last time remember we were talking about
the intermolecular forces and again we
said that these intermolecular forces
are much weaker than chemical bond
energies chemical bonds but they
determine very important physical
properties of the compound particularly
the liquids and what kind of physical
properties for the liquids particularly
Mountain points for the solids again
melting points boiling point and for the
liquid vapour pressure viscosity surface
tension and all of these are simply
determined by the intermolecular forces
and let's go through again the types of
intermolecular forces one is the
dispersion forces in fact this is also
called London forces dipole-dipole
forces and hydrogen bonding and ion
dipole forces
now we talked about this dispersion
forces and dispersion forces we said are
found almost in all substances and it
operates between all kind of molecules
and dipole-dipole forces are only
present between two polar molecules and
hydrogen bonding is a special type of
force attraction force between the
molecules with where you should have a
hydrogen bonded to a most
electronegative element those are three
actually fluorine oxygen and nitrogen
and you should also have the lone pair
electrons on these most electronegative
elements and we try to explain those
last time remember and the ion dipole
forces are the forces between an ion and
a dipole and this is usually present in
solutions because you can have the ions
in solutions and you also have dipole
molecules in solutions and when you have
iron ions in solutions and the dipole
polar molecules actually in solutions
then you will have ion dipole
attractions let's go quickly again we
explain how dispersion forces or London
forces operates between molecules
particularly important for nonpolar
molecules if you have a non-polar
molecule or nonpolar antem the
attraction force is only actually
dispersion forces between them and again
dispersion forces we said he depends on
the number of electrons in the atom or
the molecule size of the atom and shape
of the atom again the more electrons you
have in the atom or the molecule that
means the greater dispersion forces will
exist in that compound and the shape
also affects the dispersion forces too
and this is again polarizability also
determines the
dispersion forces between nonpolar
molecules if it is easy to polarize a
molecule that means move the electron to
more one side and have Delta positive
and Delta negative side of the molecule
or the atom in that case you will have
larger dispersion forces actually now
again dipole-dipole interactions are
only present between two polar molecules
and again positive side of molecule
attracts the negative side and this is
the dipole-dipole interactions or
dipole-dipole forces again as
intermolecular forces now as the gain
last time we said dipole-dipole
interactions is simply determined by the
dipole moment of the molecule the higher
the dipole moment the stronger will be
the dipole dipole forces in that
molecule and depending on those forces
again dipole moments and I pulled up all
interactions the boiling point of the
compounds changes as we will see later
more the higher the attraction between
the liquid molecules the higher will be
the boiling point of the molecule now
which have a greater effect this is
again when you have all of these
intermolecular forces dispersion forces
dipole-dipole hydrogen bonding which is
more important which is dominant which
has more effect now if the molecules it
says are comparable size and shape
dipole-dipole interactions were likely
to be dominated force that means if you
have two molecules and you try to
compare the intermolecular forces
between those two molecules and if the
molecules have about the same molecular
weight that means about the same size
then you look at whether the molecules
are polar molecules if one is polar the
other one is nonpolar then the
intermolecular forces in the polar
molecule will be greater than
nonpolar molecules because you assumed
there that the London dispersion forces
in fact will be the same because they
have the same molecular weight the same
size about the same number of electrons
probably now if one molecule is much
larger than the other dispersion forces
will determine its its physical
properties particularly when we have
very large molecules dispersion forces
becomes the dominant force determined in
the physical properties of the compound
now again this simply shows how the
hydrogen bonding changes the physical
property of the liquids in fact in this
case the boiling point of the water
molecule hydrogen fluoride and ammonia
is much higher than what we expect
actually comparing with the others in
the same group and the reason is because
they have hydrogen bonding in fact
hydrogen bonding is possible between two
water molecules or two hydrogen fluoride
and two ammonia that simply makes the
boiling point of these compounds of
these liquids in fact much greater than
what we expect this is again explains
how the hydrogen bonding we talked about
that last time now this is the ion
dipole interactions again when you have
for example sodium chloride this is
solid crystalline solids and you
dissolve it in water simply in water
then we write it again as followers it
dissolves in water we have sodium plus
ions and plus chloride minus ions in
aqueous solutions this 8 2 simply says
that it is water the solvent in this
case is water now these simply means
this the equated sodium plus ion is the
water molecules these are their h2o
water molecules over here h2o
and another h2o like this and remember
the water molecule
are polar molecules what is that means
one side is positive the other side is
