Intermolecular Interactions Flashcards
what is the most important intermolecular force that occurs between molecules
- London forces
- aka dispersion forces
what is the reason for other intermolecular forces between molecules arising
it is due to the permanent dipoles that exist in some molecules
why arent diatomic molecules like nitrogen polar
- because the atoms have the same electronegativities
- meaning that the electron density is symmetrically distributed throughout the molecule
how would a diatomic molecule end up making a dipole on its own
- the electron densities of both atoms fluctuate over time
- if the electron density becomes unsymmetrical at any instant a dipole is generated
- as one is now partially negatively charged and the other is positive
what would a diagram of an instantaneous dipole look like
- the electron density of one of the atoms in the bond would increase
- noting it with a sigma - sign on its sound
- the electron density on the other atom would be smaller
- with a sigma + sign next to it
- then the dipole sign (arrow with a line in the middle) pointing from the o+ atom to the o-
how would an induced dipole form from the instantaneous dipole from before
- when the sigma+ end of the first molecule is close to another molecule, its electron density would be pulled to the direction of the o+ region
- this is because the shared electron would be attracted to the partial positive charge
- this results in a partial negative charge on the side of the second molecule closer to the o+
- leading to the induced dipole being formed
how would those two molecules then interact with each other
- they arrange themselves so that they favorably interact with each other
- where the positive end of one is closer to the negative end of the other
- this would then be the London force of attraction between them
how does the strength of london forces change as the number of electrons in the molecule change
an increase in the number of electrons in the molecule lead to strength of the attractive london forces to increase
how can you use that relationship to explain why the strength of the london forces for the noble gases increases down the group
- the noble gases have more electrons as you go down the group
- they exist as monatomic molecules, so london forces are the only intermolecular forces of attraction between them
- as there are more electrons in the molecule it leads to stronger london forces
how does this decide the trend in boiling points of the noble gases
- their boiling points increase as you go down the group
- this is because the strength of their london forces increases
- leading to more energy required to break them
why does an increase in the electrons in a molecule result in the increase in the strength of the london forces
- more electrons in a molecule lead to greater fluctuation in electron densities
- this leads to larger instantaneous and therefore induced dipoles being created
- resulting in stronger partial charges so therefore stronger london forces
what do london forces depend on
the shape and size of the molecule
are london forces always present between moelcules
yes
why are the favorable interactions between permanent dipoles less than the interaction between instantaneous-induced dipoles
- with permanent dipoles, one region of the molecule is always positive and the other is negative
- the molecules can favorably interact with each other when two opposite sides attract
- but sometimes two sides of the same charge meet so they repel
- however with induced dipoles they are always aligned so that the interactions are favorable
can a permanent dipole induce a dipole in a nearby molecule
yes
