Lesson Video: Reaction Rate Measurements | Nagwa Lesson Video: Reaction Rate Measurements | Nagwa

# Lesson Video: Reaction Rate Measurements Chemistry

In this video, we will learn how to perform different experiments whose results can be analysed to calculate the rate of reaction.

17:54

### Video Transcript

In this video, we will learn how to perform different experiments whose results could be analyzed to calculate the rate of reaction. When we carry out a reaction, what we’re doing is turning reactants into products. When we begin our reaction, we have an excessive reactants and no products. Remember, of course, that we could have any number of different reactants. As the reaction progresses, we gradually convert our reactants into our products. How fast this process occurs is our rate of reaction.

In chemistry, it’s often useful to be able to measure this reaction rate. And there are lots of ways we can do this. We could measure the rate at which we lose one or more of the reactants. Alternatively, we could measure the rate that we form one or more of our products. The method with which we choose to do this depends on the state or properties of the substance we are watching. For example, if we’re measuring the production of a gas, we might want to use a gas syringe. Regardless of which method we use, we’ll often end up with an answer in similar units. Generally, the units we use for the reaction rate is some kind of mass or amount over time. Examples include grams per second, centimeters cubed per second, moles per second, or, if we’re carrying out our reaction on an industrial scale, even tons per day.

So now let’s have a look at some examples of ways that we could measure our rate of reaction. One factor which we know affects the rate of reaction is the surface area when we’re dealing with solids and sometimes liquids. So let’s have a look at an experimental setup that could help us to measure this effect. Let’s use an example of the reaction of a carbonate with an acid, which produces a salt, water, and carbon dioxide. In order to set up this experiment, we need to remember that the law of conservation of mass means that the mass of all of the reactants added together should equal exactly the mass of all of the products added together.

We should note, however, that in this example reaction, one of our products is a gas. Of course, it’s pretty difficult to try and weigh a gas. We could try and capture it and measure its volume, but in this example, there might be an easier method. If we place our two reactants into a conical flask and don’t stop at the top, what will happen is as we produce our products, the carbon dioxide gas will escape. If we could measure how quickly this carbon dioxide is produced and escapes, we could measure the rate of this reaction. To do this, we could simply place our reaction vessel onto a balance. As our reaction progresses and we lose CO2, what we’re doing is losing mass. The mass, of course, isn’t destroyed. It’s simply lost to the atmosphere.

We can watch the rate or speed that this mass decreases, taking measurements every 10 seconds, say. And from this, we can work out the rate of reaction. Once the mass of our reaction vessel stops decreasing, we know that the reaction has gone to completion. What we’ll end up with is a graph a bit like this, showing the decrease in mass over time. But remember that we wanted to use this to investigate the effect of surface area on rate of reaction. So what we’ll need to do is set up another version of this experiment. But this time, we grind the calcium carbonate into really small pieces. What we’ll discover is that by grinding the calcium carbonate, we increase the surface area. And therefore, the rate of reaction increases.

Of course, in order to make sure that it’s fair, we need to have several of our variables controlled. Some of the important control variables in this experiment include the mass of this calcium carbonate that we start with, the volume and concentration of the hydrochloric acid that we use, the temperature, et cetera. In this case, the independent variable or the variable that we’re trying to test is the surface area. And of course, our dependent variable is what we’re measuring, which is the mass of our reaction vessel. Now let’s look at a different type of experimental setup for measuring rates of reaction.

Let’s now have a look at the effect of acid concentration on the reaction of magnesium metal with hydrochloric acid. When we add the state symbols to our reaction equation, we can see that one of our reactants is a solid and one of our products is a gas. We could measure the production of the hydrogen gas in a similar way to what we saw previously or perhaps with a gas syringe. However, in this example, there might be an easier way. In this reaction, our magnesium metal is a solid. And as the reaction progresses, that solid will disappear. So we could simply measure the time it takes for all of our magnesium ribbon to disappear in different concentrations of acid. What we will need is a variety of reaction vessels with hydrochloric acid at different concentrations and some magnesium ribbon.

But again, let’s think about our control variables. We must make sure that we’re using the same volume of acid in each of our experiments and again the same exact amount of magnesium ribbon. We’ll also want to try to keep the temperature the same across all of our experiments. Once we have a different concentration of acid in each vessel, we can drop a piece of magnesium into one and time how long it takes to disappear. We can then repeat this for all of our concentrations.

In this example, the time taken for the magnesium to disappear is our dependent variable. And our independent variable is the concentration of acid. If you did this experiment, what you should see is that as the concentration of the acid increases, the magnesium ribbon disappears faster. So the rate of reaction has increased.

Now let’s see how we could explore the effect of temperature on rate of reaction. Let’s have a look at the example of the disappearing cross experiment. In this reaction, sodium thiosulfate and hydrochloric acid react to form sodium chloride, water, sulfur dioxide, and sulfur. The trick to working out how to measure the rate of this reaction comes by looking at the states of each of our reactants and products.

Both of our reactants are dissolved in water, so this makes them aqueous. Sodium chloride, of course, is also aqueous. And we have water in its liquid form. Sulfur dioxide is a gas. And sulfur is a solid. This, of course, means that it does not dissolve in water. Because our sulfur does not dissolve in water, it precipitates out of solution, which means that we’ll be able to see it. As the reaction progresses and we produce this sulfur, we will see the solution turn a cloudy yellow white. So how can we use this to measure the rate of reaction?

