>> host: Welcome back
to "Chemistry Matters"
and the third video
on our Unit 8 playlist:
Thermodynamics.
In the previous video,
we learned that heat
and enthalpy are the same
under constant pressure,
which means under
our normal atmospheric
conditions, and that a process
can be either endothermic,
meaning it absorbs heat
from its surroundings,
or exothermic,
meaning the process
gives off heat.
We also saw first-hand
that different substances,
while at the same temperature,
can feel colder
or warmer than each other,
and can actually melt ice
at different rates.
And speaking of that,
by now you should have
developed an explanation
for why the metal surface
in our lab melted ice
more quickly
than the plastic.
In this unit,
we're going to learn
why different substances
transfer heat
at different rates
and efficiencies.
We'll also learn about
calorimetry, a method
of measuring the quantity
of heat transferred
in a process.
Right now, let's head back
to our classroom to see
how our students explained
the melting ice mystery.
>> professor: You've had time
now to develop a scientific
explanation for the observation
that when they're at
the same temperature,
the metal block
melts the ice faster
than the plastic block.
So who would like to present
their explanation
for that first?
>> student: We will.
We thought about how metal pans
are used to cook food.
It must be because the metal
transfers heat so well.
>> student: So we think
the metal block transferred
its heat to the ice
better than the plastic.
>> student: To put it
in science terms,
our hypothesis is
that different materials
have different
heat conductivity,
kind of like
electrical conductivity.
>> professor: That's
a good start.
What you're talking about
is thermal conductivity,
which is a measure of
the ability of a material
to transfer heat.
The higher the thermal
conductivity of a substance,
the faster and more efficiently
it will transfer heat.
And the thermal conductivity
of metals are quite a bit
higher than that of plastics.
So how about you guys?
How did you explain the fact
that the metal block
melted the ice faster
than the plastic.
>> student: I think
our explanation
is pretty similar to theirs.
We did a little research
and read that plastic
is a huge molecule,
with all of its atoms
bonded to each other.
>> student: Right.
And the metal is more like
a bunch of unbonded atoms
whose electrons are free
to move around.
Heat is transferred
when particles collide
with each other,
We drew our model
to show the atoms electrons
in the metal
moving more easily
than the molecules
in the plastic.
>> professor: Good.
You guys are off
to a great start.
Could you explain your model
to us.
>> student: We drew our model
to show the electrons
in a metal are freer
to move around
than the electrons
in a plastic.
And because
there's more movement,
they can transfer energy,
in this case heat,
to the ice cube better.
>> professor: Great.
Both teams are off
to a great start.
Heat is in fact conducted
through substances by way of
molecular collisions
and molecular vibrations,
as you mentioned.
The greater the temperature
of an object,
the higher
the kinetic energy
of its atoms and molecules.
Heat can be conducted
through the transfer
of this kinetic energy.
For example,
the faster the particles are
moving near the source
of energy,
some of their kinetic energy
is then transferred
to the particles
that are moving slower,
until all the particles
have the same energy,
and are therefore
the same temperature.
And some materials
transfer kinetic energy
more effectively than others,
and therefore they have
a better heat conductivity.
But there's another
property at play here
besides heat conductivity.
If we were to measure
the temperature
of the two blocks
after a certain period of time
in contact with the ice cube,
the temperature
of the metal block
would be significantly
lower than that of
the plastic block.
You might be able to tell this
just by touching the blocks
for yourself.
>> student: Yeah, I can tell.
The metal plate
feels a lot colder
than the plastic one.
>> student: Is that because
the metal gave more energy
to the ice cube?
>> professor: Yes,
that's right, Caleb.
The difference in temperature
between the two blocks
is related to the property
of a specific heat capacity
that I mentioned earlier.
This tells us
how much heat energy
it takes to change
the temperature
of a substance
by one degree Celsius.
Specifically,
the heat needed
to raise the temperature
of one gram of a substance
by one degree Celsius.
As I mention is
the specific heat capacity.
