vapor pressures of liquids last time and
what we said that if you have a liquid
in a container in a beaker and left on a
table it will evaporate eventually that
means that the liquid will be converted
to the gas form of that compound now we
were trying to understand how this
simply takes place in evaporation
process and what we said again last time
that molecules in a liquid have kinetic
energies and these kinetic energies are
distributed again as some of the
molecules have low kinetic energy some
of the molecules has higher kinetic
energies and most are sort of between
intermediate now the molecules of the
liquid
so the liquid molecules at the surface
will go from liquid to the gaseous state
that means they will evaporate only if
their kinetic energy is high enough to
break off intermolecular forces from the
liquid and go to the gaseous state now
again this is the distribution of the
kinetic energies for example in this
case suppose that the molecules which
have kinetic energies greater than this
value here will be capable of escaping
from liquid to the gaseous state that
means they will evaporate and as soon as
one liquid molecule at the surface which
a high kinetic energy goes into the
gaseous phase if we still have liquid in
the container that will be immediately
replaced replaced by the liquid
molecules in the bulk so therefore if we
consider the surface of the liquid
actually and as long as we have a liquid
and if the temperature is the same that
means that the percent of the molecules
at the surface which have enough kinetic
energy to escape from the liquid will be
the same and this number will determine
what we call the rate of evaporation of
the liquids now what is the rate of
evaporation rate of evaporation simply
means that the number of liquid
molecules that goes from liquid to the
gaseous state per unit time so if we
simply look at the rates of the
evaporation and the rates of
condensation again
as we talked about this last time now
since the rate of evaporation will be
constant again as long as you have
liquids in a container and the
temperature is the same why because the
number of molecules at the surface
liquid molecules at the surface which
have kinetic energies greater than
actually that to break off the liquids
will always be the same and that means
that the rate will be the same now in a
closed system again suppose that at the
beginning we have no gaseous form of
that liquid above the liquid here so
that means that rate of condensation
that means the molecules going from
gaseous to the liquid is zero but as the
time goes on that means more and more
gas molecules simply deposit at the
beginning collects at the beginning of
the at the top of liquid actually more
and more goes from liquid to the gaseous
state then we will have collisions
within those gas molecules and due to
that collusion there's going to be a
kinetic energy transfer from one
molecule to the other molecule then we
will have a gain kinetic energy
distribution in the gaseous phase and
that means that the sum of the molecules
actually will have low kinetic energy
some will have higher kinetic energies
those molecules now which have lower
kinetic energy than the minimum kinetic
energy that required to escape from the
liquid when those simply comes close to
the surface they will be attracted by
the liquid molecules and that means they
will go back to the liquid state from
gas to the liquid and this is
condensation so as the number of gas
molecules increases above the liquid
and more and more gas molecules will go
from the gas to the liquid state that
means condensed that means that the rate
of condensation will increase as the
time goes on again rate of condensation
means that the number of gas molecules
that goes into the liquid phase per unit
time right so now when we reach to this
point here after time T let's say that
means that the rate of condensation and
the rate of evaporation becomes equal
what does that means as I explained last
time that means that in one second for
example if hundred molecules evaporate
and going into the gaseous phase and at
the same time hundred molecules
condenses goes from the gases to the
liquid state so that means we have an
equilibrium between the liquid States
and the gaseous state and this
equilibrium is called dynamic
equilibrium why because the evaporation
and condensation process still go on but
the amount of gas above the liquid and
the amount of liquid here stays the same
right and this is simply an equilibrium
and dynamic equilibrium now at that
stage actually when we reach to the
equilibrium the pressure of the gas
above this liquid which is equilibrium
with the liquid is called the vapor
pressure of that liquid vapor pressure
of that liquid now let's think about
that how the vapor pressure of the
liquid is affected by intermolecular
forces as we can simply show it here
that if the intermolecular forces in the
liquids increases that means that this
minimum kinetic energy goes up here like
this that means that the number of
molecules at the surface which is
capable of escaping from the liquid to
the gaseous state will decrease that
means that the rate of evaporation will
decrease that means that we will reach
to the equilibrium point in a shorter
time that again means at equilibrium we
will have less gases gas form of that
compound above the liquid at equilibrium
what does that mean that means low vapor
pressure or low vapor pressure so this
simply means that the stronger the
intermolecular forces between the liquid
molecules the lower will be the vapor
pressure of that liquid right that's we
have let's keep that in mind now suppose
that we increase the temperature I have
it I guess yes another slide which shows
better sort of now suppose that we
simply have increase the temperature of