negative so when you have sodium ion in
solution positively charged sodium ion
in solutions then the Delta minus side
of the water molecule will simply
interact with the sodium plus cation in
fact and we have ion dipole interactions
actually right so let's say this is
Delta minus Delta plus Delta minus and
Delta plus remember the water molecule
one side a little bit positively a
little bit negative so when you have
actually iron in water and water
molecules will surround this ion and
there's going to be attraction between
that ion and the water molecule which is
a polar molecule and that's why we call
that ion dipole interactions actually
this ion dipole interactions determines
the solubility affects the solubility of
the solid that dissolve in water
if this attractions in fact myung dipole
attractions is not present then the
sodium chloride will not dissolve in
water
Mata's alva and usually the ionic
compounds dissolve in polar solvents
such as water and the reason is because
ion dipole interactions is very strong
interactions as a matter of fact okay
now this is again summarizes
intermolecular forces we should probably
look at here
dispersion forces only nonpolar
molecules again if you have a non-polar
molecule such as methane and bromine
molecules then the only intermolecular
forces that operates in that compound
those compounds will be dispersion
forces and the
dispersion forces again changes between
0.1 to 30 kilojoules per mole 30
kilojoules per mole again remember the
chemical bond the weakest bond
corresponds to about one hundred and
hundred and fifty kilojoules per mole so
what does that mean that means that the
dispersion forces dispersion are much
weaker than chemical bond forces
actually now if you have a dipole-dipole
molecule again polar molecules then we
have the dipole dipole forces will
operate now remember even input between
two polar molecules dispersion forces
also present their dispersion forces
always there but when you have simply
polar molecules dipole dipole
attractions then the the dispersion
forces is not that important in that
case now again between the polar
molecules as over here the energies of
the attraction corresponds to about
between 2 and 15 kilojoules per mole
again you see that the range of
dispersion forces is bigger than the
dipole dipole interactions I should say
particularly if you have large molecules
big molecules than the dispersion forces
or the energy corresponding to that
dispersion forces attractions is much
higher than the dipole dipole forces
attraction energies now hydrogen bonding
again only present in compounds that has
those hydrogen bonded to the most
electronegative element and also a lone
pair and those most electronegative
element if you look at the amount or the
the range of the energy corresponding to
hydrogen bonding between 10 and 40
kilojoules per mole what does that means
that simply means that among these three
years the most important interactions is
the hydrogen bonding intermolecular
interactions is the hydrogen bonding
because you see it goes up to 40
kilojoules per mole for
now I enjoy both forces as I try to show
you over here and that is pretty strong
also such as it says sodium chloride
dissolved in water right and the
energies corresponding to that ion
dipole force is about 50 kilo joules or
higher so that's pretty strong as a
matter of fact now if you have ionic
compounds in solid a game like potassium
bromide ammonium nitrate these are eye
on ik compound solid and the ionic bond
energy here remember is determined by
the lettuce energy is about 150
kilojoules per mole again you see this
is considered as a chemical bond ionic
bond much higher than the other
intermolecular forces that causes the
interaction energies again now our
liquid property is affected by
intermolecular forces remember we said
that these forces intermolecular forces
are much weaker than the chemical bond
that is intramolecular forces but they
do have very great effect on their
physical properties and what are these
physical properties boiling point
actually we talked about that a little
bit and the mountain point of solids and
vapour pressure we will talk about here
and the boiling point
viscosity surface tension and capillary
action particularly these three boiling
point viscosity surface tension
capillary
relates to the liquids more to the
liquids as a matter of fact now let's
look at these now viscosity is simply
define resistance of a liquid to flow is
called viscosity right now here we have
simply same diameter holes here these
are the same diameter pipes like this
and we have oil simply falling down here
pouring down here and you see again this
oil has much higher
viscosity than this one because if you
start filling them at the same time you
see much less liquid oil passes through
the same hole same sized hole here than
here that means that this oil is
resistance to flow and we call it more
viscous than this so if he compared the
viscosity of these two oils this is
actually more viscous or higher
viscosity this is lower viscosity again
now Miss Cassity increases with strong
intermolecular forces and decreases with
higher temperature now this is really
important
the stronger the intermolecular forces
between the molecules of the liquid the
higher will be the viscosity right and
the weaker the attraction between the
molecules the lower will be the
viscosity again now these two actually
sae 40 and sae 10 do you know what they
are they're actually if you ever used if
you ever put engine oil in your engine
car engine right so you use the oil
which have sae 40 this indicates the
viscosity of this oil or sae 10 now