The standard units for
specific heat capacity
are Joules per gram
degrees Celsius.
Now, substances with a low
specific heat capacity
are more sensitive
to changes in temperature,
like the metals here
at the bottom.
Substances with a higher
specific heat capacity,
they're more resistant
to changes in temperature.
Water, for example,
has a very high
specific heat capacity,
and so it's quite resistant
to changes in temperature.
And again, look at the metals
and how low their specific
heat capacities are,
this explains why
they're so quick
to change in temperature
in the presence of heat.
It takes more than eight times
as much energy to raise
one gram of water
by one degree Celsius
than it does
one gram of iron.
That's a lot more energy!
>> host: You might be wondering
why this is important.
Why should you care about
specific heat capacity?
For one thing, the effects
of specific heat capacity
are all around us--
literally!
The air around us
has a relatively low
specific heat capacity,
and as a result,
changes temperature
quite readily.
It's the difference
in specific heat capacity
of land and air versus water
that explains
why the temperature
of the ocean
changes slowly
and very little,
while the temperature
of the surrounding land
and air changes rapidly,
and by much greater amounts.
The consistent temperature
of Earth's water
helps keep the Earth's
entire climate steady.
The specific heat
of a substance
must be taken into account
before producing
almost anything.
For example, cookware
is made with materials
that have a very low
specific heat capacity,
so that they heat up quickly
and make cooking faster,
although their handles
and many cooking utensils
are often made
of materials with
high specific heat capacity
to protect us from burns.
Substances that must be
kept cold are put in
materials with high specific
heat capacities,
such as Styrofoam,
so lots of energy is needed
to raise their temperatures.
Now that we have
an understanding
of the importance
of a material's
specific heat capacity
to the amount of heat
transferred during a process,
and we've seen the effects
of specific heat capacity
on solids, like metal
and plastic,
we can turn our attention
to the specific
heat capacity of gases.
The composition of gases
in our atmosphere
plays a key role
in the earth's climate,
so it's very important
to understand the role
of these gases
in heat transfer.
The specific heat capacities
of gases in general
are much lower than
most liquids and solids.
For example,
the heat capacity of air
is about a quarter of
the heat capacity of water,
which explains why
the temperature
of our atmosphere
is much more likely
to undergo large
temperature variations.
Earth's temperature
depends on the balance
between energy entering
and leaving our atmosphere.
The gases in our atmosphere
directly absorb some of
the sun's radiation,
as well as absorbing
some of
the reflected radiation
from our Earth.
This "trapping"
of the sun's radiation
by the atmosphere
is referred to as
"The Greenhouse Effect,"
and it's the primary
mechanism for maintaining
our climate
at a moderate temperature.
If not for the greenhouse
effect, the temperature
of our Earth
would fluctuate wildly.
For example, the moon,
which is about the same
distance from the sun
as Earth and has almost
no atmosphere,
varies in temperature
from a high
of 121 degrees Celsius,
to a deep-freeze low of about
negative 150 degrees Celsius
once the sun goes down.
The chemistry of
our atmosphere must maintain
a balance or equilibrium
between the incoming radiation
from the sun and the outgoing,
reflected radiation
from the Earth's surface.
Absorption of some of
the sun's radiation
during the day
protects the earth
from too much heat,
and the release of heat
from the Earth's surface
provides warmth at night
once the sun sets.
Atmospheric gases such as
water vapor, methane,
also known as natural gas,
and carbon dioxide,
are primarily responsible
for the absorption
of reflected radiation
from the Earth,
and thus, are our most
important greenhouse gases.
However, if the atmospheric
greenhouse gases absorb
too much radiation,
the average temperature
of the Earth may continue
to rise as more and more
heat gets trapped
in the Earth's atmosphere.
You may have experienced
the warmer feel
of a very humid day,
due to the ability
of water molecules,
the most prevalent greenhouse
gas, to absorb heat radiation.
Similarly, the coolness
that descends
over dry climates at night
is a result of the lack
of water vapor.
The variation
in the concentration
of greenhouse gases
has an important effect
on our climate.
