Alright, you gotta know what time it is. It is time to finish the chem walkthrough. We are going to finish what we started. The unit reviews are finally coming to an end. Hello everybody!
It's finally time to finish this off with Unit 5 Part 2 because in the previous one I originally thought it would be a short tips video but I ended up only covering half of the stuff in this unit so we are going to finish this off with Part 2. Basically the two things that we didn't cover so far are Collision Model and Catalysis so this shouldn't take too long. Let's get through it! Alright, so the first thing we gotta talk about is Collision Theory, and Collision Theory is actually kinda cool! It basically helps us explain why reactions occur at a certain rate. So essentially what the theory says is that in order to react, like let's say we have the reaction A plus BC yields AB plus C.
So essentially you have an A colliding with a BC, and then they turn into an AB and a C. So essentially there's three criteria in order for this reaction to occur according to collision theory. So first thing, the reactants actually had to collide.
Wow, collision theory? Who would have known that the reactants actually had to collide? This is crazy.
And then they also need to have enough energy for the reaction to occur. They had to hit each other with enough speed for the reaction to happen. Otherwise the bonds would not change between the A and the B and all that nonsense.
And the third and final criteria we got to satisfy is we got to make sure they hit at the right angle, okay? Like if you hit a baseball and you hit it at the wrong angle, it's just gonna go way out of the way and you're not gonna get any home runs or nonsense. But if you hit it right, then you get a home run. So, A and B, C have to collide at the right angle for them to get a home run and become A, B, plus C. So like, it kinda makes sense if you look at it this way, right?
Let's say that A collides with B, C but it's flipped this way. Then there's like, it doesn't make sense that this A is just magically gonna transport and form a bond with B. It's just gonna hit the C and bounce off.
So in order for them to actually create the reaction, the B has to be over here, they have to collide over here, and the C is over here. So there is a specific direction in which they have to collide. So some people like logic, I kind of like logic because it's so much easier to think about things, but some people like equations, and equations are usually easier to memorize through logic, but if you can memorize the equation, then the logic falls. So there is an equation for collision theory, and it's called the Arrhenius equation.
Now my teacher did not talk about this one at all, and you don't have to use this mathematically, I'm pretty sure, on the AP. but like just knowing it and if you're good at equations if you were able to memorize it it'll explain a lot of why collision theory works. Essentially what it says is k is equal to a e to the negative e a over r t.
So let us go through this and explain what everything means. First guy over here is called k and you probably recognize that as a reaction rate we're talking about kinetics. Then this guy over here is called the frequency factor and as the name I suggest it has to do with how often collisions occur and how often they occur in the right orientation. So this deals with the first and third criteria for our collision theory, which is sometimes called F.
It's basically a measure of what fraction of collisions have enough energy. So this right here is E, which is like 2.7 or the constant E. And then this E over here, the EA, is activation energy.
Activation, E for energy, epic. This right here is the gas constant at 8.31 joules per Kelvin, mole or whatever nonsense. And then this is the temperature in Kelvin. And basically what activation energy is, is how much energy it takes for the reaction to happen. Because remember, the second condition for our collision model is that they have to have enough energy to cause a reaction.
So the reason why this makes sense is because if you have a higher activation energy, right, you need more energy for this to happen. So that means that less of the collisions are going to have enough energy. So as you increase this, this becomes more negative, and then this overall F becomes smaller.
Makes sense. As you increase temperature... this thing decreases. So overall this f becomes bigger because your exponent is getting less negative so this is going to get bigger. And that makes sense right?
As you have a higher temperature more of your collisions are going to be higher energy. So that's basically the logic behind this equation if you want to memorize the equation and go to the logic. Now you can do some really nasty like graphing stuff but I'm like 99.9% sure you don't have to worry about that.
So we're not going to worry about it. Exactly. So now let's talk about reaction coordinate diagrams. Basically They look like this. This over here is the E of the system.
This over here is the progress of the reaction, which is also called the reaction coordinate, which is why they're called reaction coordinate diagrams. And basically they usually look something like this and this should be a flat line. So basically your reaction starts off at the reactants, right? You start with your reactants and you turn them into products. So at the beginning you got reactants and you got products.
However, as you can see The reactants first have to go up here, they have to increase their energy before they go into the product. So you can think of it like a hill, right? So you start with a ball, that's your reactants.
Then you have to excite these reactants, and they have to be moving fast enough so that they can get over this hill. And once they get over this hill, everything goes down from there and they reach the product. The height of this hill, or the energy required for the reaction to take place, is called Ea, activation energy.
Wow, when did we see this before? Yes, that's right, in the Arrhenius equation, holy moly. And this makes sense right, because you need a certain energy for a reaction to occur.
Now what about this height, between the reactants and the products, what does this signify? Well this is called the delta E of the reaction, or you probably recognized it from before, it's also called the delta H. So essentially, if your reactants have more energy than your products, then you must have released some energy in order to get to your products. So what is that?
That is called an exothermic reaction, and that's basically when delta E is negative, because you're releasing energy. What happens if our diagram look like? Wait up, wait for it, wait for it. Wablamo!
Wablamo! Okay, I can't draw. This is so hard.
Wablamo? Okay, you know what? Good enough.
So, your products are here, reactants are here. Now, our delta E is still the same, kind of, but you're going from lower energy to higher energy. So, if you're gaining energy, you've got to take in energy from your surroundings.
