Energy changes in chemical reactions (part 2)

0:00- [Instructor] Certain chemical reactions are endothermic, like the ones that are carried by plants, versus other chemical reactions like burning a marshmallow are exothermic. And this is pretty useful, we can use it to build stuff like cold packs and hot packs.

0:14But how do we control how much energy is absorbed or released and how quickly it is absorbed and released in these chemical reactions? That's what we wanna talk about in this video, so let's begin. But first, a quick recap. In endothermic reactions, energy is absorbed from the surrounding into the system, but where does the energy go?

0:32Well, turns out that certain energy amount of atoms have higher chemical energy compared to others. And so when you go from here to here, energy is stored in this particular arrangement. In an exothermic reaction, energy is released from the reaction system into the surrounding.

0:45And where does the energy come from? Well, that stored energy is now released to the surrounding, giving us an exothermic reaction. But now coming back to the big question, how do we control how much energy is released and how quickly this energy is released? So let's think about that.

1:00So we need to control two things, the amount of energy that's absorbed or released and how fast it is absorbed and released. So let's start with thinking about this one. How do we control how much amount of energy is absorbed or released? And let's you know, for the sake of an example, consider a hot pack.

1:14Inside a hot pack, let's say this is the reaction that's happening. We have ion that reacts with oxygen to form ion three oxide. It's an exothermic reaction, so it releases energy. So the energy goes from the reaction system into the surrounding. Again, why is it exothermic?

1:28Well, because it turns out that this arrangement of atoms has a higher chemical energy compared to this arrangement. Now what happens when you go from here to here? Well, certain bonds are broken and then new bonds are formed. Breaking bonds is like kind of like pulling magnets apart.

1:41It requires energy. So breaking bonds in this particular case requires about this much amount of energy. This is the amount of energy that you have to supply. But then, when new bonds are formed, it releases energy. Kind of like when magnets stick to each other, they release energy.

1:54Over here, this is the amount of energy that is released. So you can see the energy output is more than the energy that you have supplied. That's what makes it an exothermic reaction. But let's take some numbers. Okay? So let's say the energy that you supplied was 10 units, and the energy that was released was 30 units.

2:09What's the balance? The balance is that overall, 20 units of energy was released. This is the energy that is released over here. We call that as the net energy that got released in this particular reaction. So in our example, which is just a conceptual example, okay, we see that when we react four atoms of ion with three molecules of oxygen, that's our reactant, when we take this much reactant to form two formula units of ion three oxide, 20 units of energy is released.

2:40And again, we can show it this way schematically. The question now is what would happen if we took twice the amount of reactants, how much energy would be released? Why don't you pause the video and think about this? Alright, let's see. Since we have twice the amount of reactants, we have to break twice the amount of bonds as before, which means we have to supply twice the amount of energy as before, but we also get twice the amount of products, so twice the amount of energy is released as well.

3:12As a result, look, the net energy release is also twice the amount as before. Which means, by increasing the amount of reactants, I have increased the amount of energy. This is how we can control the amount of energy that is released or absorbed, if you're dealing with endothermic reactions.

3:28The way to control that is by changing the amount of reactants. So by changing the amount of reactants, you can control the amount of energy absorbed or released. Okay, that's one way to do that. What else can we do to control this? Well, let's see. When we used ion, this was the amount of energy that was released, right?

3:42But what if you use a different substance altogether? Different substances have different structures, and so the energy levels will also be different. And as a result, the amount of energy released would also be different. So for example, instead of ion, this is how much we get, this is the amount of energy we get for ion, but instead of ion, what if we burnt wood?

4:01Well then because the structures are different, it turns out that we'll end up getting lesser energy than before. On the other hand, if we use natural gas, the structures are such that a lot of energy is released when we burn it. And so using different substances is another way of controlling how much energy is released.

4:18So the second way of doing that is by changing the type of substances that we are using. Okay, now the last question for us is how do we control how fast or the rate at which the energy is absorbed or released? To answer this question, we need to remember an extremely important thing about chemical reactions.

4:32The reactant particles need to collide with each other to break their bonds. For example, take a look at these chemical reactions that are happening, okay? Now look at what happens when they collide. Can you see that? Ooh, did you see that? Look, look, when they collide, look, this bond broke and new bonds were formed.

4:50So collision is the key for chemical reaction to happen. And during this chemical reaction, if this was an endothermic reaction, it'll absorb energy from the surrounding. And if it is an exothermic reaction, it will release energy into the surrounding. So to control the rate at which energy is absorbed or released, we need to control how many collisions are happening per second.

5:09More collisions per second means more energy absorbed or released. Less collisions per second means less energy absorbed or released. So now the big question is how do we control the amount of collisions happening per second? The first one would be, again, changing the amount of reactants.

5:22Let's look at what happens if you have a little bit of reactance. Let look at the number of collisions that are happening, how quickly they're happening. Okay, now let's consider what happens if you have a lot of reactants. Ooh, can you see? So many more collisions are happening.

5:35Boom, boom, boom, boom. So many collisions are happening per second compared to this one. So if we increase the amount of reactants, not only will we get a lot more energy than before, like we saw earlier, but we also get that energy much quicker. So that's one way of controlling how fast energy is absorbed or released, by increasing or decreasing by changing the amount of reactants that we have altogether.

5:56Okay, what else can we do? Well, the second thing we can do is change the temperature. Because you know, a temperature is a measure of average kinetic energy. Again, let's consider what happens when at at low temperature, the average kinetic energy is low, so you have less collisions happening per second.

6:10But if you have a higher temperature, well you have a higher kinetic energy, so there'll be moving faster. And you can see, more collisions will be happening per second. But higher temperature will only increase the rate, not the amount of energy transfer. Okay, what else?

6:23The last thing you could do is change the distribution of particles altogether. For example, if you have reactants like this, they're distributed like this, then you can see the collisions can only happen over here. The chances of these, you know, molecules colliding with these molecules is much lower.

6:36So the amount of collisions happening per second over here would be lower. But what if the particles are distributed like this? Ooh, if the reactants are more uniformly distributed, more chances of collisions. And so you'll have more collisions happening per second, more energy absorbed or released per second.

6:51Well, we can do this by stirring the reactions of the particles come in contact more often. And if one of our reactants is so solid, well we can crush or break up the solid so that more particles are exposed to the other reactants. Again, this only increases the rate of energy transfer, but not the amount.

7:06Long story short, certain chemical reactions release energy, we call them exothermic reactions, and others absorb energy, we call them endothermic reactions. How do we control the amount of energy release or absorbed? Well, one way is by changing the amount of reactants, more reactants, more energy absorbed or released.

7:20And another way is by changing the type of substance we're using altogether. Secondly, the rate at which the energy is absorbed or released is controlled by the number of collisions that is happening between the reactant particles per second. And again, that can be changed by changing the amount of reactants or changing the temperature, or just changing the distribution of the particles.

7:39More uniformly the reactants are distributed, more is the collision rate, more is the energy absorbed or released.