Rate of Reaction

Hey, guys, In this brand new set of videos, we're gonna take a look at the rates of chemical reactions. Now, we're gonna say that chemical kinetics is basically the study of reaction rates. And when we say rates, we're really talking about speed. We're going to say the word kinetic comes from the Greek word kinesis. And in Greek, kindness is just means motion. And remember, attached to motion is speed. So chemical kinetics is looking at How fast are reacting are poor products are changing over a period of time. So that's all kinetics really is. And that's what rate is. So rate is tied to speed and we're gonna say, up to this point, hopefully we all remember Stoke Yama Tree. So we've learned how to calculate the limiting reactant amount. We've learned to calculate the theoretical yield. Now it's up to us to figure out the rates of these reactions. Figure out How fast are my compounds reacting in my balanced chemical equation?

Now we're gonna say fundamentally when looking at any balanced equation here we have a simple one. A is changing into be okay now. It could be any type of Compound B could be any type of compound, but fundamentally, this is a balanced chemical equation. We're gonna say when looking at any chemical equation, we're gonna say in the simplest way, it's just are reacting. It's breaking down to give us products. And if we take a look at the time elapsed when we start out at zero seconds, right initially all we have are the white balls. Those are a So in the beginning, before reaction is even allowed to start, we have Onley reacted and take a look over time. What begins to happen over time, you're gonna get Mawr and Mawr of compound be our product for me. Now, the mawr our product forms, the less reactions we're gonna have around because remember, the products are made from our reactant is breaking down. Eventually, we'll say that our reaction reaches completion. So we're gonna say when our reaction reaches completion. Basically, almost all of the reactions are gone and all we have our products. We're gonna say for reactions that go to completion. We have one single era going forward. Later on, we're gonna learn about reactions that don't reach equilibrium that don't reach completion but actually reach equilibrium at those states. We're not gonna have a single arrow going forward. We're gonna have to arrows. One's going forward and one's going backwards. We're gonna say, when we have double arrows were at equilibrium. When we get to states of equilibrium, we don't get rid of our reactions. Eventually, we're gonna have some reactant and products basically living in harmony with each other, living in equilibrium. For now, we're just gonna be worried about reactions going to completion. But eventually we're gonna get to the point where we should realize that sometimes chemical reactions go to equilibrium.

Now, remember we said that chemical kinetics is basically looking at how fast our reactant or products change over time, and we should realize that when it comes to a chemical reaction, a reaction could either go slow or it can go incredibly fast. And we're gonna say that there are certain factors that influence if it'll go slow or go fast.

the first factor is concentration. Now we're gonna say in order for a reaction to occur, molecules must collide. So, molecule A. Has to combine with molecule B. Now, what we should realize here is increasing the number of molecules in a container, we're gonna have more of them bouncing around so there's a greater chance of them colliding. So we're gonna say adding more molecules inside the container, increases their collisions, and as a result we have a greater chance of them sticking together, them sticking together to form our product is the reaction. We're also gonna say they not only need to collide with each other, but they need to collide with each other with sufficient energy and they have to hit each other in the correct spots. So they have to hit with sufficient energy and correct orientation in terms of sufficient energy, just think of it as this, you have two cars going head to head To each other. If they're going at five mph, they're gonna hit each other, but they're not gonna be able to stick together. But let's say both cars are both going at 100 MPH towards one another. That head on collision will force them to be basically smashed together in the same way our molecules do the same thing, molecules have to be moving incredibly fast in order to stick to one another. Otherwise their collision is going to be elastic meaning they'll just bounce off each other. Also, if you guys are bio majors, just remember, we also talk about activation sites, basically they have to eat each other in the correct spot, and in that way they'll stick together again. If they don't hit each other in the correct spot, they're just gonna bounce one another, bounce off of one another.

the second factor that we have to look at is surface area. So basically we're gonna say the greater the surface area, the greater the chance the reaction will occur and the faster that reaction can occur. So we're gonna say here, the frequency of collisions increases with increasing surface area. Now, if we take a look at these two compounds, Both have four carbons, but they're arranged differently in the first one, it's kind of linear, it's just a straight chain, but in the other ones, it's shaped like a square. So it's cyclic, it's a ring. What you should realize here is linear structures have more surface areas to react. So you always want a linear structure. So the structure on the left would have greater surface area and therefore its rate would be faster than the cyclic one. Let's say we were comparing two compounds, neither of which were cyclic. Let's say we had a second compound that we're comparing. And let's say we're comparing this one here with our same structure we said earlier in this case, this would still have more surface area because again, it's linear. This right here kind of branches off, we call that a branch group. This is the linear part of our structure. But this piece here is kind of sticking out, We're gonna say with increased branching, there's less surface area. So again, it's better to be linear with no branching groups. And it's also better to be linear and not cyclic, like we have here. So this structure in the middle would be the one with the most surface area

The next factor is gonna say is temperature. Now we're gonna say increasing the temperature, increases the rate of the reaction by increasing the energy and frequency of collisions. Think about it like this, if we increase the heat around a container that's filled with gas molecules, those gas molecules start to get excited because what's happening is they're going to absorb the thermal energy from the heat source. They then convert this thermal energy into kinetic energy energy of motion. And in that way they're gonna move a lot faster. And again, what do we say? We say? You have to move with enough energy so you have to be moving a lot faster in order to collide hard enough to stick together. And if you're moving faster, you have a greater chance of hitting one another within a given amount of time. So increasing the temperature will increase the energy and the amount of collisions we get within a snapshot of a moment.

finally the fourth thing that can affect our rate, which can either make it fast or slow. We're gonna say a catalyst. A catalyst can increase the rate of the reaction by decreasing the energy of activation. Now here we're gonna have is our reactant line and over here we're gonna have our product line and basically we say that this is called an energy diagram. Basically our reactant are starting off here at around energy of 50 kila Joel's. The reactant then have to travel up here to the top. At this very point up here, this is called our transition state and just realize a transition state is a hybrid. The transition state looks a little bit like the product, a little bit like the reacted. Now when you're at the transition state, you have two possibilities. You can either slide back down to become a reacting again. Or if you have enough energy you can tip over and slide down to become a product. We're gonna say the distance from the top of the hill, all the way down to the bottom of the hill where the reactant line is that is our energy of activation or e a. The amount of energy it takes to climb up to the top and we're gonna say the way a catalyst works is the re catalyst changes the reaction pathway. It actually makes the hill shorter. You still end with the same energy for your products, but you don't have to travel up as high to get to the transition state. So it lowers the energy of the transition state. So you get to the top of the hill faster and there were, and that way you can slide down the hill faster to become a product. That's how a catalyst will work. So I just remember the four factors that influence the rate of a reaction. If you can manipulate these guys in a certain way, you can make your reaction faster, so increasing concentration, increasing surface area, increasing temperature or adding a catalyst. All will make your rate faster by doing each of these different factors. So just remember the four factors involved for speeding up a chemical reaction.