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.