Intermolecular interactions Flashcards
What are intermolecular forces?
Forces that act BETWEEN simple molecular substances
Not within the chemical bonding
3 types of intermolecular forces
Induced dipole-dipole interactions (London/ dispersion forces)
Hydrogen bonds
Permanent dipole-dipole interactions
Where do London forces act between?
Between ALL atoms and molecules, even noble gases
What causes London forces?
Attraction between instantaneous dipole and inducing a temporary dipole
How instantaneous forces are formed
Electrons move around within orbitals quickly
At any point, the electrons are more likely to be on one side than the other = instantaneous dipole
How instantaneous dipoles induce a temporary dipole
The difference in charge across the atom in an instantaneous dipole can cause electrons in adjacent atom to move and induce a dipole
This attraction = a London force
What is the overall effect of London forces?
Atoms are attracted to each other despite dipoles being created and destroyed all the time
What do London forces hold molecules in?
A lattice arrangement which is a simple molecule structure
Strengths of London forces depends upon?
How large the molecule is because having larger electron clouds means stronger induced dipole thus London forces
Larger surface area = more molecular surfaces of contact = stronger London forces
How does the strength of London forces effect melting and boiling points?
Stronger London forces means more energy to overcome London forces thus higher mp/bp
Dependent upon surface area of molecules and having larger electron clouds
How do alkanes having a longer carbon chain effect it’s mp/bp
Between alkane molecules are London forces
Longer carbon chain means larger molecular surface so more electrons to interact
Thus stronger London forces which require more energy to overcome and melt/boil
How do alkanes being branched affect its mp/bp
Branching means alkanes pack less close together so less points of contact
Fewer London forces form = less energy to overcome and melt /boil alkane
Permanent dipole-permanent dipole interactions
Occurs when a molecule with permanent dipole (because it’s a polar molecule) interacts with another permanent dipole molecule/ induces a dipole and thus electrostatic attraction between
Do molecules have permanent dipole-permanent dipole interactions as well as London forces?
Yes they occur at the same time in a molecule with a permanent dipole
Mp/bp with molecules with permanent dipole-permanent dipole interactions between them
Genuinely higher than molecules with only London forces because they are stronger
Hydrogen bonding
When hydrogen covalently bonds with N, O, F
A polar bond is formed because they are very electronegative
And because hydrogen has high charge density due to its small size it forms bonds to the LONE PAIRS on other N, O, F atoms
What has to be present for hydrogen bonds to form?
Hydrogen bonded to nitrogen, oxygen or fluorine
Why does hydrogen bonding only occur with N, O, F?
Because they are so electronegative
Polarises the covalent bond to hydrogen and because H is small but has a large dipole, it can form hydrogen bond to lone pairs
What does the hydrogen form bonds with to the other molecules in hydrogen bonds
LONE PAIR OF ELECTRONS in N, O, F
When drawing hydrogen bonding diagram
Bond is to the lone pair of electrons
The bond is from covalent bond to hydrogen bond
=180°
What substances have hydrogen bonds?
Water
Ammonia
Hydrogen fluoride
Any organic substance with alcohol group/amine group
Do hydrogen bonds effect properties?
Yes largely because they are the strongest
If a substance can form hydrogen bonds this means compared to other substances…
It will have a much higher melting/boiling point because it’s the strongest type of intermolecular force so more energy required to break tgese
Boiling points of group 7 hydrides
High in HF
Massive decrease down to HCl
Slowly increases to HBr then HI
Why does hydrogen fluoride have a much higher bp than other group 7 hydrides?
Because it can form hydrogen bonds between each molecule, as well as other intermolecular forces
Which is the strongest intermolecular force thus requires most energy to overcome
Why does the bp increase from HCl to HBr to HI?
Because although there is an decrease in induced dipole interactions (due to smaller dipoles) there are more electrons per molecule thus stronger London forces which require a lot of energy to overcome
Bp in group6 hydrides
Very high in H2O
Sudden decrease down to H2S
Then increases slightly to H2Se then H2Te
Why does H2O have highest bp?
Because it can form hydrogen bonds
Why does the bp increase from H2S to H2Se to H2Te
Because although there is an decrease in induced dipole interactions (due to smaller dipoles) there are more electrons per molecule thus stronger London forces which require a lot of energy to overcome
