>> host: Welcome back to
"Chemistry Matters," Unit 8,
and our discussion of
Chemical Thermodynamics,
aka thermochemistry.
Have you thought of
a question and experiment
that will allow you to learn
more about the hand warmer
or cold pack processes?
Just as with all science,
your question may evolve
as you learn more about
the process and thermochemistry
in general.
At the end of this unit,
you'll have the opportunity
to develop your own cold pack
or hand warmer,
so you may wanna make notes
related to the process
you're studying,
as we move through the unit.
For now, let's head back
to our classroom
so we can see how your question
and proposed experiment compare
to those of our students.
>> professor: Who'd like to
share their question first?
>> student: Our question is,
how much heat is generated
from a instant hand warmer?
>> student: We plan to
investigate our question
by measuring the temperature
change before and after
activating the hot pack.
>> professor: Good plan.
And later on, we'll learn
about the relationship
between temperature and heat,
and you'll see that we can
use temperature measurements
to calculate the amount of heat
that's been transferred.
How about you guys?
What did you come up with?
>> student: Our question is,
why does the cold pack
get cold?
And we wanna answer this by
comparing temperature changes
caused by dissolving
different ionic compounds.
>> student: We thought that
maybe if we tried dissolving
different ionic compounds
in water, we might
be able to understand
why some absorb heat
and why some give off heat.
>> professor: It's good to see
that you're thinking
like scientists.
I can easily imagine that
these are the same types of
questions that the scientists
who made these discoveries were
actually asking themselves.
Keep your questions in mind
as we discuss how scientists
study the flow of heat
during chemical processes,
as well as physical processes.
When you understand the methods
that are used by thermochemists,
you'll be better prepared
to develop your own plan for
developing cold and hot packs
at the end of the unit.
>> host: Good questions.
Were yours similar?
Before we go further,
let's make sure
you're equipped with
a thermochemist's vocabulary.
Thermochemical changes
are usually described
in terms of a "system"
and "surroundings."
The system is the chemical
reaction or physical process
being monitored.
The surroundings are everything
else around the system,
which might be water,
air, packaging, etc.
This terminology is important
because it's not possible
to measure the temperature
of chemical reactants
and products directly.
Instead, scientists monitor
temperature changes in
the immediate surroundings.
All phase changes
and chemical reactions
are accompanied by
changes in energy,
often in the form of heat
being consumed or released.
Exothermic is the term we use
to describe a process that
produces or gives off energy
to its surroundings.
Our hand warmers are an example
of an exothermic process.
The heat exits
the hand warmers
into its surroundings,
causing the surroundings,
in this case, water,
to heat up.
Endothermic is the term used
to describe the process that
takes in or absorbs energy
from its surroundings.
For example, your hands
become the surroundings
if you're holding
the cold pack.
The heat enters the cold pack
from your hands.
Your hand will actually
become lower in temperature
during an endothermic process.
Notice here that 'H'
stands for enthalpy,
which is the potential energy
of the system.
And the change in enthalpy,
Delta H,
is the difference in enthalpy
between reactants and products.
In the first graph,
the reactants have more energy
than the products.
That difference in energy
is released as reactants
converted to products.
This graph shows
an exothermic reaction.
The second graph
shows the opposite,
an endothermic reaction.
In an endothermic reaction,
the products have more energy
than the reactants.
So energy was gained
by the reaction system.
For our purposes,
under constant
pressure conditions,
which are our normal conditions,
Delta H equals heat transferred,
or q.
But it's important to remember
that under other conditions,
such as in a sealed container
that's pressurized,
or under a vacuum,
Delta H is not equal
to heat transferred.
Most reactions are,
in fact, carried out
under constant pressure,
which simply means that
the process occurs open
to atmospheric conditions.
The amount of heat transferred
during a process is related
to the change in temperature,
and can be measured
using a piece of equipment
called a calorimeter.
A calorimeter is
a measuring device
in a container that's insulated
to minimize heat exchange
with the environment.
Any heat that's evolved
or absorbed during the process
will be recorded as
a change in temperature.
The relationship between
heat absorbed or evolved
during a process is given
by the formula
q equals m-C-delta-T,
where 'q' is the amount of heat
absorbed or evolved,
measured in joules,
'm' is the mass in grams
of the substance
gaining or losing heat,
'C' is the specific
heat capacity of the substance
measured in joules per gram
per degree Celsius,
and Delta T is the change
in temperature measured
in degrees Celsius.