Although water vapor,
carbon dioxide, and methane
all occur naturally
in our environment,
the prevalence
in the atmosphere
has increased since
the industrial revolution
caused by an increase
in the burning
of fossil fuels.
The burning of coal, oil,
natural gas and wood
have resulted in an increase
in carbon dioxide
in our atmosphere
of more than 30% since
the industrial revolution.
Average temperatures
in the U.S. have increased
by about 1.5 degrees Fahrenheit
during that same time period.
Climate change
such as prolonged
periods of heat,
heavy downpours, and floods
are consistent
with the increased levels
of carbon dioxide
in our atmosphere.
So what does all of this
have to do
with thermochemistry?
To find out, let's go back
to our classroom
for an experiment
in thermochemistry
and how it helps us
understand the role
of greenhouse gases
in our environment.
>> professor: Scientists
have played an important role
in environmental stewardship.
Studies of atmospheric gases
have helped us understand
the greenhouse effect.
Scientific models
of our climate,
based on the known
chemistry of the atmosphere,
allow us to predict
what might happen
if greenhouse gases
continue to increase.
Today you're going to
take on the role
of environmental scientists
exploring the contribution
of atmospheric gases
to the greenhouse effect.
Each group has three empty
plastic bottles,
each of which will be filled
with a gas to represent
our atmosphere.
The gases we'll study today
will include water vapor,
methane, and carbon dioxide.
Now if Krishna and Wyatt
will compare water vapor
to methane,
while Caleb and Alexis
will compare water vapor
to carbon dioxide.
Each group will saturate
the air inside the bottles
with the two gases
that I mentioned.
The third bottle
in front of each group
will simply contain the air
as it already does,
and this will serve
as a control.
In order to investigate
the ability of each gas
to affect temperature change,
the gas-filled bottles
will be placed close
to an incandescent light,
which gives off most
of its energy in the form
of infrared light, or heat.
Notice that I already
put a hole in the bottle caps
so that you can
insert a thermometer,
which will allow you
to monitor the temperature
of the gas in the bottle
over time.
Let's go over the procedure
you'll use to test your
assigned greenhouse gases.
To make carbon dioxide:
To fill a bottle
with methane, or CH4:
Natural gas is less dense
than air, so the bottle cap
thermometer and clay
should all be added
while the bottle is held
upside down.
Use caution.
Methane gas
is extremely flammable.
Special care should be taken
to keep the methane away
from any source of flame.
And to get water vapor
into the plastic bottle,
place a piece
of soaking wet sponge,
at least three centimeters
in diameter,
at the bottom
of the bottle.
Then again, tighten the cap,
insert the thermometer,
and seal with clay.
Lastly, your third bottle,
our control, already has
room temperature air in it.
So you'll want to insert
a thermometer
into that bottle
and seal it as well.
After all the bottles
are ready,
record the initial temperature
for the gas in each bottle.
Your initial temperature
will simply be
room temperature,
and it will likely
be between 20 and 25
degrees Celsius.
Place the bottles
30 centimeters away
from the incandescent light,
one on each side.
and it's important that
all the bottles are the same
distance from the light.
Use the data table
to record the temperatures
in each bottle every minute
over a period of 25 minutes.
Heating the gases
inside the bottles
with incandescent lights
models the heating
of our earth's atmosphere
by the sun.
We'll mimic the sun setting
by turning off
the incandescent light
and monitoring temperature
for an additional ten minutes.
When you've finished
collecting data,
enter it into
this spreadsheet.
We'll combine all of
our results on one page,
so we can compare the effect
of the different
greenhouse gases
on the temperature
of the atmospheres
inside the plastic bottles.
>> host: We'll take a break here
to let you perform
this greenhouse gases lab
and record your results
on the datasheet
that you can find
in our "Chemistry Matters"
Toolkit for this unit.
And when you're done
and you're ready to compare
your results to the results
of our students,
you can rejoin me
at the top of
the next video
on the Unit 8 playlist.