the system that means we increase the
temperature where we have these
allenmeyer flasks and the liquid in it
close what will happen increase in the
temperature will increase the kinetic
energy of the molecules in the liquid or
in the gas phase now increase in the
kinetic energy of the molecules at the
surface means that we will have more
molecules which is capable which are
capable of simply escaping from the
liquid to the gaseous States that means
that the rate of evaporation
at higher temperature will be like this
up here right higher rate of evaporation
at higher temperatures because more
molecules have larger kinetic energy
that minimum kinetic energy to escape
from the liquid and that means we will
have a equilibrium point over here means
that we will have war gas about the
liquid at the equilibrium point that
means that they higher the vapor
pressure so the result is or we can
generalizes again as follow increase in
the temperature will always increase the
vapor pressure an increase in the
temperature will always increase the
vapor pressure off the liquids now how
much we can increase the temperature of
the liquid and the vapor pressure here
again for three compounds four compounds
we have die ethyl ether and we have
ethyl alcohol water and ethylene glycol
now as you see again for example if you
consider water if you increase the
temperature the vapor pressure of water
will increase increase increase for
example at about 82 degrees centigrade
the vapor pressure of the water will be
about 400 nanometers now increase in
that vapor pressure increase and it will
be 760 millimeter mercury which is one
atmosphere and the temperature you see
again here is 100 degree centigrade so
again we define the normal boiling point
of a liquid and that is the temperature
at which the vapor pressure of the
liquid becomes equal to one atmosphere
or 760 millimeter mercury so when we
simply say the vapor pressure of the
normal normal boiling point of a liquid
we mean that at that temperature the
vapor pressure of that liquid will be
equal to one atmosphere so again as we
see here the boiling point of water is
100 degrees centigrade now if we look at
the ethyl alcohol the boiling point of
ethyl alcohol it reaches to 760 that is
about 80 degree centigrade little less
than 1879 now if you look at the boiling
point of diethyl ether and you go down
over here it is below 40 degrees
centigrade the boiling point but if you
have ethylene glycol that's probably
much higher there again now what all of
these means if you simply compare the
intermolecular forces what we said about
the temperature dependence of the vapor
pressure can you tell me which one has
strongest among these three for example
which one has the strongest
intermolecular attraction forces diethyl
ether or water water right why because
we said if the intermolecular forces are
stronger the vapor pressure is lower and
that simply means that it will reach 200
degrees which is the boiling point here
at higher temperature so the water
actually has stronger intermolecular
forces than ethyl alcohol and also
diethyl ether in case of diethyl ether
no almost the weakest intermolecular
forces now do you remember again what is
the main intermolecular forces in water
what kind of hydrogen bonding that is
the dominant actually we know that the
dipole-dipole are also present there
we know the London forces are also
present there but the dominant
intermolecular force remember hydrogen
bonding is much stronger than the other
two so the hydrogen bonding is
in water now in fo alcohol do we have
hydrogen bond in there too
yes we also have hydrogen bond in there
because we have hydroxide o H group
there which have the hydrogen bonded to
the oxygen and lone pair electrons on
the oxygen
but again the hydrogen bonding
capability between two ethyl molecules
is less than the hydrogen bonding
capability between two water molecules
water molecules may make more hydrogen
bonding again now in diethyl ether we
have oxygen we have two methyl groups
bonded to ethyl groups to the oxygen but
there is no hydrogen bonded so in that
case only dipole-dipole and London
forces are applicable for diantha ether
and it has the lowest vapor pressure at
a given temperature and it has no
highest vapor pressure at a given
temperature and it has the lowest
boiling point so again we can summarize
it as follow the stronger the
intermolecular attraction between the
molecules of a liquid higher will be the
boiling point right the high level
higher will be the boiling point
temperature and how about the vapor
pressure the vapor pressure will be
lower
just the opposite thing right just the
opposite
okay now this is again an equation which
relates the vapor pressure of the liquid
to temperature and Delta H vaporizations
of that liquid and it is called phases
clapeyron equations as we see here now
this p is the vapor pressure of the
liquid logarithm natural logarithm of
the vapor pressure now this is the
enthalpy of vaporization of the liquid
again and T is the temperature or gas
constant this is constant and by simply
using this equation we can generate
actually this equation which is more
useful let me
write it down low grating p2 over p1
vapor pressure of liquid 1 and liquid 2
is equal to minus Delta H vapor pressure
divided by R and over here 1 over t2
minus 1 over T 1 ok so this simply
allows us to calculate the vapor
pressure of a liquid provided that we
know the vapor pressure at a given
temperature and also Delta H
vaporization Delta H vaporization we can
put the values let's say P 1 Delta H
vaporization x' and t2 then we put the
t1 find the pp ones or so on in other
words this allows the thing as I said as
I told you to calculate the vapor
pressure of a liquid at any given
temperature provided that we know the
