Reaction Rate - Online Tutor, Practice Problems & Exam Prep

The reaction rate, symbolized as v, measures the speed of a chemical reaction, defined as the change in product concentration over time. It can be expressed as $\frac{∆[\mathrm{Product}]]}{∆t}$. Typically, the focus is on product concentration, as reactants decrease over time. The slope of a concentration vs. time graph indicates the reaction velocity, which decreases as equilibrium is reached, highlighting the role of enzymes in accelerating reactions without altering equilibrium.

Reaction Rate

In 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 the lowercase 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 the lowercase 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 reactants 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, biochemists tend to focus 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 molarity, and standard units of time are seconds.

And so what's important to note 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 y = mx + 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, 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, 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 going to be on the y-axis, so it's going to be the change in product concentration. So that's exactly what we have here, over the run. And the run is going to be on the x-axis, horizontally. And so that's going to be 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, 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 rate, so between these two points over here, notice that the reaction rate or the slope of the line is just equal to 0, where the line is just a straight horizontal line.

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 to equilibrium faster. And so this point here, the reason that the curve levels out like this and the slope gets to 0 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.

Reaction Rate Example 1

Alright. So here we have an example problem that wants us to calculate the reaction rate for this following reaction here, where we have reactant a being converted into product b. And notice that we're given the initial concentration of a as 1 molar, the initial concentration of b as 0, and the final concentration of b after just 2 seconds as 0.02 molar. And so we've got these 4 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 the 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 we are given both the final and the initial concentration of our product. And so all we need to do is take the difference between these 2. Remember to do the final minus the initial to get this change in product concentration.

So the final concentration is 0.02 molar, so we have 0.02 molar. And then, we're going to subtract the initial concentration, which is 0. And this is going to be over the change in time, which is 2 seconds, so we can put 2 seconds here. And of course, 0.02 minus 0 is just 0.02. And so, all we need to do is 0.02 divided by 2, which is equal to 0.01, and the units are going to be molarity 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 can 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 in our next practice video. So I'll see you guys there.

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

Here’s what students ask on this topic:

What is the definition of reaction rate in biochemistry?

The reaction rate, symbolized as v, measures the speed at which a chemical reaction proceeds. It is defined as the change in the concentration of products over a given time interval. Mathematically, it can be expressed as:

$\frac{∆\left[\mathrm{Product}\right]}{∆t}$

This means that the reaction rate is the ratio of the change in product concentration to the change in time. It is typically expressed in units of molarity per second (M/s).

How is reaction rate related to the concentration of reactants and products?

The reaction rate is directly related to the concentration of reactants and products. As the reaction proceeds, the concentration of reactants decreases while the concentration of products increases. The reaction rate can be determined by measuring the change in product concentration over time. For reactants, the rate is often expressed with a negative sign to indicate the decrease in concentration:

$\frac{∆\left[\mathrm{Reactant}\right]}{∆t}$

However, biochemists typically focus on the change in product concentration to determine the reaction rate.

What units are used to express reaction rate?

The reaction rate is expressed in units of concentration over time. The standard unit of concentration is molarity (M), and the standard unit of time is seconds (s). Therefore, the reaction rate is typically expressed in molarity per second (M/s).

How does the slope of a concentration vs. time graph relate to reaction rate?

The slope of a concentration vs. time graph represents the reaction rate. In such a graph, the concentration of the product is plotted on the y-axis, and time is plotted on the x-axis. The slope of the line between any two points on the curve is equal to the reaction rate. Mathematically, the slope (m) is given by:

$\frac{∆\left[\mathrm{Product}\right]}{∆t}$

As the reaction progresses, the slope decreases, indicating a decrease in reaction rate until equilibrium is reached.

Why does the reaction rate decrease over time in enzyme-catalyzed reactions?

In enzyme-catalyzed reactions, the reaction rate decreases over time because the concentration of reactants decreases as they are converted into products. Additionally, as the reaction approaches equilibrium, the rate of the forward reaction decreases. Enzymes speed up the reaction to reach equilibrium faster but do not change the equilibrium position. Therefore, the reaction rate decreases until it reaches zero when equilibrium is achieved.