That's why it's called endothermic. You're taking it into yourself. Endothermic. and that means that delta E is greater than zero.
And what would be the Ea of this reaction, the activation energy, is basically the size of the hump. And remember, it's the height from the reactants, not the product, because the reactants have to roll up the hill. You don't have to roll the products backward up the hill, that's not how it works. Activation energy of the forward reaction is from the reactants to the top of the hill.
If you wanted the reverse reaction, then sure, you could take the products and roll them up the hill that way, but if they're asking for the forward reaction, you have to do it from the reactants to the top of the hill. Alright, one last thing to mention about this. This over here, the apex, when your ball gets to the top of the hill, this is called the transition state. Or the activated complex. And basically the way to remember this is like, it's halfway between the reactants and products, so it's called the transition state.
It's transitioning between the two. You can also think of it as an activated complex because now that it's reached this position, the reaction can occur. Epic, we figured out how to read reaction coordinate diagrams, now let us talk about the next thing we gotta talk about. Wait, that was so useful.
I literally said the same thing. in like 600 words. Epic. Catalyst is finally the beautifulest word in the history of English.
No that would probably be rendezvous. Actually does that count as an English word? Dude I don't even know anymore.
God dang it. So basically catalysis is basically when a catalyst does something right and the definition of catalyst is something that's not consumed that also speeds up the reaction. That's probably the least eloquent way to word it but that's how I like to think about it. I'm not trying to come off as a knockoff of Khan Academy but I kind of am.
So like, you know, why can't I use this stuff, right? So this is basically the example that Khan Academy used to explain catalysis, and I'll try to explain it in my own way. So, basically, this reaction occurs.
Wow, that was eloquent. You're decomposing hydrogen peroxide into water and oxygen. So, let's look for the catalyst. So H2O2 is clearly not going to be our catalyst because it's consumed. What about I-?
So, okay, nothing happens over here. I-is not consumed here, it's not consumed there. And well blammo, it comes out at the other end.
It gets eaten and then gets pooped out. That's crazy. You do see that it does get converted to something else in the middle, but it still remains the same at the very end of the reaction.
So basically, just because I-was in the beginning and it came out untouched at the end, it's called a catalyst. And it speeds up the reaction. So you know that H2O2 is a reactant, H2O2 is a reactant, this guy is a catalyst, this guy is a catalyst.
These guys are products. So then what is this guy doing over here? What is this troll IO-bro doing over here?
Dude he's so lame dude, he gets produced and then just gets consumed immediately after. Well that's why he's called an intermediate. And an intermediate is basically something that's created in a reaction, but it's consumed during the reaction as well. And that makes sense right?
It's not at the beginning, it's not at the end, so it's gotta be an intermediate. Alright Epic, now there's only one thing you gotta know more about catalysis, and they're basically different types of catalysts. First one are enzymes.
These guys are just biocatalysts. If you've taken any bio whatsoever you probably know that. Biocatalyst.
Like DNA polymerase, anything that ends with A is caspase. Like, I don't know, what else? DNA ligase, helicase, pl-I'm probably bringing back very good memories right now.
And then a completely different categorization of them is homogenous, which basically means it's in the same phase as the reaction. So if your reaction is occurring in an aqueous solution, then your catalyst should also be aqueous. And then heterogeneous is basically... Dude, I don't even know if I'm saying it right. I suck at English, but anyway, heterogeneous basically means that it's a different phase from the reactants and an example of that is when you have like a surface Heterogeneous catalysis is literally just called surface catalysis because it's like if you do a gaseous reaction over a surface The surface could help catalyze the reaction.
So it's basically a different phase So if your reaction has to do with gases and you're using a solid catalyst, then that's a heterogeneous catalyst All right, one last thing we got to talk about. The ectrophotometry is going to be so epic Literally what spectrophotometry is doing is you take a vial of solution, you shine a light through it That's an arrow and you measure how much light comes out. Now obviously some of the light is going to get absorbed by the liquid.
So you start with I0 here, you end with I1 and I1 is less than I0. So it makes sense right? If you do I1 over I0 is what fraction of the light is transmitted? So it's called transmittance.
If you take the negative log of this you get something called absorbance. And this makes sense right? As you increase your transmittance your absorbance should decrease. So absorbance is basically a measure of how much is absorbed.
Transmittance is how much is transmitted. transmitted I can't speak but makes sense but the interesting one is absorbance because we could write a very cool equation called Burt Lambert law a is equal to abc oh my god technically it's also epsilon lc which is more descriptive but abc just sounds cooler and basically a is absorptivity b is path length and c is your concentration and then this over here is your absorbance but basically the only thing you got to take away from this equation is that your absorbance is proportional to your concentration. So if your AP test gives you like spectrophotometry data and it's like the absorbance is like 0.6 then you know you can just literally treat that as your concentration because if you multiply by a constant it will give you your concentration.
So like you know how I was talking about finding experimental reaction orders like they give you a bunch of concentrations and you have to find the ratios between the two? You could literally do the exact same thing with absorbances so you could literally pretend that absorbance just means concentration. Alright Epic, we are finally done with AP Chemistry! Well, I mean technically there's other units, but we covered everything that's on this year's AP exam, so...
What can I say? That was kinda epic! Alright, as always, if you enjoyed the video, leave a like and subscribe for more.
Let me know if you guys want more of these kind of videos. Let me know in the comments what specific kind of videos you guys want. But other than that, thank you guys for watching again!
See you guys next time!