Lesson video

Hello, I am Mrs. Adcock, and welcome to today's lesson.

Today's lesson is on the rate of a chemical reaction, including graphs.

We are going to be looking at how we can calculate the rate of a reaction, how we can then use graphs to help us calculate the mean rate of a reaction, and how we can calculate the instantaneous rate of a reaction.

Today's lesson outcome is "I can describe what happens during a chemical reaction as it progresses over time and interpret graphs showing how the instantaneous rate of the reaction changes." Some of the keywords we will be using in today's lesson include rate of reaction, mean rate of reaction, instantaneous rate of reaction, tangent, and gradient.

Here you can see each of those keywords written in a sentence.

It would be a good idea to pause the video now and read over those sentences.

You might even like to make some notes so that you can refer back to them later in the lesson if needed.

Today's lesson on rate of a chemical reaction, including graphs, is split into two main parts.

First of all, we are going to be focusing on calculating mean rate of reaction.

And then we are going to move on in the second part of the lesson to look at calculating instantaneous rate of reaction.

Let's get started on the first part of our lesson on calculating mean rate of reaction.

During a chemical reaction, reactants are converted into products.

Here we can see an example of a chemical reaction.

We have calcium and hydrochloric acid reacting together to produce the products calcium chloride and hydrogen.

Calcium and hydrochloric acid are our reactants, and they react together to produce our products, which are calcium, chloride, and hydrogen.

We've just looked at what the reactants and products are in a chemical reaction.

Now your turn to have a go at this question.

What are the reactants in the following reaction? Zinc and nitric acid react together, and we have zinc nitrate and hydrogen.

Are the reactants, a, zinc; b, nitric acid; c, zinc nitrate; or, d, hydrogen? The correct answers are zinc, so that's a, and, b, nitric acid.

So, zinc and nitric acid, they are our reactants.

They react together to produce zinc nitrate and hydrogen, so they are the products.

Well done if you correctly identified the reactants in that reaction.

The rate of a chemical reaction is the speed with which a chemical reaction takes place, converting the reactants into products.

Some chemical reactions have a low rate of reaction, and some chemical reactions have a high rate of reaction.

Here we can see an image showing some rust, and the rusting of iron is a slow reaction and has a low rate of reaction.

Here we can see a Bunsen burner, and the combustion of methane is a fast reaction, and this reaction has a high rate of reaction.

Time for another question: which reaction has the highest rate of reaction? Look in the table there.

You can see we have reactions A, B, and C, and we've got the time taken for the reaction to complete, in seconds.

The reaction that has the highest rate of reaction is reaction A.

Well done if you've got that correct.

The reason we know that reaction A has the highest rate is because that reaction was completed in the lowest amount of time.

It only took 30 seconds for reaction A to complete; however, if we look at reaction C, it took 240 seconds for that reaction to be complete.

Reaction C would have the lowest rate of reaction out of those three reactions.

The rate of reaction doesn't stay the same throughout a reaction, but it varies throughout a reaction.

Reactions are often fastest at the beginning.

And then they slow down until the reaction is complete.

We can determine the mean rate of a reaction by measuring the amount of reactant used in a given time.

We can use this equation to calculate the mean rate of reaction.

The mean rate of reaction is the amount of reactant used divided by the time taken.

We can also determine the mean rate of reaction by measuring the amount of product that's formed in a given time.

We would then use this equation here, where the mean rate of reaction isn't the amount of reactant used, but we are now looking at the amount of product formed divided by the time taken.

So you can use either of those equations, the amount of reactant used or the amount of product formed divided by the time taken, to work out the mean rate of reaction for a reaction.

How can we measure the mean rate of a reaction: a, mass of reactant made divided by time taken; b, mass of reactant used multiplied by the time taken; c, mass of product made divided by the time taken; or, d, time taken for a reaction? The correct answer is c.

Well done if you chose c.

To measure the mean rate of a reaction, we do the mass of the product made divided by the time taken.

Another way we can calculate the mean rate of reaction is to do the mass of reactant used divided by time taken.

This is similar to answer a, but answer a says, "The mass of reactant made," and remember we don't make the reactants.

We use the reactants, and we make our products.