enthalpy of vaporization of that liquid
for example if I ask you this let me
what is the what is the vapor pressure
of water at 65 degrees centigrade all
right now can we calculate it and Delta
H vaporization is let's say it is given
this is about 47 kilojoules I'm not sure
about kilojoules per mole I'd say for
water I can't remember the summer about
there I like that now can we calculate
the vapor pressure of
water at 65 degrees centigrade by using
this equation okay let let's see now
tell me then let's assume that
temperature t1 is less than temperature
t2 and let's call the temperature t1 is
equal to 65 degrees centigrade now p1 is
we don't know that's what we count we're
supposed to calculate then what we need
here T 2 and P 2 corresponding to this
back actually the vapor pressure of
water at t2 and whatever that Peters do
we know these t2 n p2 for water for
example peep yes exactly
760 or one atmosphere that's that's it
that's it so if you just simply put the
values here t2 in this case let's say
that 100 degree centigrade but it has to
be simply put in Calvin these you have
to be put in Kelvin remember have to put
in Kelvin so this is going to be 373
Kelvin like this t2 and p2 is 760
millimeter 1 atmosphere 760 millimeter
mercury or one atmosphere now if I put
these values I know the Delta H
vaporization and I know simply the t2
value 100 degrees I know the vapor
pressure at that temperature and I can
put the T 165 calculated p1 clear
actually the question is we can just ask
you given this value here we
to calculate the vapor pressure of water
at that temperature or any liquid
actually right and indicate so remember
you always have this that means if you
know the boiling point of that liquid
you know that the vapor pressure at that
temperature is also 760 millimeters
mercury or one atmosphere so using that
information you can put the values and
calculate in fact the the vapor pressure
in this case water at 65 degree
centigrade clear so we can we can simply
use the crisis clapeyron equation to
solve problems like this ok now phase
diagrams are also important for
substances and it simply tells us or
shows the relationship between the
pressure temperature and the state of
matter right pressure temperature and
the state of matter that means if
somebody says what is the state of this
compound at this temperature and this
pressure you need to look at the phase
diagram of that compound of that
substance if you have a phase diagram of
that substance then you can simply find
out at a given temperature and given
pressure what will be the state would be
solid will it be liquid with be gas or
it may be actually the equilibrium
between those states right between
liquid and gas and between solids and
the liquid and solid and the gas and so
on this is actually a phase diagram
again for a compound doesn't tell us
exactly
the compound is what what the substance
is again but it says let's read it again
here a phase diagram is a graph of
pressure versus temperature for a
substance it shows Mountain boiling and
sublimation points at different
pressures the triple point and critical
point also right now let's look at this
figure here now in a phase diagram
we actually have lines like this and we
also have the areas and we also have the
intersections of these three lines here
there's only one point like that right
now the simply area shows the single
state of the substance
for example this part here is the solid
state for this substance this is the
liquid state this is the gaseous state
what does that means that means is this
for example if you just take this point
and come down over here find the
temperature and count down the pressure
here that simply means that at this
pressure and this temperature that
substance will only be in the solid
state that's it right or for example if
you pick up a point here take the
pressure of this point and the
temperature and at that temperature and
pressure that means that the substance
will only be found in liquid and the
same thing is true for the solids so
liquid solid gas excuse me gaseous phase
over here now how about the point on the
lines for example the line which
separates the liquid and a gas if you
just take this point here for example
and take the pressure and the
temperature this pressure actually
corresponds to the vapor pressure of
this liquid at this temperature so as
the temperature increases you see the
pressure increases the vapor pressure
increases or if you take a point over
here and that point here
chili represents the equilibrium between
the solid and liquid take that
temperature and come down over here any
point here is the equilibrium between
solid and liquid any points here on this
line equilibrium between liquid and the
gas here any point between the solid and
the gas and if you go from this state to
that this is depositions again right and
this is this way is meltin this way is
freezing again phase changes this way is
vaporization condensation this is all
how about that that is the triple point
what does that mean is that there's only
one point like that actually in every
phase diagram and what does that mean
that simply means at this temperature
here if you take the temperature there
and this pressure three phases are found
in equilibrium with each other so you
can have gas phase you can have the
liquid phase and you can have the solid
phase all in a closed system equilibrium
so as long as that temperature and
pressure will never change that means
that we will have the liquid gas and the
solid of that substance at that
temperature and pressure now how about
this point here this point is the
critical point we have defined that
again remember about this temperature
for example you will never be able to
liquefy a gas and make it liquid and we
will simply have the super critical food
