, SiH4, GeH4 and SnH4) are tetrahedral molecules and
do not have a permanent dipole. The increase in boiling point is due to the
increase in the number of electrons in the molecules which increases the
strength of the Van der Waal forces of attraction.
- Group 5, 6 and 7 hydrides all have a permanent dipole. This additional force
of attraction increases the boiling points of these molecules in comparison to
the Group 4 hydrid
i) H2O, HF and NH3 have higher than expected boiling points.
The boiling points of H2O, HF and NH3 are higher than expected. This is
because of the hydrogen bonds between the molecules of these compounds and so
a large amount of energy is required to break the hydrogen bonds between the
molecules.
If we only considered van der Waals forces for the hydrides of Groups 5, 6 and 7 of
the Periodic Table then the boiling temperatures of the first hydrides of the Groups
would be expected to be lower than they are.
Why has H2O a higher boiling point than NH3 and HF?
The two lone pairs of electrons on each oxygen atom can form a H-bond with
another water molecule and the two polar hydrogens can form H-bonds with
surrounding water molecules. Hence each water can potentially form four hydrogen
bonds which leads to a higher boiling point.
In HF there is one polar hydrogen which can form one H-bond per molecule so its
boiling point is lower than water.
In NH3 there is one H-bond per molecule as only one lone pair on the nitrogen – in a
group of ammonia molecules there are not enough lone pairs to satisfy all of the
hydrogens.
properties of liquids
Miscible - liquids that mix in all proportions
Immiscible – liquids that do not mix
Liquids often mix because of their ability to form the same intermolecular forces
between the molecules. Solids are soluble in a solvent because they have similar
bonds between their molecules.
The term ‘like dissolves like’ is a phrase often used to explain miscibility of liquids
and the solubility of a solid in a solvent. ‘Like’ in this case means that the bonding
between the particles in one substance is similar to the bonding between the
particles in another substance.
When answering questions use eg. Polar dissolves polar OR non-polar dissolves
non-polar.