which one is more viscous this one is
more viscous right higher viscosity this
one is lower viscosity
now since the this Cassetti decreases
with increase in temperature let me ask
you this one of this oil is used in the
wintertime the other one is used in the
summertime which one for the winter and
which one is for the summer
first one is for what summer right
because then the temperature is high in
the summer so the viscosity will drop
and this one is for the winter right
usually again right now they have the
engine oils which comprises both of
these properties one kind so you put
that in the summer and in winter too but
in all times you have to change the
engine oil into winter time and also in
a summer time so because you have to use
different viscosity oils again in the
summer time you use high viscosity oil
in a very time you use low viscosity oil
to make the engine properly work
function again right okay again you see
here the viscosity of series of
hydrocarbons the viscosity increases as
we go down why because the
intermolecular forces increases what
intermolecular forces are present among
these hydrocarbons dispersion forces
right only dispersion forces because
these are not polar molecules they're
nonpolar molecules only intermolecular
forces the dispersion and remember the
larger the molecule the higher the
dispersion force means that they came
for example here have highest
intermolecular forces and its viscosity
is highest higher the viscosity actually
higher the intermolecular forces or
vice-versa now surface tension and is
another property of liquids which is
determined actually by the
intermolecular forces here again we have
water and you have a bug which simply
stays on the water it can walk on the
water without immersion into liquid
water as a matter of fact because water
says act as if has a skin on
due to extra inward forces on its
surface it says those forces are called
surface tension let's try to examine
what is surface tension means what is
the origin why we have surface tension
right off the liquids and how it changes
now let's consider this suppose that
this is a liquid and these are the
molecule may be just water okay this is
just water in a beaker like this and
there's our this is the surface of the
liquid and this is the bulk inside the
ball now since there is intermolecular
forces present between the molecules of
the liquid and now let's consider two
molecules one is on the surface this
molecule here the other one is in the
ball this molecule here and think about
the forces intermolecular forces that
acting on those now if you consider this
one here so this molecule is attracted
by a force with this molecule and this
molecule this molecule that molecule and
so on now if you sum up all of these
attraction forces the net result will be
zero right now that means that there is
no net force acting in any direction for
this molecule which is in the ball in a
liquid now consider the molecule at the
surface this molecule so this molecule
will be attracted by this molecule here
in this direction and that directions
this direction and that direction if you
take the combinations of all of these
forces it will not be equal to 0 it will
be equal to a net force which will be
downward here what does that means that
means that all the molecules at the
surface of the liquid will be pulled in
by a force into the liquid right so now
this simply creates a tension at the
surface and and that simply makes the
surface tension possible again
so now that
is why actually liquid molecules
particularly have strong intermolecular
attractions try to take the shape of a
sphere if you take it water drop and the
shape of the water drop sort of
spherical right spherical or if you take
liquid mercury and just drop it it is
almost spherical right almost vertical
why because the intermolecular forces
try to minimize the surface area of the
liquids right because all the molecules
at the surface are pulled in so the
liquid in fact which have strong
attractions try to minimize its surface
area and the minimum surface area for a
given amount of substance will only be
obtained if the shape of that substance
is sphere right so that's why actually
strong intermolecular attractions if
present in a liquid and it's drop will
be more like a sphere now in the mercury
cases mercury has very very strong
internal inter atomic attractions there
in the liquid mercury
and if you take a liquid mercury drop
and put it on the table it will be
almost spherical almost spherical but
the water is also like that so actually
the surface tension related or the
measure or I should say off the surface
session is the amount of energy required
to create unit amount of surface from a
liquids right and that creating a
surface will require energy because all
of the molecules at the surface will be
pulled into the liquid so the liquid
actually will try to minimize its
surface and the energy required to make
that surface area is the viscosity
actually the unit of the viscosity is
also a meter per Joule per meter per
against second
okay clear now again how does this
surface tension changes as you can
imagine the stronger the intermolecular
forces between the liquids the higher
will be the surface tension of that
liquid right the high will be the
surface tension of the liquid now here
we have two kinds of interactions or
attractions cohesion and adhesion these
two terms now intermolecular forces that
bind similar molecules to one another
are called cohesive forces what does
that mean that means if you consider
liquid water in a beaker a glass of
beaker right now they're the attraction
between two water molecules is cohesive