properties over here and that is the
critical temperature now this is the
phase diagram of water a little bit off
shape the ones that I get used to
actually but you see again the critical
temperature and the critical pressure
for the water is pretty high right and
why because very strong intermolecular
forces which is the hydrogen bond in the
liquid case but one point here and that
is
the line between the solid and the
liquid this is more like not you know it
is inclined in this direction sort of
like rather than that actually I should
probably show you a little bit
exaggerated that and it is actually like
this or the water more like that this is
the solid this is the gas this is the
liquids like this now usually usually
this line that is the equilibrium
between the solid and the liquid is not
in listening time not sort of this
direction but rather than this direction
like this
and the reason for that is hydrogen
bonding hydrogen bonding and that
actually has a lot of important results
for example ice skating on the ice is
only possible because that this line
inclined in that direction rather than
that way why let's try to explain that
now this is remember the temperature
temperature here this is the pressure
increase in temperature increasing
suppose that we are somewhere over here
or just like right there and there's
temperature let's say this is minus
let's say 10 degree centigrade like that
and that pressure is about somewhere
about let's say 1 atmosphere okay like
this now this part is simply at this
point you have only ice and now you're
standing on ice with the ice-skaters ice
skaters remember those are special kind
of shoes and you simply have
and irony l-- right under those very
thin very narrow
you probably remember the skate ice
skating skates now when you just stay on
the ice like that Hama skates
what does that means we apply pressure
to their right where wherever the skates
are they are very actually narrow
they're about two or three millimeters
wide right
those skates lies there Steel's actually
now when you have the pressure on that
area on the ice
what do that means the temperature is
the same you go in this direction right
now what does that means going in that
direction temperature the same going
increasing the pressure simply this way
we actually go from solid to the liquid
state right that means that a very thin
layer of liquid water will form right
under your skates blades off this case
actually right now that makes the ice
slippery actually you have a liquid
between your skate and the solid and
that makes it slippery and because of
that slip furnace actually you're able
to skate on the ice but you won't be
able to skate for example in any other
solid materials right now as soon as you
go to a different place then when the
pressure simply took off from that point
it immediately freezes back again but
wherever you go with this skate
that means there's always a very thin
layer of liquid water between your
skates and the eyes and that makes it
slippery and that makes the ice skate
and possible
now so that means if you take a nice and
just apply pressure it will melt and
this is again causes the density of ice
at zero degree centigrade less than the
density of liquid water at zero degree
centigrade there is very few I guess
bismuth is another substance that has
this property that means lower density
in the solid than in the liquid usually
the density in the solid is higher than
the liquid why because when you have
solid water ice the space between the
water molecule increases compared to the
space between the water molecules in the
liquid state why because the water
molecules arrange themselves in a such a
way to get the maximum hydrogen bonding
between each other and you have these
hexagon type shapes of the water
molecules and that's what the snowflakes
you know have like very nice shapes and
so on that's due to the hydrogen bonding
that arrangement of the water molecules
to get maximum hydrogen bonding creates
some empty spaces between the water
molecules what does that means that
means compared to the liquid that means
that the density of the ice becomes less
than the density of the water right
that's what it is now
that causes and other things actually
that makes possible the life in the
North Sea and the South Sea and appalls
North Pole and South Pole in the water
why as follows now suppose that the
density of water ice actually is greater
than the had been greater than the
density of liquid at zero degree
centigrade what would happen since the
freezing of the
see for example North Sea or at North
South Pole we'll start on the top of the
sea water right now as soon as it forms
solid ice
since the simply density of life if it
had been greater than water it would
have simply sink down to the bottom of
the sea and this will continue like this
until the whole North Pole sea or the
South Pole sea will be frozen solid ice
what does that means that means there
would be no life there aquatic life
right but since the density of ice at
zero degree centigrade is less than the
density of the water as soon as the ice
forms on the surface of the sea it stays
there and then it forms again just gets
thicker and thicker and thicker but
never ever gets to freeze the whole sea
right because after let's say 1 meter
thick ice formed on the top that
insulates the water below and the water
below stays always above zero degrees
centigrade and there is aquatic life
continues there that's another thing and
that is also due to the hydrogen bonding
you see again there's a lot of things
that you can simply explain by the
hydrogen bonding and they're good things
as a matter of fact ok ah yes now this
is the phase diagram of carbon dioxide
you can simply examine these and you see
again unusual features of carbon dioxide
