So we've been talking kind of at a very macro level about heat
transfer by using an example of heat being transferred
between our hand and a glass of water.
So now let's go more of a micro level.
Let's talk about heat transfer and chemical reactions.
So when we're talking about heat transfer and chemical reactions
we're talking about the term called enthalpy.
Now it turns out that if we think about chemical reactions
as our system, so go ahead and write that here, chemical
reactions can either release or absorb
energy that can change the temperature
of its surroundings.
Which we generally think of when we're
talking about chemical reactions as the solution
that the chemical reaction is taking place in, and of course
if the chemical reaction is taking place in our body
the surrounding is really just our body,
which you can think of really as just a big bag of water.
So a question might be on your mind.
Why did chemists come up with this fancy term enthalpy
to describe the heat transfer of chemical reactions?
It turns out that enthalpy is a very useful quantity
to calculate for many chemical reactions
because not only does it tell us something about heat transfer,
but it also is a component of Gibbs free energy, which
is an important parameter that chemists
use to determine whether or not a reaction will take place
or not.
Generally speaking, I think it's OK to conceptualize
with some simplification that enthalpy is essentially just
a fancy term to describe heat transfer for chemical
reactions.
And because we can never define an absolute quantity of heat,
because remember heat is the amount of energy transferred,
it's not an absolute term, enthalpy is always
referred to as a change in enthalpy,
and oftentimes it's written as delta H.
And specifically this change in enthalpy
describes the change in heat energy.
That is whether heat was lost or gained
from the perspective of the system.
So even thought that there is this kind
of important interplay, this conservation of energy
between the system and the surroundings,
this term enthalpy is really just telling
us what's happening from the perspective of the system.
And I think this will make more sense
as I give you an example below, but before I do that I just
want to note what the units of enthalpy are.
And the units of enthalpy are joules
per mole of reactant in the chemical reaction.
And this of course makes sense because joules
is a unit of energy, and we're talking
about heat, which is a form of energy.
And it's notable to know that having it per mole
allows us to take the amount of whatever
is reacting into account.
This is something that we can't really
take into account if we just, say,
measure the change in temperature
that occurred over the course of a reaction.
All right, so now let's go ahead and take a look at an example.
So I'm going to go ahead and scroll down.
Let's go ahead and use an example
that you will be fairly familiar with.
So when our bodies get super hot to cool ourselves
down our bodies essentially evaporate
water from the surface of our body
to help cool ourselves down.
And this is really just a fancy way
of saying that we start sweating.
Right?
Now the chemical reaction for sweating
we can really think about as just essentially
the evaporation of water.
So that is to say taking water from the liquid phase
and turning it into water vapor, or water in the gaseous phase.
The change in enthalpy for this particular reaction,
and note that I'm saying for this particular reaction,
for our system, is defined as the enthalpy
of our product, which is our water in its gaseous form,
minus the enthalpy of our reactant, which
is water in its liquid form.
This of course is a way to conceptualize enthalpy,
but remember it's really actually not
possible to measure an absolute quantity of enthalpy
for anything.
We can only ever measure this change
in enthalpy, which involves monitoring
the change in temperature of the surroundings
during the course of a reaction as well
as some other considerations that we're not
going to go into in this video.
But just to note that it's really the change in enthalpy
that we're measuring and not these absolute quantities,
even though technically this is how
we're conceptualizing enthalpy.
So the key idea here about this process of evaporation
is that it requires energy to occur.
Just think about boiling water.
In order to get water into its gaseous phase
you need to heat it up.
So the fact that we're adding energy to our system
should tell you something about what
this sign of this change in enthalpy should be.
The products are essentially gaining more energy
in the form of heat than the reactants.
So we say that in a process that absorbs
heat the change in enthalpy is greater than 0.
In other words, the change in enthalpy is positive.
And whenever a reaction has a positive change in enthalpy,
which by definition means that heat is being absorbed,
we call it an endothermic reaction.
And the way that I like to remember this is that endo is I
think a Latin prefix for within.
So heat is going in to our system, which
is our chemical reaction.
So you're probably wondering at this point,
well, if we're absorbing heat, how
is this connected to us cooling off?
Well, this is kind of the important point
that I alluded to before.
This change in enthalpy only tells
us what's going on with our system, which of course is
our chemical reaction, but our body in this case
is our surrounding.
So we can use this relationship above that tells us
that the heat lost or gained by the system is equal
and opposite to the heat lost or gained by the surroundings.
So in this case we know that our system
is absorbing heat, which means that our surroundings,
our body, must be losing heat.
And that's how we cool down.
Now you can also imagine that we have chemical reactions
in which the change in enthalpy is less than 0,
and we call these types of reactions exothermic reactions.
And the way I kind of think about these
is that ex, this ex term is kind of a Latin prefix for out of.
So heat essentially is going out of the system
instead of being absorbed.
One example of a very prevalent reaction in our bodies
that is exothermic is the hydrolysis, or reaction
with water, of ATP, which is of course the energy
currency of our cells.
So ATP reacts with water, and it loses a phosphate group
to become ADP and a free phosphate group.
And this reaction has a negative change in enthalpy,
or in other words it releases heat.
The fast that this reaction is exothermic
is physiologically significant for many reasons.
But one of those reasons is when we're talking about shivering.
So we all know that we shiver when we're cold.
And the reason we do that is because we
want to warm ourselves up.
And that heat energy is indirectly
tied to this hydrolysis of ATP which releases heat and allows
our muscles to contract to warm us up.
So just to wrap things up here, I
think the key takeaway is that enthalpy describes
heat transfers for chemical reactions,
and notably it's from the perspective
of the chemical reaction, not the surroundings.
And so chemical reactions can either lose heat, in which case
they are classified as exothermic reactions
and having a negative enthalpy, or they
require an input of heat, and they're
classified as endothermic, and have
a positive change in enthalpy.