intermolecular forces Flashcards
what are the 3 types of intermolecular forces
london forces
permanent dipole-permanent dipole
hydrogen bonds
what does the strength of a london force depend upon
number of electrons
shape of the molecule
what is an instantaneous dipole
at any instant there may be more negative charge in one area of a molecule than another and more positive charge in another area
how do london forces form
- uneven distribution of electrons
- results in an instantaneous dipole in the molecule
- this induces a induced dipole on an adjacent molecule
do long thin molecules have stronger or weaker london forces and why
stronger
larger instantaneous dipoles
how do electrons affect strength of london forces
more electrons
stronger london forces
where do permanent dipole forces form
between polar molecules
define a hydrogen bond
a strong permanent dipole - pd attraction between the lone pair of electrons on a very electronegative atom (N F or O) and a H atom directly covalently bonded to another very electronegative N F or O atom
how do hydrogen bonds form
O N and F atoms have lone pairs
they attract electrons from the small hydrogen atoms so strongly that they leave exposed protons.
Protons have a high charge density and strongly attract lone pairs from neighbouring O N and F atoms
what is the order of strength of intermolecular forces
pd-pd < london forces < hydrogen bonds
why is ice less dense than water
there are more extensive permanent hydrogen bonds in ice than water
H bonds hold the molecules apart
what happens when ice melts (why does density then increase)
some of the hydrogen bonds break
the structure collapses in on itself
increases the density
what is the rule for solvents and solutes
if the total energy required to separate the solute particles and to separate the solvent molecules is less than the energy released when new forces between the solute and solvent are formed, then the substance will dissolve.
why do polar substances dissolve in polar solutes
polar substances contain pd-pd or H bonds as well as london forces
when polar solvent dissolves polar solute the new intermolecular forces that could be formed would be pd-pd or H bonds and london forces
solute would dissolve as energy required to overcome forces < energy released when new forces are made
why do non polar substances dissolve in non polar solvents
non polar substances contain only london forces.
when a non polar solvent dissolves a polar solute the new intermolecular forces formed will be london forces.
solute would dissolve as energy required to overcome forces < energy released when new forces are made