forces actually right now the
intermolecular forces that bind a
substance to a surface surface are
called adhesive forces so again the
attraction force between the water
molecule and the glass if the water in a
beaker glass beaker for example that
kind of force is called adhesive forces
now depending on the intermolecular
forces actually sometimes we have
cohesive forces are greater than
adhesive forces and sometimes adhesive
forces are greater than cohesive forces
now that results what we simply know as
the capillary action of these liquids
now what is a capillary action the
capillary action is again as follow
suppose that you have a liquid in a
beaker right if you dip into that a very
small diameter tube glass tube into that
that's called the capillary actually and
what you will see depending on whether
the adhesive forces greater than
cohesive or
greater than adhesive you may have that
the liquid will start rising up in that
capillary tube or it may simply go down
below the surface area in that capillary
tube let's see how that is possible
again this is what the capillary action
here now we have water and the mercury
again two liquids now in the water cases
this surface is convex type that means
like this fashion like this now here in
the mercury it is convex concave type it
is upwards like this now again in the
water case the adhesive forces that
means the attraction between water
molecule and the glass surface which
have also molecules there right is
greater than the cohesive forces between
two water molecules so what really
happens because of this the water in
fact try to increase the contact area
with the glass because the more contact
area between the water and the glass
means stronger attractions and that also
corresponds more stable system energy
that will simply the energy will be
lower in that case now on the other side
over here in the mercury case so the
cohesive forces that means the
intermolecular force between the mercury
atoms is much greater than the mercury
and the glass molecules and that means
that the mercury try to minimize the
contact area with the glass and the
mercury and that's why it has the
concave type shape over here meniscus
actually these are called the meniscus
and that's what happens now what does
that simply means again in capillary
actions so if you have water here and
the mercury here two liquids and if I
dipped a capillary tube into the water
again I will see that the water inside
the capillary tube will rise up to a
certain level right now in the case of
mercury the inside the capillary the
mercury level will go down right this is
what is called the Kepler reaction and
it's all again depends on the magnitude
of adhesive versus cohesive forces
adhesive and cohesive forces and it
again depends on intermolecular forces
again right usually if you have a very
strong cohesive forces intermolecular
attractions intermolecular forces in a
liquid then it will behave like a
mercury and again if the intermolecular
forces are weak it will behave like
again water but we should always compare
it with the adhesive force adhesive
force that means the liquid molecules
and the surface of the container or the
tables again okay now remember again all
of these properties are also affected by
temperature
just like in viscosity here the
capillary action is also affected by the
temperature also okay now phase changes
again is simply conversion from one
state of matter to another is called
phase change what does that means that
means going from solid to liquid or from
liquid to gas or just the other way
around from gas to the liquid from
liquid to the solid or solid to the gas
gas to the solid are called phase
changes so actually states here the
state of the matter is called face the
same things actually some kind of phase
changes or state changes and so on now
energy is either added or released in a
phase change so we should know whether
we are we're going to add energy or get
energy from the phase changes in other
words we have to know whether the
process of phase change will be
exothermic or endothermic right now we
can actually guess that now phase
changes Mountain freezing vaporize and
condensed ins subliming deposit and
these are the phase changes here are the
definitions of those actually if you go
from this is the liquid over here and
this is the gas okay
liquid and this is the solid and a gas
between the liquid and the gas right
here now if you go from liquid to the
gas it's called vaporization now if you
go from gas to the liquid it is called
condensation which one is exothermic
which one is endothermic vaporisation is
what exothermic or endothermic
endothermic now the from gas to the
liquid condensation is exothermic right
energy will be released or energy will
be absorbed
now here between the liquid and the
solid we have freezing and melting or is
called fusions the same things actually
now if you go from solid to the liquid
it will be endothermic process energy
have to be absorbed now if you go from
liquid to the solid freezing will be
exothermic now then if you have gas to
solid skip in the liquid phase that's
called actually sublimation and the
reverse of that is called depositions
this is going to be actually exothermic
and this is going to be endothermic
process and the turbine process now okay
let's look at the the energy change
change of state actually and we said the
process will be exothermic or
endothermic now we're saying how about
the amount of heat released or amount of
heat absorbed and that corresponds to
the energy changes here again
we summarized it the heat of fusion is
the energy required to change a solid
that it is mountain point to a liquid