The term I just mentioned,
specific heat capacity,
is a property of matter that
conveys something most of us
understand intuitively.
That a change in temperature
is related to heat transferred.
But the concept of
specific heat capacity,
represented by 'C'
in this formula,
is a bit more complex and
requires further explanation.
So let's head back to
our classroom so we can see
firsthand the important role
that specific heat capacity
plays in the transfer of heat
from one substance to another.
For this lab,
our students will be recording
their observations on forms
that you can find in the
Chemistry Matters Toolkit
for this unit.
You can use them when
making your own notes.
>> professor: Before I explain
what specific heat capacity is,
I think it will be helpful
to explore its effects.
For now, simply use your senses
to observe the properties
of these two blocks.
And so, I'm gonna give
each group two blocks.
And you have a form,
similar to the one I have here.
And I'd just like you to
discuss with your partner
some observations
that you would make.
You know, some similarities
and some differences
between the two blocks.
>> student: Well,
both blocks are black.
That's a similarity.
>> student: But feel them.
One of them feels a lot colder.
I think they're at
different temperatures.
>> student: This block
weighs a lot less.
I think they're made
of different stuff.
>> student: You're right.
This one feels like metal,
I think.
The metal one feels colder.
I wonder what this one's
made of.
>> professor: I'm hearing
great observations.
Let's explore some of those.
It looks like that you all
noticed that the colors
and the shapes of the blocks
are the same.
And it also looks like
you've all noticed that they're
made of different materials.
So let's talk about that
for a moment.
Krishna, why do you think
they are made of
different materials?
>> student: Mainly because
one of them feels a lot
less dense than the other.
>> professor: Okay.
That's a good observation.
They are different materials.
One is a metal
and one is a plastic.
What's another difference
that you noted?
Alexis?
>> student: Well,
this one feels colder
than the other one.
The metal one, I think.
>> student: Yeah, they're at
different temperatures.
>> student: Well,
I don't think they're
at different temperatures,
because they've just been
sitting here in the room.
I think the metal one
just feels cold.
Kind of like how a tile floor
feels cold on your feet.
>> professor: That's an
excellent analogy, Krishna,
and you're right.
Both of the blocks
are at room temperature.
Would anybody like to try
to explain why it is that
the metal block feels as if
it's at a lower temperature?
>> student: I'll bet
it has something to do with
specific heat capacity.
But I don't know how or why.
>> professor: Well, you're
on the right track, Wyatt.
So let's try an experiment
that might help us understand
specific heat capacity better.
I'm gonna give each group
two ice cubes.
But first, I want each team
to predict what will happen
when you place the ice cube
on the different blocks.
Record your prediction
on the sheet that I gave you,
that looks like this.
When you're ready,
place one ice cube
on each of the blocks,
and then watch closely.
>> student: I think the ice
will melt more slowly
on the metal block
because it's colder.
Well, it's not colder.
It just feels colder.
>> student: That sounds good.
I'll write it down.
>> student: What do you think?
>> student: I think
they'll melt at the same rate
because both blocks are
at the same temperature.
>> student: That makes sense.
Let's go with that.
>> student: Look!
The ice on the metal block
is melting a lot faster.
>> student: That's weird.
The metal feels colder.
Why would it melt the ice
more quickly?
>> student: Ours is doing
the same thing.
How is that possible?
Both blocks are at
the same temperature.
>> professor: So, take a minute
and record your observations,
and then brainstorm
with your partner
to come up with
a scientific explanation
of what you just observed.
And I'll give you a hint
to help you construct
your explanation.
Every substance has a different
ability to conduct heat,
which affects how quickly
heat can travel
through that substance.
So, when you've agreed
on an explanation,
draw a diagram representing
what happened
on a molecular level.
>> host: Did you predict that
ice melts faster on metal
than on other materials?
That's really surprising
to most people.
And it's a mystery
that'll take some thought
to figure out.
Can you think of an explanation?
I'll give you some time
to develop your own explanation
of what's happening.
Remember to draw a diagram
or a model of what
you think is happening
on a molecular level.
And if you'd like to try
this experiment,
but don't have the exact
melting blocks that
our students used,
you can find a similar
experiment in the
Chemistry Matters Toolkit
for Unit 8.
Come back and join us
for the next video
in the playlist
after you've had a chance
to develop your own explanation
and model, and carried out
the experiment yourself,
and we'll compare notes.