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Course: High school chemistry > Unit 3
Lesson 4: Energy of chemical reactionsEndothermic and exothermic reactions
Chemical reactions can result in energy being released (exothermic) or energy being absorbed (endothermic). We can use the law of conservation of energy to determine how that energy is transferred between a system and its surroundings. Energy diagrams visually represent potential energy changes during a reaction, providing quantitative information about the bond-breaking and bond-forming process. Created by Sal Khan.
Want to join the conversation?
- Is there a reaction that releases the same amount of energy as was originally put in?(8 votes)
- “Yes! A reaction that releases the same amount of energy as is put in is called an isothermal reaction. An example of this is water boiling. The reaction will happen at a constant temperature (at 100∘C) and, therefore, will be an isothermal process. This is true for all phase change reactions.“
Quoted TheterrorIs_Rael
I just put this here so it’s easier to see -π(4 votes)
- I am confused. Sal says that you add energy to a chemical mixture to get them to react, but when you add energy to, say, hydrogen peroxide, it decomposes. Can someone explain please?(3 votes)
- Sal is generally correct. Adding energy to a chemical mixture can help start reactions by providing the necessary energy to overcome initial barriers. Whether the reaction proceeds or not depends on the nature of the reaction and the presence of activation energy barriers.(3 votes)
- Are graphs inverted?
In an endothermic reaction there will be less released energy than what you put in, so the new potential energy should be lower than initial point.
In an esothermic reaction there will be more released energy than what you put in, so the new potential energy should be higher than initial point.(3 votes)- Sal is right here, you’re the one with it backwards. There’s only a limited amount of energy in each of these equations, so the higher the output of energy, the lower the potential energy remaining.(3 votes)
- Now I understand "Endothermic and exothermic reactions" in a nutshell!(1 vote)
- Is there a reaction that releases the same amount of energy as was originally put in?(1 vote)
Video transcript
- [Narrator] So what we have
depicted here is a reaction. I have a beaker. In that beaker, I have molecules, I have these purple molecules. I also have these blue ones. And if I were to just leave
this beaker at room temperature in my laboratory, nothing
is going to happen. These molecules aren't going
to react with each other. But then if I add energy to
this, if I were to heat it up, then I can get it to react. And so that's what we have over here. The energy is used to break these bonds and then the bonds can reform
to form different molecules. So we have a reaction here. Here are the reactants,
here are the products. And I'm also going to
assume that as these bonds, as these new bonds form,
it also releases energy. And what we're going to
think about in this video is whether the energy that is released from this
reaction is greater than, or less than, than the
energy that we put into it. And depending on the answer to that, we would label this as
either an endothermic or an exothermic reaction. So let's think about
this a little bit more. So if you have a system,
and you might say, "What's a system?" Well, a system is really just a fancy way of saying the thing
that you are focused on. And you can define that
system in any which way. Here we will call our system this beaker that has the solution inside of it. So our system is going to include that, but it's not going to include the outside, the burner that we're
using to create the heat. It's just that beaker. So this is the system in either situation. And then we could also label
the things outside the system. We could call that the surroundings. Hopefully, that's an
intuitive thing to call it. And when we light the flame
underneath this beaker, were transferring energy
from the surroundings into the system. The law of conservation of energy, which is also known as the
first law of thermodynamics, tells us that energy can neither
be created nor destroyed. So if we're getting energy into a system, it has to come from someplace. In this situation, the energy is coming from our surroundings, but as we know, we're not just putting
energy into the system. When these new bonds are formed,
it's also releasing energy. In a situation where it's
releasing less energy than was put in, I'll do that with a less
wide arrow, like this, this we would call an
endothermic reaction. Even though some is being
released, on a net basis, you're putting more in than
you are actually getting out. Now, if you had it the other way around, where if you had your system here and you're putting in a certain amount of energy into the system, but you're getting a lot more
energy out of the system, I'll do that as a big arrow, like this, that's an arrow, that's not a house, if you have a big arrow, like this, well, now, this we would
call an exothermic, exothermic reaction. Let me box these off, so
that we don't get confused. Now, we can also understand this in terms of potential energy and how the reaction progresses. So on the horizontal axis, we'll call this reaction
progress, reaction progress. And then the vertical
axis is potential energy. Now, our system, before
we start heating it up, our system is going to be
at some potential energy and maybe it's just happy right there. But then we start putting
energy into the system. We're doing this right
over here by lighting that burner under it. So as we put energy into the system, the potential energy goes up. And that energy is used to
essentially break the bonds of these reactants. And then once those
molecular parts are all free of each other, they can
now reform new bonds. And if we're in an exothermic reaction, those new bonds are going to
be at a lower potential energy than the original bonds of the reactants. So one way to think about it here is, at this phase, we are putting in energy. This is the energy we're
putting into the system. And you can calculate it by
thinking about this arrow. And then once those
molecular parts reform, you are releasing energy. And so you could think of that height as the release of energy. And if you thought about
what the net energy that gets released is, well, that's going to be the difference between the potential energy as you're entering into the reaction and then your final potential energy. So this distance right over
here is the net energy. And in this case, it is the net energy that has actually been
released from the reaction. So this right over here
is an exothermic reaction. You might be wondering, "Well, "what would an endothermic
reaction look like?" Well, in that situation, and I'll just draw it really fast, it would have the same
labels on the chart, you would be putting energy in. So let's say, you have some molecules, energy in to break those
bonds, the bonds are broken, and now things reform, but they get to a higher potential energy than you were before. So here we're putting in a lot of energy, but then when those bonds,
when we have new bonds forming, that's not quite as much as
the energy that we put in. And so there's a net
amount that we put in. The net amount that we put
in is gonna be the difference between that and that. It's going to be this
height right over here. So this is exothermic and this is endothermic right over here.