now this is important right so when we
talk about the heat of fusion of water
and this may be actually specific heat
of fusion type or molar heat of fusion
type what is the molar heat of fusion
type that simply means that the amount
of heat actually required to melt 1 mole
of ice to 1 mole of liquid at what
temperature 0 degree centigrade melting
point so we should remember that this is
what it is at mountain point of the
liquid actually or if you're talking
about the specific heat of fusion that
will be one gram of ice actually melting
again at 0 degrees to have 0 degrees 1
gram water that is the specific heat but
usually we use molar heat of fusion or
molar heat of vaporization now the heat
of vaporization is the energy required
to change a liquid at it is boiling
point to a gas that's and other things
again when we talk about when we give
you the molar heat of vaporization for
water that means it is the amount of
heat required to convert one mole of
water at hundred degrees centigrade to
one mole of gas vapor water at hundred
degree centigrade but usually when we
simply use this heat of vaporization or
heat of fusion we don't really pay that
much attention about the melting points
or the boiling point sometimes we say
the chain
change of the heat of fusion or heat of
vaporization with temperature is very
small so we use that for all
temperatures as a matter of fact or at
least for certain range of temperatures
now heat of sublimation is again the
same changes of solid directly to the
gas the same things again now if you
have the reverse of these processes then
the same amount of energy will be
released to the surrounding again we
have here heat of fusion heat of
vaporization heat of sublimation for
different molecules butane diethyl ether
water and mercury now the question is
what factor determines the magnitude of
heat of fusion or heat of vaporization
or heat of sublimation the main things
is intermolecular forces that act in
operating in that liquids or in that
substance the higher the intermolecular
forces I should say the stronger the
intermolecular forces the higher will be
the melting point or the and also the
higher will be the heat of fusion and
the higher will be the heat of
vaporization high will be the heat of
sublimation 's right so these are all
determined by intermolecular forces
because the stronger in fact forces
acting between the molecules it will be
more difficult to make it liquids from
solid to liquid what does that means
that means you have to give more energy
and that means that these values heat of
fusion heat of vaporization heat of
sublimation will be higher you can
actually compare these over here to in
terms of intermolecular for example
mercury has the strongest intermolecular
forces and you see again all of these
values are highest almost for the
mercury than the water and continues
like that now heating curves again are
given simply the state of the substance
temperature and also versus the
he added to that in other words for
example let's consider the heating curve
for water suppose this is for water and
we take let's say 18 grams of water at
minus 25 degrees centigrade let's say 18
grams becomes 1 mol of water at minus 25
degrees centigrade what does that mean
that minus 25 degrees centigrade it is
ice right solid so if you give heat to
this one mole of water simply heat then
the temperature will increase and until
what until zero degrees centigrade over
here zero degrees again is the melting
point now then if you keep giving heat
to that one mole of water zero degree
centigrade ice actually then it will
melt and at this point here it will be
liquid water at zero degree centigrade
again the amount of heat that given at
zero degree centigrade to ice water is
all used up to melt that water and the
temperature does not change right the
temperature does not change and then you
simply from that point on when all of
the simply ice is converted to the
liquid water if you keep giving heat to
that the temperature will increase the
water temperature will increase increase
until it reaches to the boiling point
actually which is hundred degree
centigrade under one atmosphere pressure
atmospheric pressure and then when you
reach there at this point the given
amount of heat if you keep giving heat
to the system will be used to evaporate
that water to the gas and when you reach
to point a here actually then all of the
liquid will be in the form of vapor and
if you keep keep giving heat to that
then the temperature of that vapor gas
water actually will increase as a matter
of fact now if this is one
moral of water for example this heat
this amount of heat from B to C will be
equal to the molar heat of fusion of
water right molar of heat of fusion that
means that the amount of heat must be
given to one mole of ice at zero degree
centigrade to convert that to one mole
of water at zero degree centigrade now
this amount over here again this heat
from D to e actually corresponds to the
molar heat of vaporization of water
right as you see again molar heat of
vaporization is always greater than this
you can also calculate the amount of
heat used to go from A to B using
specific heat of ice actually and the
amount of water and you can also
calculate the heat given from C to e
again by using specific heat of water
and here by using the specific heat of
gas you can calculate the amount of heat
given to that system so this is called
the heating curve and it can be done in
fact for any substance now suppose that
you have a vapor form of water in a
container and suppose it's sent in a
cylinder right which has a piston and
suppose that the vapor is at hundred