The units for rate of reaction depend on the units of the reactant used or the product made and then the time.

Time is typically measured in seconds.

Let's look at some examples.

In the table here, we can see, in the first column, we've got the units of the reactant used or the product made, and they vary, so we could use the units grammes or centimetres cubed or moles.

Our units of time, we said they are typically seconds, but we can see in that column, sometimes, we might have measured in minutes.

And then to work out the units of rate of reaction, we take the units of the reactant used or the product made, and we divide it by the time.

So in the first row, you can see we've got grammes, and the units of time were second.

So when we work out the units of rate of reaction, we do grammes divided by seconds, so it would be grammes per second.

In the second row, the units of the reactant used or the product made were centimetres cubed, and the units of time were seconds.

So the units of the rate of reaction will be centimetres cubed divided by seconds so centimetres cubed per second.

In the third row, we can see we've done exactly the same, and we do moles divided by seconds, so this is moles per second.

And in the bottom row, because the units of time were minutes, in this case, the units of rate of reaction will be centimetres cubed per minute.

So for example, if our product made was a volume that we measured in centimetres cubed, then we would be able to see how much of that product we are making in centimetres cubed per minute.

Which unit could be used for rate of reaction: a, grammes per second; b, centimetres cubed; c, moles per second; or, d, grammes per centimetre cubed? Remember we are not looking for the units of the reactants that were used or the products that were made and we are not looking for the units of time.

We're looking for units that could be used for rate of reaction.

The correct answers are a and c.

A and c show the units that could be used for reactant used or product made divided by the time, so we have grammes per second and moles per second.

Well done if you chose answers a and c.

We are going to have a go at calculating the mean rate of a reaction.

I'll have a go first.

And then it'll be your turn to have a go.

So I do: when heated, 1.

5 grammes of calcium carbonate took 40 seconds to decompose.

Calculate the mean rate of reaction.

First of all, we are going to look at the amount of reactant that has been used, and this was 1.

5 grammes.

Then we are going to look at the time taken, and the time taken was 40 seconds.

Finally, we can work out the mean rate of reaction by doing the amount of reactant used divided by the time taken, so we have 1.

5 grammes divided by 40 seconds, and that gives us a mean rate of reaction of 0.

0375, and the units will be grammes per second.

It's your turn to have a go.

You've got five grammes of magnesium ribbon that took 30 seconds to react fully with hydrochloric acid.

You need to calculate the mean rate of reaction.

Remember to include the units once you have calculated the mean rate of reaction.

Pause the video now, and have a go at answering this question.

Let's see how you got on.

So, first of all, we should've looked at the amount of reactant used, and that was five grammes, then considered the time taken, and the time taken was 30 seconds.

Then finally, to work out the mean rate of reaction, you do the amount of reactant used divided by the time taken, so this will be 5 divided by 30, which gives us an answer of 0.

167, and the units are grammes per second.

Well done if you got that question correct.

We are going to have another go at calculating the mean rate of reaction, but this time, we've been given examples where we've not been told the amount of reactant used, but instead, we have information on the amount of product produced.

I do: in a reaction between sodium carbonate and sulfuric acid, 45 centimetres cubed of carbon dioxide gas is produced in 30 seconds.

Calculate the mean rate of reaction.

First of all, let's look at the amount of product formed.

The amount of product formed is 45 centimetres cubed.

Then we look at the time taken, and this was 30 seconds, and finally, to work out the mean rate of reaction, we do the amount of product formed divided by the time taken, so this will be 45 centimetres cubed divided by 30 seconds, and that gives us an answer of 1.

5 centimetres cubed per second.

Your turn to have a go.

In a reaction between aluminium and hydrochloric acid, 25 centimetres cubed of hydrogen gas is produced in 20 seconds.

Calculate the mean rate of reaction, and again, remember to include the units.

Pause the video, and have a go at this question now.

Let's go over the answer.

First of all, you should have identified the amount of product formed, and that was 25 centimetres cubed.

The time that was taken was 20 seconds, and therefore, to work out the mean rate of reaction, we do 25 centimetres cubed divided by 20.

That gives us an answer of 1.

25 centimetres cubed per second.

Hopefully, you got that question correct.

