1

concept

## Reaction Rate

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and this video, we're going to begin our lesson on reaction rate. So the reaction rate is also commonly known as the reaction velocity and is symbolized with lower case letter V. And really, all it is is the speed that a reaction proceeds as written from the left to the right of the reaction. And it's typically expressed as the change in the product concentration over a given time interval. And that's exactly what we're saying. Over here. It's the ratio of the change in the concentration of products over the change in time and so down below. We have all of these equations to express the reaction velocity or the reaction rate, which we already know is symbolized with lower case letter V. And all it really is is the change in the concentration of some substance that participates in the reaction over a change in time. And so technically, we could look at the change in the concentration of reactant over a change in time to get the reaction velocity. But we have to remember that the concentration of reactant is decreasing over time as the reaction proceeds. And so that means we need to remember to include the negative sign here. But as we'll see moving forward in our course, the reactant concentration is not typically the main focus when it comes to the reaction velocity. And instead, biochemist tend to focus on the change in the concentration of products over the change in time when it comes to reaction velocity. And so what I want you guys to know is that the reaction velocity or the reaction rate is equal to the change in the concentration of product over the change in time. And so because the reaction rate or the reaction velocity is equal to the change in concentration over the change in time, it's expressed in units of concentration over units of time, and so standard units of concentration are molar ity, and standard units of time are seconds. And so what's important to know is that in a graph that plots the concentration of product on the Y axis and the time on the X axis, the slope of the line between any two points in our curve is equal to the reaction velocity. And so before we actually take a look at our curve down below, I want you guys to recall from your previous courses that the equation for a line is why equals m x plus B, where the m here is the slope of the line and really all the slope is is the rise over the run and so up above. We said that the, uh in a plot that has the concentration of product against the time that the reaction velocity is equal to the slope. And so here we have that the slope is equal to the reaction velocity and again up above. We said that the reaction velocity is equal to the change in concentration of products over the change in time. And so now, taking a look at our graph down below notice that our curve is this green curve that we see here. And if we take the line that forms between any two points in our curve, we can get, uh, the reaction velocity through the slope of that line. So early on in our reaction here, you can see that we can get the line between these two points, will give us this black line here, and the slope of this line will equal to the reaction velocity and again the slope is just the rise over the run. And so you can see that the rise is gonna be on the y axis. So it's gonna be the change in product concentration. So that's exactly what we have here over the run, and the run is gonna be on the X axis horizontally. And so that's gonna be the change in time. And so this ratio right here gives us the reaction velocity. And so what I want you guys to note is that typically reactions. Uh, the reaction rate of an enzyme catalyzed reaction will actually decrease over time. And so notice that if we take the points later in our curve that the slope of the line is actually going to decrease. So now notice that the slope gets lower and if we move on later to our graph, notice that the slope gets even lower and so the slope will continuously decrease and the reaction rate will continuously decrease over time for a typical enzyme catalyzed reaction. And so notice it gets to a point where the reaction rate So between these two points over here, notice that the reaction rate or the slope of the line is just equal to zero, where the line is just a straight horizontal line. And so the reason for that is because equilibrium has been reached and so remember that react enzymes do not change the equilibrium of a reaction. Instead, enzymes speed up chemical reactions so that they can get Thio equilibrium faster. And so this point here, the reason that the curve levels out like this and the slope gets to zero is because equilibrium has been reached. And so this is something important to note as we move forward in our course. And that concludes our lesson on reaction rate, and I'll see you guys in our next video.

2

example

## Reaction Rate Example 1

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all right. So here we have an example problem that wants us to calculate the reaction rate for this following reaction here where we have reacted a being converted into product beat and notice that were given the initial concentration of a as one Moeller the initial concentration of B as zero and the final concentration of B after just two seconds as 0.2 Moeller. And so we've got these four potential answer options below. And so what we need to recall from our previous lesson video is that the reaction rate or the reaction velocity is symbolized with letter V, and it's commonly expressed as the change in the concentration of the product over the change in time. And so notice that the product of our reaction is B and so were given both the final and the initial concentration of our product. And so all we need to do is take the difference between these two. Remember to do the final minus the initial to get this change in product concentration. So the final concentration is 0.2 Mueller. So we have 0.0 to Mueller and then we're going to subtract the initial concentration, which is zero. And this is going to be over the change in time, which is two seconds so we can put two seconds here. And, of course, 0.0 to minus zero is just 0.2 And so all we need to do is 0.2 divided by two, which is equal to 0.1 And the units are going to be molar ity per second so we can put that in over here. And so notice that this answer option here matches with answer option A So we could go ahead and indicate that a Here is the correct answer for this example problem. And we'll be able to get some practice utilizing and calculating the reaction rate and our next practice video. So I'll see you guys there.

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Problem

Calculate the reaction rate for A → B, given that [A] _{i} = 6.3 M, [B] _{i} = 0 & [A] _{f} after 4.8 seconds = 1.14 M.

A

1.69 M/s

B

2.53 M/s

C

0.24 M/s

D

1.08 M/s