degrees and the pressure of the vapor is
one atmosphere now if you simply
increase the pressure by pushing the
piston down on that water vapor at
hundred degree centigrade what will
happen the water will condense and we
will have liquid waters now if you keep
that piston there and increase now the
temperature of the system then again
that water actually condensed liquid
water will be evaporated again you will
have a gasp all gasp but now the
pressure will be greater than one
atmosphere right now if you increase the
pressure again again you will have some
liquid water forming and the gas and
liquid water but now the pressure will
be higher now if you increase the
temperature again that condensed water
will evaporate back and you'll law you
will have in the cylinder all gaseous
water now if you increase the pressure
then you will still have liquid water
liquid and the gas if you continue doing
this increase in the pressure forming
the liquid from the gas increase in the
temperature evaporate in that liquid
increase in pressure and so that you
will reach to a temperature after which
doesn't matter how much pressure we
apply to the gaseous water vapor you
will never able to get liquid water
right liquid water and this temperature
actually for water is about six hundred
and forty seven point six degrees this
corresponds to about 370 degree
centigrade that means about that
temperature doesn't matter how much
pressure you apply to the gas you will
never able to liquify it that means you
won't get the liquid form of that
compound now that temperature is called
the critical temperature now the
pressure at that temperature of the gas
is called the critical pressure so every
liquid has this simply liquid critical
pressure and the critical temperature
critical temperature now can we relate
this critical temperature and critical
pressure to the intermolecular forces
yes
the stronger the intermolecular forces
that means higher will be the critical
temperature and the critical pressure if
you have weaker intermolecular forces
than the critical pressure and
relatively temperature will drop down so
now if you go above the critical
pressure and the critical temperature
what we have there is supercritical
fluids in other words you won't have a
liquid you won't have a all gas but the
state will be actually the substance
will be sort of between liquid and a gas
behaves like a liquid and behaves like a
gas but there will be no phase
boundaries like separations off the
phases like liquid or gas and this is
simply called supercritical fluids and
it is very very useful in chemistry
sometimes to use that as a solvent to
carry out certain separations and
reactions and so on ok now this gives
you again the critical temperature of
the substances and the corresponding
critical pressures again if you compare
these values this increases as you go
down and again the intermolecular forces
actually increases as we go down the
Grieg of attraction intermolecular
attractions increases as we go down and
this is reflected in the critical
temperature and critical pressure now
our vapor pressure okay I guess I can
let's say we try to explain this vapor
pressure of liquids
now this thesis simply gives you the
distribution of the kinetic energy among
the molecules in the liquid and in the
gas also it's very similar like this
this shows number of molecules this is
kinetic energy and what did that means
for example the number of molecules
which have kinetic energy this value
will be equal to that and this value
will be equal to that so again you see
the number of molecules which have the
kinetic energy this correspondent to the
maximum will probably the highest number
highest number and so on and some
molecules will have very high kinetic
energy some molecules will be low
kinetic energy some will be sort of
intermediate now if you increase the
temperature what happens the average
kinetic energy increases you see the
shape of the curve would look like this
what does that means that means that the
average kinetic energy of the molecules
at higher temperatures will be higher
than the average kinetic energy at lower
temperatures actually this is lower than
that again so that is how the kinetic
energy distribution is among the
molecules in the liquid and it's also in
a gas or base a little bit different
shape but very similar shape lines okay
now suppose that I have a beaker and I
put water on it just water this is
liquid water right like this and now if
I have that and put it there that much
water it half-full and come back after
about a week or so what will happen
let's say ten days later one month okay
that means all of those will evaporate
right we'll go from gases to the liquid
to the gaseous state evaporate but
suppose that I have liquid in a beaker
like this water let's say now I put it
on a table here then I put a jar on it
like this a glass jar dumbbell shape
that means I close it this is the glass
again jar put it on that and suppose
that again I have no water gaseous water
molecules here you can assume that
vacuum there or but doesn't have to be
vacuum but just say that there's no
water there then if I come back about
two days later what will happen if the
temperatures are the same
actually the level here will go a little
bit downward now if I come about ten
days 15 days and then we'll still be the
same if it is sealed again that means
it's closed and the temperature is the
same around surrounding temperature is
the same that means they will there will
be water in that beaker forever as long
as the temperature is the same as long
as the system is sealed right now let's
try to examine what is really going on
why when it's open there will be no