Here is a worked example calculating the mean rate of reaction for the reaction shown on the graph below.

This time we haven't been given the data in a written form.

We've been given the information about this reaction as a graph.

We can see the volume of carbon dioxide that has been made, and we can see the time along the x-axes.

We can use this graph to work out the total volume of product that has been made, and the total volume of product made is 60 centimetres cubed.

We can then use the graph to work out how long the reaction takes to complete, and the reaction takes 70 seconds to have reached completion.

We can then work out the mean rate of reaction for this reaction.

So, the mean rate of reaction is the amount of product made divided by the time taken, so that's 60 centimetres cubed divided by 70 seconds, and that gives us an answer of 0.

86 centimetres cubed per second.

Having a look at this graph, see if you can work out how long did this reaction take.

Is it, a, 25 seconds; b, 35 seconds; or, c, 45 seconds? If you look along the x-axes, you can see the time in seconds.

We know when the reaction is complete, because no more carbon dioxide gas is being made, so the volume of product made stays constant.

And it stays constant from 35 seconds onwards, so at 35 seconds, this reaction was complete.

And then the volume of carbon dioxide made stays the same, even at 40 seconds and 45 seconds.

So the correct answer, "how long did this reaction take?" is b, 35 seconds.

Time for our first practise task of today's lesson.

For this practise task, you need to calculate the mean rate of reaction for the following reactions and remember to include the units in your answer.

So, you have got a, b, c, and d, and in each question, remember to use that equation.

The amount of reactant used or the amount of product made divided by the time taken is how we calculate the mean rate of reaction.

Pause the video now.

Have a go at answering this question.

And then come back when you're ready to go over the answers.

Let's see how you got on.

Question 1 a, the mean rate of reaction is the amount of product made divided by time.

It says, "A reaction lasts 10 seconds and during the reaction, 20 centimetres cubed of hydrogen is produced," so to work out the mean rate of reaction, we do 20 centimetres cubed divided by 10 seconds, and that gives us an answer of two centimetres cubed per second.

In b, a student investigates the rate of decomposition of hydrogen peroxide.

When they added 1.

5 grammes of manganese oxide to 100 centimetres cubed of hydrogen peroxide, they collected 48 centimetres cubed of gas in six minutes.

We've been given some additional information here that we did not need to use, so hopefully, you identified the information that was important.

The mean rate of reaction is, again, the amount of product made divided by the time taker, and the amount of product made was 48 centimetres cubed, and that was made in six minutes.

So we do 48 centimetres cubed divided by six minutes, and that gives us an answer of eight centimetres cubed of product made per minute.

1 c, a reaction produced 0.

60 grammes of carbon dioxide in 45 seconds.

We are gonna use the same equation again to work out the mean rate of reaction; therefore, we do 0.

60 grammes divided by 45 seconds, and that gives us an answer of 0.

013 grammes per second.

And for d, eight grammes of magnesium took 32 seconds to fully react with 25 centimetres cubed of hydrochloric acid, and it says, "Give your answer in grammes per second." Again, we didn't need to use all the information in the question.

So, hope you have identified that we focused on the eight grammes of magnesium that reacted, and this took 32 seconds to fully react, so we do eight grammes divided by 32 seconds, and you should have an answer of 0.

25 grammes per second.

Well done if you correctly calculated the rates of reaction and you managed to work out the correct units.

For the second part of this practise task, you need to calculate the mean rate of reaction shown on the graph below, so we can see we've got a reaction.

We've got the volume of carbon dioxide that's been made in centimetres cubed, and we've got the time in seconds.

If you pause the video now, use this graph to get the data that you need to work out the mean rate of reaction for this reaction.

Let's see how you got on.

To calculate the mean rate of reaction, we need to know the total volume of product that has been made and the time that the reaction took to complete.

So, the total volume of carbon dioxide made is 180 centimetres cubed.

The reaction took 65 seconds to complete, and we're able to work this out 'cause we can see the point where we have made our final volume of carbon dioxide.

And then after this, as time goes on, no more carbon dioxide is being produced, because the reaction is complete.

You may have a slightly higher value for the reaction time.

Anything between 65 and 70 seconds would be correct.

