Hi. It's Mr. Andersen and this is chemistry
essentials video 53. It's on the enthalpy of reaction which is the amount of energy
taken in or released during a chemical reaction as bonds are broken and new bonds are formed.
And look no farther than nitroglycerin for a reaction that creates a huge amount of energy.
It was used by Alfred Nobel, a scientist and explosives expert, but it was highly unstable
and his brother was killed in one explosion. So he invented dynamite which essentially
stabilizes nitroglycerine by wrapping it in some kind of a cellulose. And so it was used
not only in mining and construction but it was also used in war, leading some people
to call Alfred Nobel "The Merchant of Death", which made him feel a little bit bad. He put
all of his money into a trust and then created the Nobel Prizes which are famous to this
day. And so in the enthalpy of reaction what we're looking at are the reactants and what
are called the bond energy, the amount of energy it takes to break the bonds and the
reactants. And then the amount of energy that is released in the products which is essentially
the negative bond energy. And you can think of putting these on a balance beam. And if
there's more reactant energy consumed we would call that an endothermic reaction. If there's
more released we call that an exothermic reaction. Now remember these reactions sit in their
surroundings. And since energy can neither be created nor destroyed, in an endothermic
reaction energy, thermal energy, is going to go from the surroundings into the reaction.
That's going to cause that reaction to feel cold. And the opposite if we're looking at
an exothermic reaction. Now if we're looking at the amount of enthalpy reaction there is,
Hess's Law is something that kind of lays outside of what you should know in AP Chemistry.
But there's a few ideas that you should really be able to apply. And so if we look at an
exothermic reaction, like this thermite reaction, we can look at the energy of the reactants
and the energy of the products. And you can see that this is a downhill reaction. In other
words we're releasing energy. And that energy is called the enthalpy of reaction. Now it's
given off from that reaction as heat into the surroundings. If we were to actually measure
it, that's going to be the change in enthalpy which is simply the enthalpy of the products
minus the enthalpy of the reactants. In this case it would be negative 850 kilojoules per
mole. Now if I were to show you an enthalpy diagram it would simply look like this. An
enthalpy diagram is different than an energy diagram in that we simply put the enthalpy
of the reactants and the enthalpy of the products. Since we're subtracting the reactants from
the products, in other words since the products in this case is lower than the reactants we're
going to get a negative value, negative 850 kilojoules per mole. Now what does that mean?
Where does that energy go? It's going to go as thermal energy to the surroundings. Now
something interesting is that if we actually turn that reaction around and make the reactants
the products, look what happens to the change in enthalpy. It stays the same. The only thing
that changes is going to be the negative value that was in the front. And if we swap it around
again it's going to be that negative again. And so this is the first of Hess's ideas that
you should understand. That is if we reverse the reaction then we have to reverse the sign
in that change in enthalpy. If we were to look at an endothermic reaction, in this case
we have an uphill reaction where the products actually have more energy than the reactants.
And so this is going to be our enthalpy of reaction. It's moving from the surroundings
as thermal energy into that reaction itself. We could measure it and we would find that
it is a positive value. Now how do we measure that? We're going to use a calorimeter remember.
We're going to have the reaction take place surrounded by water. We can measure the change
in that heat of the water and we can measure that enthalpy change. It's a positive value
or an uphill reaction and the reason why is that our products have a greater enthalpy
than our reactants. Now we don't have to know what that energy is to begin or at the end
we just have to know what that change is in order to measure enthalpy. Now what's interesting,
if we take a look at a reaction we find that it's usually made up of a number of smaller
reactions. And so in this Born-Haber cycle what we're doing is taking lithium solid and
combining it with fluorine gas and we're making lithium fluoride. Now if you look at that
change in enthalpies, it's very small. And it's going to be an exothermic reaction because
it's negative. But if we look at all of the steps of this reaction, this would be an endothermic.
This would be endothermic. This would be endothermic. And then we have two exothermics. And so this
is the next of Hess's law that you should be able to apply. And that is that the enthalpy
of reaction is equal to the sum of all of the reactions that make up that reaction.
So lots of times in chemistry you'll be given a target reaction and then you're going to
have parts of that reaction. And your goal is to figure out the change in enthalpy of
that whole reaction. It's pretty simple algebraically to solve this as long as you remember those
two parts of Hess's Law, that if we reverse the reaction we have to reverse the sign.
And then the sum of all the parts is equal to the sum of the whole. And so if I look
at this equation right up here I've got my carbon on the left side. And so that's good.
But you can see here that I have my carbon monoxide on the left side and I eventually
want it on the right side. You can see here that I have two exothermic reactions. And
so the first thing that I could do is I could simply switch this reaction around. That's
totally legal. Watch what happened to my delta H. My delta H now is a positive value and
so I can change the reaction around and then I'm good to go. If you look at this I've got
carbon dioxide on the left, carbon dioxide on the right side and so I can actually cross
those off. And now I'm kind of left with almost just what I want. There's one problem here.
I could move my carbon down here. I could move my carbon monoxide here. Now I'm left
with my oxygens. Well on the left side I have one O2. On the right side I have a half of
an O2. And what am I looking for? A half of O2 on the left side. So what I can do is I
can simply cross that off. And now I've just got a half O2 on the left side. And so now
I've totally got my reaction. All I do now is I simply add these up. So algebraically
I can add these up and this is going to be the delta H of that whole reaction. Let's
try a little bit harder one. Now you could pause the video at this point. And you could
try this on your own. That would be a good thing to do. I'll wait for you. Okay. So let's
get at it. So here's our target equation right down here. And so you can see here that I
have my NO on the left side. My nitrogen monoxide. And so that's good. I'm going to leave that
there. But what I really want is my oxygen all by itself on the left side here. So I'm
going to take this and I'm going to turn this equation around. You can see that I got a
negative value right here. So that helps out a little bit. You can see here that I have
to have two ozones on the left side. And so if I could get those to cancel out that's
going to help me a little bit. So I could switch that around. So these are going to
cancel. So that's good. What am I missing then? I've got my nitrogen dioxide on the
right side. Well you can see here that I really want just one of these on the left side. And
so what I could do is I could take that whole equation times a half. Well if I take that
whole equation times a half, watch what's going to happen to my delta H. I'm going to
have to take that times a half as well. Okay. So let's see how we're doing so far. I've
got my ozones. I can get rid of those. Now if we get to the oxygen, this is really convenient.
You'll find this is lots of times very convenient. On the right side now I've got one O2 plus
a half. So that's three halves of an O2 which is the same that I have on the left side.
So I could cross out all my oxygens. Now I simply add that up. There's my target reaction.
And now I simply add up all my delta Hs. And that's going to be the overall exothermic
reaction that I'm looking at. Okay. So did you learn qualitatively and quantitatively
patterns in enthalpy of reaction? Again it's simply a balancing act. We look at the energy
it takes to break the reactants. The energy formed by the products. And whichever way
that balance tips, it's going to tell us what kind of a reaction it is. And then we combine
reactions to figure out the sum of all of the parts. And I hope that was helpful.
