Energy changes in chemical reactions (part 1)
About this video
Some chemical reactions absorb energy (endothermic, like photosynthesis) and others release it (exothermic, like burning a marshmallow). The video explains this with chemical energy stored in arrangements of atoms: breaking bonds needs energy and forming bonds releases it, so the overall balance decides whether energy is absorbed or released. It follows energy from the Sun into sugar and back out when it burns, uses hot packs and cold packs to show energy moving between a system and its surroundings, and ends with the law of conservation of energy.
You'll be able to
- explain that breaking bonds requires energy and forming bonds releases energy.
- classify a reaction as endothermic or exothermic and say where the energy comes from or goes.
- describe energy flow between a reaction system and its surroundings, using hot and cold packs.
- state the law of conservation of energy.
Key words
- endothermic
- A reaction or process that absorbs energy from the surroundings; 'endo' means inwards.
- exothermic
- A reaction that releases energy to the surroundings; 'exo' means outside.
- chemical energy
- Energy stored in the arrangement of atoms in molecules.
- combustion
- A burning reaction, like sugar reacting with oxygen to give carbon dioxide and water plus energy.
- surroundings
- Everything outside the reacting system, such as the air or your hand.
- law of conservation of energy
- Energy is not created or destroyed, only transferred from one thing to another.
Transcript
- [Instructor] Consider these two processes: one where a plant is growing, this is a time lapse, and another one where we are burning a marshmallow. Both of these involve chemical changes. How do we know? Well, over here, new stuff is being produced, new leaves are being produced, and over here we can see and feel the energy change, light and heat.
But what's interesting is that this particular chemical reaction absorbs energy, whereas this one releases it. So why do certain chemical reactions absorb while others release energy? And more importantly, where does energy go or come from? That's what we're gonna figure out in this video.
Let's start with the plants growing. The reaction involved is called photosynthesis. It's where carbon dioxide reacts with water to produce glucose, or sugars, and oxygen. Now, this equation is not really balanced, but that's okay. What's important for us is that, look, the energy's on the left side, which means that energy is being absorbed over here, and that energy comes from the sunlight.
The Sun's energy is being absorbed to produce sugar and oxygen molecules. But the big question is why? Why does this reaction require or absorb energy? Well, think about it this way. Chemical reactions are basically atoms being rearranged to form new arrangements, isn't it?
And it turns out that these arrangement of the atoms have a higher chemical energy compared to these arrangement of atoms. Here's an analogy that really helps me understand this, okay? Consider a ball that is on a lower level. Now, compared to this, a ball that is at a higher level has more energy, right?
We say it has more potential energy compared to when it was over here. So just like over here, we could say the arrangement of the ball on the Earth when the ball was over here, it has lower energy, and the arrangement of the ball on the Earth when the ball is over here has a higher energy.
And so now, if you wanna get a ball from the lower level to a higher level, you need to supply that energy difference, isn't it? In a similar way, it turns out that carbon dioxide and water, this arrangement of atoms has lower chemical energy. And sugars and oxygen, this arrangement of atoms has a higher chemical energy.
So to go from here to here, to rearrange the atoms from here to here, you need to supply that difference in the energy. To be more concrete, here's how we can think about it, okay? To rearrange the atoms, you need to first break these chemical bonds. Breaking bonds is kind of like pulling magnets apart.
It requires energy, so you need to supply energy. And in this particular diagram, you can imagine that the energy supplied, the amount of energy you need to supply is about this much. The hill represents the amount of energy that you need to supply over here, okay?
And when new bonds are being formed, it's kind of like magnets sticking to each other. They release energy. When magnets stick to each other, you hear sound. That's because energy is being released. Similarly, when new arrangements are formed, they release energy, and the amount of energy released in this particular case is about this much.
You can see energy supplied is way more than the energy that is released. And so overall, you need to supply energy for this reaction to happen. Make sense? And such chemical reactions where energy is absorbed, we call that endothermic reactions. The word endo stands for the energy is going inwards.
It's being absorbed by the reaction, and where is it going? Well, it's going and it's getting stored as chemical energy in these product molecules. Okay, now a quick question. What do you think would happen if we were to run this chemical reaction backwards? What if we started with this and we ended with this?
What would happen? Well, it would now be the exact opposite, right? I mean, now the energy would be released because the product molecules have lower chemical energy than the reactant molecules. Again, you can imagine that to break these bonds you require, you need to supply this much energy.
You should always supply energy to break bonds. But when the new bonds are formed, they will release this much amount of energy. So look, a lot more energy is being released compared to how much we're supplying. Therefore, overall, the energy is released in this reaction.
And that's why such chemical reactions where energy is released are called exothermic reaction. The word exo kind of means outside, the energy is going out of this reaction. And where is this energy coming from? Well, it was stored over here, right? That energy is being released.
And that's exactly what happens when you are burning this marshmallow. By the way, this particular reaction is called a combustion reaction. And then when you're burning a marshmallow, basically the oxygen and the sugar react to give you carbon dioxide and water, and in the process, a lot of energy is released.
Now, of course, you need to supply some initial energy to start the reaction, because, you know, you need energy to break the bonds. But then once you get it started, the energy released is so much more that a part of that energy released can be used to further break more bonds and the reaction keeps sustaining itself, providing us with thermal energy and will keep on going until we run out of sugars or oxygen.
So what's interesting is that if you think about the energy flow, it comes from the Sun, the plants store that energy from the Sun into the sugar molecules, and the sugar finally finds its way into your marshmallow and when you burn that marshmallow, that same energy from the Sun is released.
That's interesting, right? In fact, most of the energy on the Earth comes from the Sun itself. Okay, finally, let's model how this energy moves around in a little bit more detail by using another example, hot packs and cold packs. Consider this reaction where iron reacts with oxygen to produce iron three oxide.
Now, it releases energy over here. That's why we've written it as plus energy, right? So this is an exothermic reaction. But the way we think about it, where is the energy being released over here? Where is it going? Well, we can think that the energy is going out of our reaction system into the surrounding.
That's another way to think about exothermic reaction. The energy is going into the surrounding, and that energy can be used up to heat the surrounding, and that's exactly how your hot packs work. This is how they can heat up the surrounding, and the surrounding over here could be your hand, for example.
And again, where is this energy coming from? Well, iron and oxygen, this particular arrangement had a higher chemical energy. When we went down to iron three oxide, it has a lower chemical energy. And so this rearrangement causes some energy to be released. Now, consider what happens when you are dissolving a solid in water.
It's not really a chemical reaction, but they can break apart, and that requires energy. So in this particular case, energy is being absorbed, but from where? Well, again, from the surrounding. And this surrounding could be anything, like, for example, from your hand.
If you're holding this system, then energy will move out from your hand and your hand will feel cold. This is how cold packs work. Notice that it's an endothermic process because energy is going from the surrounding to the system. So putting it all together, an exothermic reaction is where energy is released from the system to the surrounding.
Where did this energy come from? Well, the energy is released when the high-energy reactant molecules is rearranged to form low-energy product molecules. And the endothermic reaction is the exact opposite. In these reactions, energy is absorbed. And where does it go?
Well, the energy is stored when the low-energy reactant molecules are converted into high-energy product molecules. And so a fundamental thing we are seeing over here is that energy is not created or destroyed. It's only transferred from one entity to another.
We call this the law of conservation of energy. It's one of the fundamental laws of our universe.
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