What we’ll need is a conical flask, sometimes called an Erlenmeyer flask, a thermometer, and a piece of paper with a large black cross drawn on it. As we add the acid, we start our timer. As we add the acid, we’ll also need to swirl our flask to make sure that everything mixes. Conical flasks are ideal for swirling because you’re much less likely to splash the contents out than if you’re swirling, say, a beaker. You will then need to view your reaction from the very top of the conical flask. We’ll talk about why this is important in a minute.

As the reaction progresses and we produce our sulfur as a product, the solution will turn cloudy. As the solution turns cloudy, it will be harder and harder to see the black cross. Once the cross has disappeared completely, stop the timer. So our dependent variable or the thing that we’re measuring is the time taken for our cross to disappear from view. So what we’re really measuring here is the rate that our solution turns cloudy. So you may hear cloudiness in a solution referred to as turbidity. Turbidity is just the scientific way of saying the cloudiness of a solution.

We’ll then need to repeat this experiment at different temperatures. And this is where we need the thermometer. So our independent variable in this case is the temperature. And of course, there are some control variables, the first being the concentrations of both the reactants that we’re using. The next really important control is the view that we use in order to determine the time it takes for the cross to disappear. Our view needs to be straight down directly over the conical flask. This ensures that our experiment is repeatable. This is because for each measurement, we’re looking through exactly the same depth of water and glass.

If we were to change the direction of the view we took in order to determine when the cross had disappeared, we might start getting different results. And that’s not what we want. What you should see if you do this is that as the temperature of the solution increases, the time taken for our cross to disappear decreases. This means that as the temperature increases, the rate of our reaction also increases.

Now let’s have a look at a different experiment. This time, let’s investigate the effect of a catalyst on the decomposition of hydrogen peroxide. One potential catalyst for this reaction is manganese dioxide, MnO2. Remember that catalysts are regenerated at the end of our reaction. So this is why it’s not a reactant or a product. Once again, let’s look at the states each of our reactants and products is in. Pure hydrogen peroxide is a liquid, as is the water product. And of course the oxygen is a gas. It’s going to be difficult to measure anything to do with the two liquids. So this time, let’s look at the gas.

There are several ways we can measure the production of a gas, one of which is using a gas syringe. What you’ll need is a setup a bit like this, with a conical or Erlenmeyer flask, a stopper, and a gas syringe attached by some tubing. The hydrogen peroxide and catalyst are placed in the conical flask. This is then sealed with a bung and the gas syringe is attached. As the reaction progresses, oxygen gas is formed. Because the system is sealed, the gas only has one place it can go, up the tubing and into the gas syringe. Gradually, as the oxygen pushes its way into the gas syringe, the plunger on the syringe will move. This will allow you to take measurements of the volume of gas produced over time. What you’ll end up with is a graph showing the amount of oxygen gas produced at different time intervals.

In this example, the independent variable is the catalyst. We can perform multiple versions of this experiment with no catalyst, a small amount of catalyst, or maybe a large amount of catalyst. Our dependent variable, the thing that we measure, is the volume of oxygen gas produced. Things that you’ll want to control are the volume of the hydrogen peroxide, the temperature, and how quickly you stopper the conical flask. There are, of course, other ways to measure the production of a gas. A setup like the one shown is an equally valid method, though perhaps slightly more fiddly to set up. As the gas is generated in this example, it travels down the tube and up into the upturned measuring cylinder. As the gas accumulates at the top, it forces the water down. Using the amount of water displaced, we can work out what volume of gas is produced.

We’ve seen a few different ways for measuring the rate of various reactions. However, this is not an exhaustive list. There are many other different ways as well, for example, using colorimetry to measure the change in the color of a solution. But since we can’t explore every method in this video, let’s move on to looking at some questions.

Which of the following is not a viable unit for a reaction rate? (A) kg over s, (B) g over s, (C) h over s, (D) M over min, or (E) t over d.

Let’s start by working out what all of these symbols stand for. (A) stands for kilograms per second. (B) is similar, grams per second. (C) is hours per second. (D) is molars per minute, remembering that molar means moles per liter, sometimes written as moles per decimeter cubed. And (E) is tons per day. To work out what a sensible unit for reaction rate is, let’s remind ourselves what we mean by rate of reaction.

The rate of a reaction is the rate that to the reactants are turning into the products. To measure the rate of reaction, we could measure the loss of our reactants or we could measure the formation of our products. Either way, we’re using a similar equation. We can calculate the rate of reaction by doing the mass or amount of our product formed or the mass or amount of our reactant lost divided by time.

So the units for our mass or amount will be things like kilograms, grams, moles, et cetera. And the units of time will be seconds, minutes, hours, et cetera. So we’re looking for units along the lines of kilograms per second, grams per minute, moles per hour, et cetera. We can see that (A), (B), (D), and (E) all follow this pattern, while (C) does not. So this is our correct answer.

Let’s summarize the key points. Reaction rates can be calculated by measuring reactant loss or product formation, giving us units such as these. To set up a suitable experiment for measuring rate of reaction, consider the states of your reactants or products. This can help you pick the most appropriate setup.

## Join Nagwa Classes

Attend live sessions on Nagwa Classes to boost your learning with guidance and advice from an expert teacher!

• Interactive Sessions
• Chat & Messaging
• Realistic Exam Questions