Hi. It's Mr. Andersen and this is chemistry essentials video 33. It's on endothermic and
exothermic reactions. If you understand that therm means heat, and exo means to exit or
to move out it's pretty easy to breakdown these reactions. So an exothermic reaction
would be one where heat is being released from the reaction. So a thermite reaction
right here, you can almost feel how hot that is, is a reaction where we are releasing energy.
Releasing heat from the reaction itself. And likewise if we have an endothermic reaction,
that's going to be one where the reaction is taking in heat. And so in a cold pack like
this, there's a combination of water and ammonium nitrate. That reaction is consuming heat.
It's taking it from its surroundings and so it is going to feel cold over time. And so
you really have to understand what is the system and what are the surroundings. And
so in a reaction the system is going to be where the reaction is taking place. So in
the chemicals themselves. And then the surroundings is going to be outside of that. So if we have
a change in heat that's either going to be an exothermic or an endothermic reaction.
Now in the lab we can't really see at the level of the atoms and so we have to view
it macroscopically. And we do that by simply measuring the temperature change. And so in
an exothermic reaction we're going to find that heat is being released from that reaction
to the surroundings. And in an endothermic it's actually being absorbed. Now a really
neat way to observe this is to use a model. Is to use an energy diagram like this. And
so the way it works is that the energy progress will be going along the x-axis. And then the
amount of energy in the reactants and products is going to be displayed. And so in an exothermic
reaction, what you'll see is a downhill reaction. In other words the reactants are going to
have more energy than the products. Now where is that energy going? Where is that heat going?
It's going to the surroundings. And likewise in an endothermic reaction the reactants are
actually going to have less energy than the products. And so they're consuming that energy.
And so let me give you an example of that. Here's that exothermic reaction again. We've
got a thermite reaction where we're taking iron oxide and aluminum. And we're making
iron and aluminum oxide. Now this is a really intense reaction and it's actually, it provides
the oxygen in this form of combustion. And so this should be able to work even under
water. But if we were to look at the energy diagram of this over time, as it progresses,
we'd find that these reactants, the iron oxide and the aluminum have a higher energy. In
other words there's more energy locked in those bonds than we're going to have in the
products. And so we can see this as a downhill reaction. Now what's going on? Why do we see
this increase in energy here? Remember we have to start pressuring those bonds, put
a little bit of pressure on it before we can actually have them break apart and we get
that exothermic reaction. We call that an activation energy. So we're going to put a
little bit of energy into the system. But you can see that we're going to net energy
coming out of that. And so that's going to be measured in heat coming off that thermite
reaction. Now if we look at an endothermic reaction, again this would be like solid ammonium
nitrate and we're just combining it with water so we're going to make ammonium and nitrate
ions. In that case what we're doing is the reactants are actually going to have a lower
amount of energy than the products. And so you can see here's our activation energy,
but this is an uphill reaction. In other words it requires energy. It requires heat. Where
is it getting that heat? It's getting it from the surroundings. And so when we're measuring
exothermic and endothermic reactions in the lab, we're really stuck with the temperature
itself. And so if I wanted to measure a reaction like this, let's say I take a chemical and
I mix it with water or mix it with another chemical, I can simply use a thermometer to
measure what's going on. And so I see a decrease in that temperature. Okay. So what's happening?
It's actually consuming heat from the surroundings. And so this would be an endothermic reaction.
And so that's pretty simple. What you should be able to do after you're done is to generate
a relevant symbolic and or graphical representation of the changes. Remember we're using an energy
diagram to do that. It's all about the heat. And I hope that was helpful.