Intermolecular forces Flashcards
where can you find intermolecular forces?
between covalent bonds
where are intramolecular forces?
inside each molecule
what are intermolecular forces?
the attractions between molecules
3 types of intermolecular forces
dipole dipole attraction
Van der waal attraction
hydrogen bonds
strength of Van der waal attraction
weakest
strength of dipole dipole attraction
medium strength
strength of hydrogen bonds
strongest
when does dipole-dipole attraction happen?
when a covalent molecule has permanent partial + and partial - charges due to having large electronegativity differences
where do van der waal forces occur
do not have dipole moments, non-polar covalent
what causes van der waal forces?
flickering dipoles
what causes flickering dipoles
due to their random movement within the orbitals, there are small lengths of time when the electrons are not equally shared
what does it mean for a flickering dipole to be ‘on’
inducing a temporary dipole
what point in time do van der waal forces occur
when the 2 molecules involved are both ‘on’
why does NO boil at much higher than N2
because van der waal forces are much weaker than dipole-dipole attraction
why does boiling point increase with Mr in non-polar covalent molecules
because the number of van der waal forces increases with Mr
what causes the great strength of polythene?
it is due to millions of van der waal forces betwee the parallel molecules
boiling points of hydrogen bonding
surprisingly high BP
when does a hydrogen bond occur?
when hydrogen in a covalent bond with N,O or F in one molecule AND there is a lone pair of electrons on either N,O or F on another neighbouring molecule
example of hydrogen bonding
in water
why are N,O and F significant in hydrogen bonding (2)
- very high electronegativity values
2. smallest atomic radii
why is Cl not involved in hydrogen bonding
atomic radius is too bigh
how much stronger is a covalent bond than a hydrogen bond
10 times stronger
where do you find hydrogen bonding between N and O
in biology; proteins, DNA etc.
why is ammonia extremely soluble in water?
becuase of hydrogen bonding
why does water expand below 4ºC?
when water is about the solidify as ice, H2O molecules rotate so that they can have their maximum of 4 H bonds, for this they need more “elbow room”
why is the boiling point of water much higher than that of NH3 or NF?
there is not the maximum aof 4 hydrogen bonds in them as opposed to in water
4 properties hydrogen bonding gives water
High BP
affects density
skin on water
clustering
explain high BP of water
very high BP for a its Mr of 18
explain how h bonding affects density
solid form less dense that liquid form
explain the skin of the water
water molecules attracted to each other and causes surface tension
why does like dissolve like
polar solutes dissolve in polar solvents because they are both charged etc.
All other things being equal, boiling point increases…
boiling point increases with Mr
what gives shapes of covalent molecules?
Valence Electron Repulsion Theory
2 things that the Valence Electron Repulsion Theory states
i) Electron pairs repel each other so that they end up as far apart from each other as is possible in space
ii) Lone pairs are better at repelling than bonded pairs
why are lone pairs are better at repelling than bonded pairs ?
they are closer to the nucleus of the central atom
What gives the name of the shape of a molecule?
the number of electron pairs (lone or bonded) on the outer shell of the central atom
3 steps to determine the shape of a molecule question
- is it ionic or covalent
- draw ring diagram of molecule
- count number of electron pairs on the outer shell of the central atom
2 electron pairs on outer shell of central atom
linear
bond angle of linear molecule
180º
example of linear molecule
BeH2
3 electron pairs on outer shell of central atom
triagonal planar
bond angles in triagonal planar molecule
120º
example of triagonal planar molecule
BF3
4 bonded electron pairs on outer shell of central atom
perfect tetrahedral
bond angle of perfect tetrahedral molecule
109.5º
example of perfect tetrahedral molecule
CH4
3 bonded and 1 lone pair of electron pairs on outer shell of central atom
pyramidal
bond angle of pyramidal molecule
107º
example of pyramidal molecule
NH3
2 bonded and 2 lone pairs of electrons on outer shell of central atom
V-shaped
bond angle in v-shaped molecule
105º
example of v-shaped molecule
H2O
tetrahedral angle in NH3
107º
why is tetrahedral angle in NH3 107º?
lone pair - bonded pair repulsion is greater than bonded pair - bonded pair repulsion
tetrahedral angle in H2O
105º
why is the tetrahedral angle in H2O 105º?
lone pair - lone pair repulsion is greater than lone pair - bonded pair repulsion which is greater than bonded pair - bonded pair repulsion
diatomic molecules shape
linear
3 examples of diatomic molecules
H2, O2, N2
Shape of carbon with 4 single bonds
tetrahedral
shape of carbon with one double or triple bond
planar