water after a certain time but if it is
closed seal the same temperature but
there will always be water there now
what really happens is this let's
consider the molecules at the surface
right so remember again some of the
molecules again at the surface if you
simply look at consider the the kinetic
energy distributions among the molecules
at the surface right it would be
something like this at this temperature
for example something like that now the
molecules water molecules or the liquid
molecules whatever that is and let's say
water molecules stays in the liquid form
why because they are attracted by the
other water molecules right that's why
they're simply staying in water in the
liquid phase
but suppose that some of the water
molecules that the surface has very high
kinetic energy much higher than the
average kinetic energy is that possible
sure that would be less number of
molecules but there will be molecules
that have kinetic energy large now if
this kinetic energy large enough to make
the molecule water molecule break the
attraction forces between the liquid and
the liquid and that molecule will go
from the liquid state into the gaseous
state
right it will escape and then we'll go
into the liquid state likeness now again
the kinetic energy distributions of the
molecule at the surface will never
change as long as the temperature is the
same because when one molecule at high
high kinetic energy is Kym plie scaped
from the liquid went into the gas phase
another molecule from the bulk will come
and replace it so therefore at the
surface as long as we have water at the
surface the number of molecules which
will be capable of escaping from the
liquid to the gas will always be the
same constant again that means that the
number of molecules having kinetic
energy let's assume that that kinetic
energy you know
minimum kinetic energy to escape from
the liquid water to go into the gas is
this amount right here so that means
that at this temperature for example
that many molecules at the surface is
capable of escaping from the liquid
right and it will be the same now what
really happens as the time goes on at
the beginning we have no gas what
molecules then I will have simply
molecules gases vapor whatever you call
water molecules like this and going in
this fashion but here is the point when
you have all of these gas molecules
which have kinetic energies higher than
theirs for example either than that they
will start colliding with each other now
during the collisions there's going to
be a kinetic energy transferred from one
molecule to the other molecule so that
simply means that if I look at the
kinetic energy distribution in the gas
phase molecules here it would also be
something like this like that and that
kinetic energy probably will corresponds
to this point here this is let's say e
kinetic energy of the molecules that
they escaped from the gas from the
liquid again into the gas like this so
what does that means that means that
there's going to be some molecules
in the gas phase which have kinetic
energy less than this amount over here
now what does that means that means that
when those molecules moving around here
come and close to the surface they will
be pulled into the liquid what does that
means that means condensation right so
now here I have evaporation and then I
will have condensation as the time goes
on now I can simply show that by a graph
again as follow the rate again let's say
this is the rate here increase in this
fashion and let's say this is the time
it goes on now the rate of evaporation
will be constant again this is the rate
of evaporation now what is the rate of
the operation means that means the
number of water molecules goes from
liquid state to the gaseous state per
second right that's what the rate is now
since the number of molecules actually
which at the surface considering the
surface which is capable of escaping
from liquid to the gas is always the
same number that means that the rate
will always be the same because the rate
will be proportional to the number of
molecules water molecules at the surface
which have kinetic energy enough to
escape from the liquid to the gas right
so that will be constant now let's
consider the condensation that means the
reverse process in other words the
molecules in a gas phase now due to
their collisions think there's going to
there's going to be kinetic energy
distribution and some of the molecules
in the gas phase will have low kinetic
energy some will be a little higher and
if the kinetic energy of the gas
molecules lower than that value again it
will go into the liquid phase pulled
into the liquid phase so at the
beginning that would be zero but as the
time goes on then it will increase and
let's call it rate of condensation I
guess there's a class here right okay
now rate of condensation means what the
number of molecules going from gas to
the liquid per second after a certain
time the rate of condensation becomes
equal to the rate of evaporation what is
that means we reach the equilibrium
that's called dynamic equilibrium from
that point on as long as the system is
closed the temperature is the same there
will be no change in the amount of
liquids and no change in the amount of
gas above it which is in equilibrium the
process will continue
that means evaporation and condensation
will continue but the rate will be equal
that means that the amount of liquid and
amount of gas does not change and the
pressure of the gas here after that
equilibrium point actually at this point
let's say t0 time is called the vapor
pressure of that liquid at that
temperature okay
so let's continue this next time any
questions okay