Rate of Reaction - Online Tutor, Practice Problems & Exam Prep

Rate of Reaction Concept 1

Hey guys, in this brand new set of videos, we're going to take a look at the rates of chemical reactions. Now, we're going to say that chemical kinetics is basically the study of reaction rates. When we say rates, we're really talking about speed. We're going to say the word kinetic comes from the Greek word kinesis. In Greek, kinesis just means motion. Remember, attached to motion is speed, so chemical kinetics is looking at how fast our reactants or products are changing over a period of time. That's all kinetics really is and that's what rate is. Rate is tied to speed. We're going to say, up to this point, hopefully, we all remember stoichiometry. 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 my compounds are reacting in my balanced chemical equation.

Rate of Reaction Concept 2

Now we're going to say fundamentally when looking at any balanced equation. Here we have a simple one. A is changing into B. Now A could be any type of compound. B could be any type of compound. But fundamentally, this is a balanced chemical equation. We're going to say when looking at any chemical equation, we're going to say in the simplest way, it's just our reactants breaking down to give us products. And if we take a look at the time elapsed, when we start out at 0 seconds, right? Initially, all we have are the white balls. Those are A. So in the beginning, before a reaction is even allowed to start, we have only reactants. And take a look, over time what begins to happen. Over time, you're going to get more and more of compound B, our product forming. Now, the more our product forms, the less reactants we're going to have around because remember, the products are made from our reactants breaking down. Eventually, we'll say that our reaction reaches completion. We're going to say when our reaction reaches completion, basically almost all of the reactions are gone and all we have are products. We're going to say for reactions that go to completion, we have one single arrow going forward. Later on, we're going to learn about reactions that don't reach completion but actually reach equilibrium. At those states, we're not going to have a single arrow going forward. We're going to have 2 arrows, one going forward and one going backwards. We're going to say when we have double arrows, we're at equilibrium. When we get to states of equilibrium, we don't get rid of our reactants. Eventually, we're going to have some reactants and products basically living in harmony with each other, living in equilibrium. For now, we're just going to be worried about reactions going to completion. But eventually, we're going to get to the point where we should realize that sometimes chemical reactions go to equilibrium.

Rate of Reaction Concept 3

Remember we said that chemical kinetics is basically looking at how fast our reactants 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 going to say that there are certain factors that influence if it'll go slow or go fast.

Rate of Reaction Concept 4

The first factor is concentration. Now we're going to 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 going to have more of them bouncing around, so there's a greater chance of them colliding. So we're going to 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 going to 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 2 cars going head to head to each other. If they're going at 5 miles per hour, they're going to hit each other but they're not going to be able to stick together. But let's say both cars are both going at a 100 miles per hour 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 hit 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 going to bounce off one another.

Rate of Reaction Concept 5

The second factor that we have to look at is surface area. So basically we're going to say the greater the surface area, the greater the chance the reaction will occur and the faster that reaction can occur. So we're going to say here, the frequency of collisions increases with increasing surface area. Now if we take a look at these 2 compounds, both have 4 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 one, 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 2 compounds neither of which was 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 going to 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.

Rate of Reaction Concept 6

The next factor we're going to discuss is temperature. Now, we're going to 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, the energy of motion. And in that way, they're going to move a lot faster. And again, what did 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.

Rate of Reaction Concept 7

Finally, the fourth thing that can affect our rate, which can either make it fast or slow, is a catalyst. A catalyst can increase the rate of the reaction by decreasing the energy of activation.

Now, what we have here is our reactant line, and over here we have our product line, and we call this an energy diagram. Basically, our reactants start off here at around an energy of 50 kilojoules. The reactants then have to travel up to the top, to this very point up here. This is called our transition state. Just realize a transition state is a hybrid. The transition state looks a little bit like the product, a little bit like the reactant.

Now, when you're at the transition state, you have two possibilities. You could either slide back down to become a reactant again, or if you have enough energy, you can tip over and slide down to become a product.

We're going to say the distance from the top of the hill to the bottom of the hill where the reactant line is, that is our energy of activation, ea, the amount of energy it takes to climb up to the top. And we're going to say the way a catalyst works is that a 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, allowing you to get to the top of the hill faster, and in that way, you can slide down the hill faster to become a product. That's how a catalyst will work.

So, just remember the four factors that influence the rate of a reaction. If you can manipulate these factors in a certain way, you can make your reaction faster. Increasing concentration, increasing surface area, increasing temperature, or adding a catalyst, all will make your rate faster by affecting each of these different factors. So, just remember the four factors involved in speeding up a chemical reaction.