Then to work out the mean rate of reaction, we do the amount of product made divided by time, so that will be 180 divided by 65, and that gives us a mean rate of reaction of 2.

77.

And hopefully, you've included the units too, which are centimetres cubed per second.

Well done for your good work on that first part of today's lesson on calculating the mean rate of reaction.

Now we're going to look at how do we calculate the instantaneous rate of reaction.

Graphs of mass, volume, or concentration versus time can show us how the rate changes throughout a chemical reaction.

Here we can see an example of a graph where we've got the volume of carbon dioxide made, and we've got the time for this reaction.

And the gradient of this graph represents the rate of the reaction, and hopefully, you will notice that the gradient changes throughout the reaction, and the steeper the slope the greater the rate of the reaction.

Time for a question: when plotting the amount of product made against time, the steeper the gradient the, a, higher the rate of reaction; b, rate of reaction remains unchanged; c, lower the rate of reaction? When we have graphs which show the amount of product made against time, the steeper the gradient the higher the rate of reaction.

Well done if you got that question correct.

You're clearly focusing well.

We can interpret graphical data to understand what happens to the rate over the course of most reactions, and for most reactions, they follow a similar shape to this graph here.

Gradient is steepest at the start of the reaction, where the rate of reaction is highest.

Then the gradient becomes shallower as the rate of reaction slows, and the rate of reaction slows as the reaction progresses.

And finally, the rate of reaction is zero when the reaction has ended, so for most reactions, we will see the rate of reaction will be highest at the start.

Then the rate of reaction decreases until it becomes zero when the reaction has ended.

The instantaneous rate of reaction is the rate of reaction at a specific time in the reaction.

The instantaneous rate of reaction can be determined from the gradient of the curve at a specific time, so unlike mean rate of reaction, where we looked at the amount of product made or the amount of reactant used over the course of the reaction, here we are looking at a specific time and looking at the rate at a specific point in the reaction.

To calculate the instantaneous rate of reaction from a graph at a specific time, we, first of all, need to draw a tangent to the curve.

And then we can calculate the gradient of the tangent.

To calculate the gradient, we do the change in y divided by the change in x.

Is this statement true or false: "The instantaneous rate of reaction is the average rate at which reactants are converted into products over a given period of time"? That statement is false.

Can you justify your answer? Is it false because, a, the instantaneous rate of reaction is the rate at which reactants are converted into products at a specific moment in time or, b, the instantaneous rate of reaction can be calculated using the overall time taken for the reaction to complete? The correct answer is a.

So, the instantaneous rate of reaction is the rate at which reactants are converted into products at a specific moment in time.

So we are not interested in the overall time taken for the reaction, and it's not the average rate.

It's the rate at a specific moment in time in the reaction.

Well done if you got that question correct.

Here we've got a worked example calculating the instantaneous rate of reaction at 20 seconds for the reaction shown on the graph below.

The graph shows us the volume of carbon dioxide made, in centimetres cubed, and the time in seconds.

First of all, to work out the instantaneous rate of reaction, we need to draw a tangent, so if we find 20 seconds on the graph, and you can see that marked there with an X, we are going to draw a tangent at this point because we want to know the rate of reaction at 20 seconds.

A tangent is a straight line that touches the curve at a specific point.

You can extend your tangent line to help you calculate the gradient a bit easier.

So, you want to find points that make it easier for you to work out the change in y and the change in x.

So, we are now going to calculate the gradient of our tangent line, so first of all, we are going to look at our change in y.

And y goes from 150 down to zero, so we have 150 minus zero, and that gives us a change in y of 150 centimetres cubed.

And then our change in x, it went from 60 seconds to zero seconds, so we do 60 minus zero, and we have a change in x of 60 seconds.

So we can now work out the gradient at 20 seconds, and the gradient of this graph will tell us the rate of reaction at that specific moment in time.

So, to work out the gradient, we do 150 divided by 60, and that gives us an instantaneous rate of reaction at 20 seconds of 2.

5 centimetres cubed per second.