cannot exist in the liquid state at
pressure below 500 5.11 atmosphere and
that is the triple point here and then
carbon dioxide sublimes at normal
pressure and the temperature you know
for example this is one atmosphere
pressure and my sphere like this carbon
dioxide will sublime at this temperature
simply from the solid to the gas that's
again the information critical
temperature
for the carbon-dioxide much much lower
you see again 31.1 and centipede that
means about thirty one point one degree
centigrade it would be impossible to
liquefy or solidify carbon dioxide it
will always be in the gaseous state now
in other things there's liquid crystals
liquid crystals now we said that the
solids the molecules atoms or the ions
that makes the solid are not free to
move and the solid they make vibration
of motions but they're fixed there and
that's why if you take a solid it put in
a container it will never take the shape
of that containers the shape will still
stay saying stay the same like that but
when you go into the liquid state then
the water molecule actually the
molecules liquid molecules are free to
move but they have to stay close
together to just slide by each other and
move freely in the liquid as a matter of
fact but sometimes some compounds
actually when you reach to the mountain
point and increase the temperature they
still keep the regular orientations or
arrangement of the molecules close to
the solids close to the solids but they
are more liquid more free to move
compared to the solid but still they're
simply arranged very regularly in that
liquid I should say liquid form what you
can call it liquid form and that means
you can look at that considering the
motions of the molecules like a liquid
but if you think about the orientation
arrangement of the molecules they look
like more crystals so these are called
liquid crystals they have actually
crystalline properties but they also
have the liquid properties you know they
also have the liquid properties for
example here we have the compound this
is methyl hexane I guess or some other
organic compound now you see between 145
degrees centigrade 179 degrees
centigrade it behaves like a liquid
crystalline phase liquid crystal that
means it behaves like a solid but it
says the liquid properties so we call
that liquid crystals now about 179 for
example it would be liquid phase now
below 145 it would be solid phase right
but between those temperature here in
that range it will behave like a solid
and like a liquid and we call that
liquid crystal
besides in listen to immediate state
liquid decreases they have some trades
of solids and some of liquids molecules
in the liquid crystal have some degree
of order that this is what it is crystal
actually now here are how the orders
means liquid crystals and they are given
different names now in nematic liquid
crystals molecules are only ordered in
one dimension along the long axis this
is the nematic like this and then
smectic liquid crystals the molecules
are ordered in two dimensions along the
long axis and in layers this is over
here like this in one layer two layers
now here is this is somatic aided
somatic see just the different
orientations this is another cholesterol
like liquid again you see the
orientation you see all of these crystal
properties but the substance is sort of
a liquid state in a liquid state okay
any questions
yes super liquids
supercritical fluids you mean super
liquid she said
ah yes yes yes yes that again I that may
be about the liquids like when you reach
to the freezing point it doesn't freeze
and goes below that temperature and that
becomes a supercooled liquid now this is
also case when we see the solubilities
right when you have a saturated
solutions at certain concentrations you
may if you just add more of that solids
or solid actually in that solutions it
does not precipitate but still stays in
the solutions that's called the
supersaturated solutions so super
liquids again maybe the liquids that
still be liquid below the freezing point
I guess that's what you're saying right
yeah but those are again very unstable
those are very very special like
supersaturated solutions super liquid or
super in fact as we see here
supercritical fluid also has very very
different properties and very unstable
you know
is there any honeys no no not the honey
I
I really don't have but they have to be
usually long molecules usually the after
we have a benzene ring and they have to
have dipole-dipole interactions as a
matter of fact so they will simply align
in one directions they will ordered in
one directions there are quite a number
of those actually yes
that can stayed in liquid at the same
time that's that's what oh yeah sure you
can you can you can now if you simply
have the triple point temperature and
pressure you will have the solid liquid
and gas in a closed system you have to
have the closed system right it has to
be closed system and there's going to be
the vapor and then is it going to be the
liquid form and the solid form actually
there are also vapor pressures of solids
which we haven't talked here just like
liquids in fact if you go to lower
temperature lower than zero degree
centigrade for example in the case of
water and the ice in a closed system
will have the vapor form of it which is
in equilibrium with the solids even
though the vapor pressure would be much
lower than the vapor pressure of the
liquids but the solid do also have vapor
pressures too so only in a closed system
only one temperature and pressure that
triple point temperature and pressure
you can have three states in equilibrium
they will be existent together in
equilibrium actually okay I guess we
will stop here let's not start chapter
twelve next hour okay let's have a break
on ten minutes please come back