Change of state example - Chemistry, Class 11 Video Lecture

I have this problem here from
chapter five of the Kotz,
Treichel, and Townsend Chemistry
and Chemical
Reactivity book, and I'm doing
this with their permission.
So they tell us that ethanol,
C2H5OH, boils at-- let me do
this in orange-- it boils at
78.29 degrees Celsius.
How much energy, in joules,
is required to raise the
temperature of 1 kilogram of
ethanol from 20 degrees
Celsius to the boiling point and
then change the liquid to
vapor at that temperature?
So there's really two parts
of this problem.
How much energy, in joules, to
take the ethanol from 20
degrees to 78.29 degrees
Celsius?
That's the first part.
And then once we're there,
we're going to have 78.29
degrees Celsius liquid
ethanol.
But then we also need the energy
to turn it into vapor.
So those are going to
be the two parts.
So let's just think about
just raising the liquid
temperature.
Let's figure out how we're
going to do that.
Just the liquid temperature.
So the first thing I looked at
is well how many degrees are
we raising the temperature?
Well we're going from 20 degrees
Celsius-- let me write
Celsius there just so it's
clear-- 20 degrees Celsius to
78.29 degrees Celsius.
So how much did we raise it?
Well, 78.29 minus 20 is 58.29.
So our change in temperature
is equal to 58.29 degrees
Celsius or this could even
be 58.29 kelvin.
And the reason why we can do
that is because differences on
the Celsius scale and
the kelvin scale
are the same thing.
The kelvin scale is just
a shifted version
of the Celsius scale.
If you added 273 to each of
these numbers you would have
the kelvin temperature, but
then if you take the
difference, it's going to be
the exact same difference.
Either way you do it,
78.29 minus 20.
So that's how much we have
to raise the temperature.
So let's figure out how much
energy is required to raise
that temperature.
So we want a delta T.
We want to raise the
temperature 58.29.
I'll stay in Celsius.
Actually let me just change it
to kelvin because that looks
like what our units are given
in terms of specific heat.
So let me write that down.
58.29 kelvin is our change
in temperature.
I could have converted either of
these to kelvin first, then
found the difference, and gotten
the exact same number.
Because the Celsius scale and
the kelvin scale, the
increments are the
same amount.
Now, that's our change
in temperature.
Now how much ethanol are
we trying to boil?
Well, it tells us
right over here.
It tells us that we're dealing
with 1 kilogram of ethanol.
And everything else they give us
is in grams. So let me just
write that 1 kilogram, that's
the same thing as 1,000 grams.
We could just write it here.
1.00 kilogram is equal to-- or
let me write it this way--
times 1,000 grams
per 1 kilogram.
These cancel out.
This is the same thing as 1,000
grams. Although the
reality is we only have three
significant digits-- this
makes it look like
we have four.
So we have 1,000 grams times
1,000 grams. And then we just
multiply this times the specific
heat of ethanol.
The specific heat capacity of
ethanol right here, 2.44
joules per gram kelvin.
So times 2.44 joules.
Let me write it this way, 2.44
joules per gram kelvin.
You see that the
units work out.
This kelvin is going to cancel
out with that kelvin in the
denominator.
This gram in the numerator
will cancel out with that
grams. And it makes sense.
Specific heat is the amount of
energy per mass per degree
that is required to push
it that 1 degree.
So here we're doing 58 degrees,
1,000 grams, you just
multiply it.
The units cancel out.
So you have kelvin canceling
out with kelvin.
You have grams canceling
out with grams.
And we are left with-- take out
the calculator, put it on
the side here.
So we have 58.29 times 1,000--
times one, two, three-- times
2.44 is equal to-- and
we only have three
significant digits here.
So this is going to be 142--
we'll just round down--
142,000 kelvin.
So this is 142,000.
Sorry 142,000 joules.
Joules is our units.
We want energy.
So this right here is the amount
of energy to take our
ethanol, our 1 kilogram of
ethanol, from 20 degrees
Celsius to 78.29 degrees
Celsius.
Or you could view this as from
293 kelvin to whatever this
number is plus 273, that
temperature in kelvin.
Either way, we've raised its
temperature by 58.29 kelvin.
Now, the next step is, it's
just a lot warmer ethanol,
liquid ethanol.
We now have to vaporize it.
It has to become vapor
at that temperature.
So now we have to add the
heat of vaporization.
So that's right here.
We should call it the enthalpy
of vaporization.
The enthalpy of vaporization,
they tell us, is
855 joules per gram.
And this is how much energy you
have to do to vaporize a
certain amount per
gram of ethanol.
Assuming that it's already
at the temperature of
vaporization, assuming that it's
already at its boiling
point, how much extra energy per
gram do you have to add to
actually make it vaporize?
So we have this much.
And we know we have 1,000
grams of enthanol.
The grams cancel out.
855 times 1,000 is
855,000 joules.
So it actually took a lot less
energy to make the ethanol go
from 20 degrees Celsius to 78.29
degrees Celsius than it
took it to stay at 78.29, but go
from the liquid form to the
vapor form.
This took the bulk
of the energy.
But if we want to know the total
amount of energy, let's
see if we can add this
up in our heads.
855,000 plus 842,000.
800 plus 100 is 900.
That's 900,000.
50 plus 40 is 90.
5 plus 2 is 7.
So it's 997,000 joules
or 997 kilojoules.
Or we could say it's almost 1
megajoule, if we wanted to
speak in those terms. But that's
what it will take for
us to vaporize that 1
kilogram of ethanol.