Why is a tangent drawn when we calculate the instantaneous rate of reaction? Is it, a, it enables us to work out the amount of reactant used at a specific time; b, it enables us to calculate the gradient at a specific time; or, c, it enables us to work out the amount of product made at a specific time? We draw a tangent when we are calculating instantaneous rate of reaction so that we can then work out the gradient of our tangent line, so the correct answer is b: the tangent enables us to calculate the gradient at a specific time.

And remember the gradient is the rate of reaction.

Well done if you got that question correct.

Time for our final practise task of today's lesson.

The graph below shows the reaction between lithium carbonate and hydrochloric acid.

You've got the volume of carbon dioxide that's made, in centimetres cubed, and the time in seconds.

And then you have got five points marked on that curve: A, B, C, D, and E.

Question a says, "At which point was the rate of reaction highest?" and you need to explain your answer.

And b is "what has happened at point E?" and again, you need to explain your answer.

Pause the video now.

Have a go at answering question 1.

Then when you come back, we will move on to question 2.

Question 2, look at the graphs showing the amount of reactant against time for reactions a, b, and c.

And this time, we've got the mass of reactant that's been used, in grammes, and we've got time in seconds.

And we've got three different reactions shown on this graph.

They are reactions A, B, and C.

Which reaction is the fastest? And explain how you can tell this from the graphical data.

Pause the video now.

Have a go at answering question 2.

And then we will move on to the final question in this practise task.

Question 3, calculate the rate of reaction at 25 seconds, so you are being asked to calculate the instantaneous rate of reaction.

We know this because you've been told to calculate the rate of reaction at a specific moment in time.

Make sure to include the units and show your workings on the graph.

Pause the video.

Have a go at answering question 3.

And when you come back, we'll go over the answers to all of the questions in this practise task.

Let's see how you've got on.

Question 1, we were asked, "At which point was the rate of reaction highest?" and you needed to explain your answer.

The rate of reaction is highest at A, and this is where the gradient is steepest.

The rate of reaction then decreases throughout the reaction.

Hopefully, you identified A and talked about that being the point on the graph where the gradient is steepest.

"B, what has happened at point E? Explain your answer." No more carbon dioxide is being made at E.

The rate of reaction is zero, as the reaction has ended.

Well done if you got that one correct.

Question 2, "which reaction is the fastest? Explain how you can tell this from the graphical data." Reaction A is the fastest.

It has the highest rate of reaction.

We know this because it's got the steepest gradient.

In A, that same mass of reactant has reacted in a shorter time.

And finally, question 3, you had to calculate the rate of reaction at 25 seconds, making sure that you included the units in your answer and showed your workings on the graph.

First of all, we need to identify 25 seconds on the graph.

And then we are going to draw a tangent at this point.

The tangent is drawn at time equals 25 seconds.

Then we can calculate the gradient of the tangent by working out the change in y divided by the change in x, and the gradient will then tell us the rate of reaction at 25 seconds.

So our change in y is 180 minus 40, which is 140, centimetres cubed, and the change in x is 50 minus five, so that's 45 seconds.

Now, you may have different values there, but hopefully, you will end up with a similar rate of reaction.

So, the instantaneous rate of reaction for the tangent line we have drawn here will be 140 divided by 45, and that gives us a rate of reaction of 3.

11 centimetres cubed per second.

You should have a very similar answer to that one there.

Well done if you drew your tangent and then calculated the gradient of your tangent with your tangent touching the line at 25 seconds.

We have reached the end of today's lesson on the rate of a chemical reaction, including graphs.

Let's just finish by summarising some of the key points we have covered in today's lesson.

Most chemical reactions start more quickly, and they slow down as they progress, and therefore, the gradient will be steepest near the beginning, where the rate of reaction is highest.

The rate of a chemical reaction is equal to the amount of product formed per unit time or the amount of reactant used per unit time.

The mean rate of a chemical reaction is different to the instantaneous rate that changes throughout the reaction.

Graphs of mass, volume, or concentration versus time can all represent the changing rate of a chemical reaction, and finally, the gradient of a rate of reaction graph can be calculated from a tangent drawn at a point on the curve.

Well done for all your hard work throughout today's lesson.

Hopefully, you've learnt lots about the mean rate of reaction and instantaneous rate of reaction and how to calculate these.

I hope you've enjoyed today's lesson, and I hope you're able to join me for another lesson soon.