Enthalpy changes during Phase Transformations Video Lecture | Chemistry Class 11 - NEET

welcome to enthalpy and phase changes we
know that a substance in the gaseous
state has more energy than that same
substance in its solid state so that
tells us that in terms of energy gases
have the most energy and solids have the
least energy so in order to change a
solid into a liquid or liquid into a gas
we have to add that energy and that
energy that's added typically comes in
the form of heat so for the solid to
liquid phase change and the liquid to
gas phase change these are clearly
endothermic processes and then implies
that phase changes in the other
direction gas is condensing to a liquid
and liquids freezing to solid that these
must be exothermic changes and we can
also throw in sublimation and deposition
sublimation is a solid becoming a gas
directly that has to be endothermic and
deposition being a gas going directly to
the solid phase so that's exothermic
because it's going down in energy so
clearly heat is involved in these phase
changes the question is how much heat is
needed for a phase change now to talk
about a phase change we have to be
talking about a specific substance
because this question is going to be
different for every substance so for now
let's use water as an example of what
happens to a substance as heat is added
here we have what's called a heating
curve for a substance this case the
heating curve is for water a heating
curve generated by measuring the
temperature of a substance as heat is
added at a constant rate so because this
substance is water there's a couple
temperatures that are important to us
first of all 100 degrees is right here
and zero degrees is right here those we
know to be the melting and boiling
points of water so that means in this
area the water is solid because it's
below zero in this area the water is
liquid because it's between zero a
hundred and above hundred is over here
where the water is a gas so before we
talk about phase changes let's talk
about where there is no phase change we
have this gas liquid in solid regions
clearly there's a temperature change
involved because during these segments
you can see the temperature is
increasing and because the temperature
change is involved we know that specific
heat capacity must be involved somehow
as well
that the equation q equals M times C
times delta T is useful for figuring out
how much heat is required for each of
these temperature changes we've already
talked about that in a previous video
let's focus on these flat regions and as
you might predict based on where we
label the regions of solid liquid and
gas these flat regions represent the
phase changes in this larger flat region
we have h2o as a liquid becoming h2o as
a gas and in the lower segment we have
h2o as a solid becoming h2o as a liquid
and these phase changes have specific
names this top region this top one is
called vaporization and this bottom flat
region where the solid is melting to a
liquid we call that fusion so this
region down here is referred to as
fusion and this upper plateau is
referred to as vaporization now a couple
things to point out about these phase
change areas the vaporization and fusion
plateaus first of all there is no
temperature change what does that mean
it means that there's no change in
kinetic energy but that should be odd
because heat has been constantly added
this whole time so if heat is being
constantly added how come the
temperature suddenly stops changing for
a period of time and that's because the
heat is absorbed as potential energy so
during a phase change there's no change
in kinetic energy but there is a change
of potential energy and that should make
sense because molecules in the gaseous
state need to be further apart than
molecules in the liquid state molecules
in liquid state need to be further apart
than molecules in the solid state and
potential energy is the energy of
position so that's a major idea for this
section that phase changes involve a
change in potential energy but no change
in kinetic energy now we're going to
focus more on this heat part we already
know how to calculate the heat during a
temperature change for those regions
where we have the solid liquid and gas
heating up we can use the equation Q
equals M times C times delta T the
specific heat equation to figure out how
much heat is used for those temperature
changes but we also need a way of
looking at how much
heat is needed for the phase change this
fusion plateau and the vaporization
plateau and we clearly can't use a
specific heat equation because there is
no change in temperature so let's take a
closer look at those two regions so here
we have the reactions for fusion and
vaporization both of these are
endothermic processes which means they
both have positive heat of reactions and
we actually call the heat of reaction
for these something special for the
solid water becoming liquid water that's
Delta H fusion the heat of fusion and
that is six kilojoules per mole and for
vaporization it looks similar we have
Delta H for the vaporization the heat of
vaporization and that's equal to forty
point eight kilojoules per mole now
these values are specific for these
reactions so a substance other than
water would have a different heat of
fusion and a different heat of
vaporization so one of these heats of
reaction tell us well the heat of fusion
tells us that it takes six kilojoules of
heat to melt one mole of ice and the
heat of vaporization tells us it takes
forty point eight kilojoules of energy
to vaporize one mole of water now
sometimes is useful to have information
in terms of moles but typically when
we're melting something or vaporizing
something we know the mass of it so it's
going to be more convenient if we know
the amount of energy per gram needed to
do these phase changes and the
conversion is fairly simple if you just
use the molar mass but for these
reactions so for water the heat of
fusion is 334 joules per gram so it
takes 334 joules for every gram of water
to melt we have a similar value for the
vaporization and that the heat of
vaporization H sub V becomes two
thousand two hundred and sixty joules
per gram so how do we use these
constants H F and H V well there are two
very similar and very easy equations
that shows how to use them Q equals M
times H F and Q equals m times H V these
equations allow you to calculate the
heat Q for a particular mass of a sample
given the amount of energy needed for
the phase change let's take a look at an
example that lets us use
one of these equations this question is
asking how much heat is absorbed to
completely melt 20 grams of ice at zero
degrees Celsius the key term for this
example is melt that tells us that we're
dealing with heat of fusion so we want
to use the equation Q equals M mass
times the heat of fusion the next thing
I want to do is identify my variables
it's asking how much heat so Q remains a
variable it also gives me the mass of
the sample as 20 grams of ice and the
heat of fusion value depends on the
substance itself so for ice for water we
said is 334 joules per gram so now that
I have my variables identified I can go
ahead and plug things in so Q equals 20
grams times 334 joules per gram which is
equal to six thousand six hundred and
eighty joules and that's the amount of
heat needed to be absorbed to completely
melt 20 grams of ice at zero degrees
Celsius
that wraps up our lesson on enthalpy and
phase changes write down any questions
you have in your notes and bring them
with you to class