Dipole induced dipole force - States of Matter Video Lecture - Class 11

in this video I'd like to address this
question what makes molecules stick
together in the liquid and solid states
let's take a look at an animation in
this animation we could clearly see that
molecule is presented by the blue dots
stick pretty closely together in the
solid state in the liquid state and the
question is well what makes them do that
as compared to molecules in the gaseous
state where they're pretty far apart and
there's essentially no interaction
between them here's an answer
there are intermolecular forces or some
cop sometimes called intermolecular
attractions among molecules and this is
essentially the sticking power between
molecules that keeps them in the liquid
or solid state intermolecular
attractions arise because of weak
electrostatic attractions between
molecules electrostatic attractions
occur when an electrically positive
reagent is attracted to an electrically
negative region so-called dipole exists
each molecule this cartoon can be an
example of a speck of solid each oval
represents a molecule in each molecule
there exist a dipole a region of
positive and a region of negative and
notice how the molecules are oriented in
the speck of solid positive end of the
one molecule is attracted or lined up
with the negative end of another
molecule this continues throughout the
speck of solid
you
a symmetrical distribution of
electrically positive regions and
electrically negative regions ie dipoles
are inherent in polar molecules hydrogen
fluoride and water are two examples
there are Lewis structures of HF and
water and below each Lewis structure is
something called an electrostatic
potential map these maps show
electrically positive regions
electrically negative regions using
electronegativity values we could
predict where the electrically negative
regions would reside in a molecule the
symbol that is used to show where the
dipole exists in the molecule the
arrowhead points towards the
electrically negative region and the
cross or the other end of the arrow
resides in the electrically positive
region of the molecule in addition to
arrows with crosses at the other end
these Delta symbols can also be used to
show where the electrically positive and
electrically negative regions are in the
molecule symmetrical distribution of
electrically positive regions and
electrically negative regions ie
no dipole are inherent in nonpolar
molecules such as bf3 and ch4 next to
each Lewis structure is the
electrostatic potential map for the
molecule notice the electrically
negative region symmetrically placed in
the bf3 molecule and boron in the center
is the electrically positive region in
ch4 carbon and hydrogen have very
similar electronegativities there isn't
too much localization of either
electrically positive or electrically
negative ranges in the molecule
well a question can arise if dipoles are
needed for electrostatic attractions
between molecules to keep them together
in the liquid and solid states and how
do nonpolar molecules those without
dipole stick together to form liquids
and solids nonpolar molecules will
induce dipoles on other nonpolar
molecules what also can occur is polar
molecules will induce dipoles on
nonpolar molecules we'll see this in the
animations that follow
the first kind of intermolecular
attraction I'll explain is induced
dipole-induced dipole this occurs among
nonpolar molecules one molecule will
induce a dipole on another molecule in
the molecule that is doing the inducing
a dipole in fact is created in that
molecule
you
to summarize induced dipole-induced
dipole molecules do not need to be
faller
an example of two nonpolar molecules
would be two nitrogen molecules and of
the intermolecular attractions I'll
present induced dipole induced dipole is
the weakest the next type of
intermolecular attraction is dipole
induced dipole this occurs between a
polar molecule which has an existing
dipole and a nonpolar molecule which
does not have an existing dipole the
polar molecule induces a dipole on the
nonpolar molecule very common example is
water and oxygen oxygen remains
dissolved in water because of because of
the water inducing a dipole on the
oxygen
you
to summarize dipole induced dipole one
molecule needs to be polar the other
molecule non-polar an example water and
oxygen and this intermolecular
attraction is also considered weak
next is dipole-dipole I have the word
regular underneath the title of dipole
dipole because there is another type of
dipole-dipole attraction that involves
hydrogen bonding we'll see that next but
regular dipole-dipole involves two polar
molecules that will not engage in
hydrogen bonding a good example is HCl
for ordinary dipole-dipole both
molecules need to be polar and because
of this there's an increase in strength
in the intermolecular attraction many
molecules that undergo dipole-dipole
intermolecular attraction undergo a
specific type which is called hydrogen
bonding in order for a molecule to
engage in hydrogen bonding a hydrogen on
one molecule must be covalently bonded
to either a nitrogen oxygen or fluorine
atom that hydrogen atom that is
covalently bonded to the nitrogen oxygen
fluorine will hydrogen bond to a
nitrogen oxygen or fluorine on another
molecule this is an electrostatic
attraction where the hydrogen is
slightly positive and is attracted to
the slightly negative nitrogen oxygen or
fluorine here's an example of two
alcohol molecules engaging and hydrogen
bonding note the yellow line between the
hydrogen and the oxygen is not a
covalent bond that simply shows the
electrostatic attraction between the
hydrogen and the oxygen but I want to
point out that the hydrogen that is
engaging in this electrostatic
attraction is covalently bonded to an
oxygen and the hydrogen is
electrostatically attracted to the
oxygen which is on another molecule
here's an example of water hydrogen
bonding with an alcohol molecule we
could see the hydrogen that is
covalently bonded to the oxygen on the
alcohol molecule is electrostatically
attracted to the oxygen on a water
molecule
here's an example of water hydrogen
bonding with an ammonia molecule the
hydrogen of the water molecule is
electrostatically attracted to the
nitrogen of the ammonia molecule here's
another example of hydrogen bonding in a
DNA molecule notice the hydrogen bonding
between the nitrogen and the hydrogen
the oxygen in the hydrogen in the last
intermolecular attraction look at is ion
dipole this occurs between an ion
whether a cation or anion and a polar
molecule
notice how the anion is attracted to in
this case a water molecule and notice
the orientation of the water molecule
the hydrogen's of the water molecule are
slightly positive and those are pointed
towards the negative anion when a cation
engages in ion dipole attractions with a
water molecule notice how the water
molecule is oriented towards the cation
the oxygen is pointed towards the cation
because the oxygen is slightly negative
this table summarizes all of the
intermolecular attractions that were
covered in this video