Lesson video

Hello, I am Mrs. Adcock and welcome to today's lesson on "Appropriate Equipment for Measuring the Rate of a Reaction." We are going to be learning about how do we choose the best equipment for measuring the rate of a reaction.

Today's lesson outcome is I can suggest appropriate equipment to measure the rate of reaction depending on the reaction.

Some of the keywords that we will be using in today's lesson include rate of reaction, volume, accurate and precise.

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 through 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 appropriate equipment for measuring the rate of reaction is split into three main parts.

First of all, we are going to be looking at measuring the volume of gas produced.

Then we will move on to look at measuring the change in mass of the reaction mixture.

And finally, we will finish the lesson by measuring the light passing through a solution.

Let's get started on the first part of our lesson, measuring the volume of gas produced.

When a chemical reaction takes place, new products are made and you can observe changes.

And these changes might be a change in colour, effervescence, which is when we observe bubbling, change in temperature or a precipitate forming.

Here, we can see a change of colour and a precipitate made during this chemical reaction.

The colour is changing from colourless to yellow, and a precipitate is forming, which is reducing the amount of light that can be transmitted through the reaction mixture.

In this clip here, we can see effervescence showing a gas is produced in this chemical reaction.

So when these two reactants are mixed together, we can see bubbling as a gas is produced.

Which of the following may occur in a chemical reaction? A, a colour change.

B, effervescence, bubbling.

C, no new products.

D, a precipitate forming.

Choose any answers that you think are correct.

The correct answers are A, a colour change, B, effervescence and D, a precipitate forming.

These all may occur in a chemical reaction.

C is not correct because when we have a chemical reaction, then our reactants react together and new products are formed.

To determine the rate of reaction, we need to know the amount of reactant used in a given time, and we can use this equation here to work out the rate of reaction.

We divide the amount of reactant used by the time taken or to determine the rate of reaction, we need to know the amount of product made in a given time.

So to work out the rate of reaction, we can calculate the amount of product made divided by the time taken to make this product.

So two ways we can work out the rate of reaction.

If one of the products is a gas, we can use an upturned measuring cylinder to measure the volume of gas produced in a given time.

Here we can see the equipment set up.

We have our reaction mixture.

In a conical flask, we have a bunk placed in the top of the reaction mixture and a delivery tube.

The gas that is produced can travel through this delivery tube, and then we have water in a trough and an upturned measuring cylinder that is filled with water.

And as that gas is produced and it travels through the delivery tube and into that upturned measuring cylinder, then the water will be displaced and we will have our collected volume of gas and we can measure by reading off the measuring cylinder, the volume of gas that has been produced.

This method, using an upturned measuring cylinder, works well for chemical reactions that produce large volumes of gas at moderate rates.

The size of the measuring cylinder should be close to the expected volume of gas produced in order to obtain accurate results.

Accurate results are results that are close to the true value, so we want our results to be as accurate as possible.

We can see in the image there that we've got measuring cylinders of all different sizes, and these are used to measure different volumes.

We want to use the measuring cylinder that is as close to the expected volume of gas produced as possible.

So for example, if the volume of gas produced is 45 centimetres cubed, we don't want to use a 10 centimetre cubed measuring cylinder, as this is too small, we will not be able to measure up to 45 centimetres cubed of gas.

A hundred centimetre cubed measuring cylinder will have large graduations, so although this will hold the 45 centimetres cubed of gas that we are likely to produce, the graduations are quite large, so we won't be able to read and get as accurate a value.

However, if we use a 50 centimetre cubed measuring cylinder, this will have smaller graduations and this improves the accuracy of the results.

So we would choose a 50 centimetre cubed measuring cylinder if the volume of gas produced is 45 centimetres cubed.

Which volume measuring cylinder should be used to measure the 85 centimetres cubed of gas produced in a reaction? Would we choose a measuring cylinder that measures: A, 50 centimetres cubed? B, 100 centimetres cubed? Or C 500 centimetres cubed? The correct answer is B.

We should use a 100 centimetre cube measuring cylinder to measure 85 centimetres cubed of gas produced in a reaction.

So well done if you chose answer B.

Option A would be too small using only a 50 centimetre cube measuring cylinder.

Option C, using a 500 centimetre cubed measuring cylinder will mean that the graduations on the measuring cylinder will be larger compared to using the 100 centimetre cubed measuring cylinder.

Gas syringes are another way we can measure the volume of gas produced during a chemical reaction.

Gas syringes are ideal for chemical reactions that produce a small volume of gas.

Here we can see the apparatus set up using a gas syringe to measure the volume of gas produced.

We have a conical flask containing our reaction mixture and then the gas that is produced will travel up through the delivery tube and into the gas syringe.

Gas syringes typically have higher precision for measuring gas volumes compared to measuring cylinders.

Precise results vary very little from the mean value.

They are very similar when repeated.

If our results are precise, this does not mean that they are necessarily accurate.

Just a reminder that accurate results means that they are close to the true value, whereas precise results means that when we repeat these results, our results are very similar to each other.

Gas syringes have small graduations, and this makes them more useful for recording precise results, and they are also designed to prevent gas lost.

Because of this, we are therefore more likely to get accurate results using a gas syringe as well.

When using measuring cylinders, the meniscus makes it difficult to make precise readings and when using a measuring cylinder, if we produce a soluble gas, then when the gas bubbles up through the water, some of this gas may dissolve and therefore it will be lost and not recorded when we are measuring the volume of gas produced.

When measuring the production of gas over the course of a reaction, the results may look similar to this graph.

We have the volume of gas in centimetres cubed on our y-axis and we have the time in seconds on the x-axis.

We can see that the volume of gas produced per second is initially very high, so when the reaction starts, we have a high rate of reaction.

As the reaction progresses, less gas is produced per second, so we can see that the rate of reaction decreases as the reaction progresses.

And we can see finally, no more gas is produced, so the volume of gas is remaining constant, and this is because the reaction has ended.

Time for a question.

Why do gas syringes typically give more precise results when measuring small gas volumes? Is it A, the meniscus can be easily read.

B, some gas may dissolve.

C, they have small graduations.

And D, little or no gas is lost.

Gas syringes typically give more precise results when we are measuring small gas volumes, this is because they have small graduations and no or little gas is lost to the surroundings.

So well done if you chose options C or D.

A, reading the meniscus, this applies when we are using a measuring cylinder and B, some gas may dissolve.

Again, we mention this because if we are producing a soluble gas, then when it bubbles through the water, if we are using the inverted or upturn measuring cylinder method, then some of this gas may dissolve.

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

Question one, is sketch a graph to show what happens to the production of gas over the course of a reaction? You'll have the volume of gas produced on one axis and the time in seconds on another axis.

Question two, describe a method using a measuring cylinder that can be used to work out the rate of reaction for a reaction that produces a gas.

Pause the video now have a go at answering these two questions, then when you come back, we'll go over the answers.

Let's see how you got on.

So hopefully when you've sketched the graph, your graph should look a similar shape to this one here.

We have the volume gas in centimetres cubed on our y-axis and we have the time in seconds on our x-axis.

And the graph shows that initially, the rate of reaction is high, and then as the reaction progresses, the rate of reaction decreases.

And finally, the rate of reaction is zero because our reaction has ended and the volume of gas produce remains constant.

For question two, we needed to describe a method using a measuring cylinder that can be used to work out the rate of reaction for a reaction that produces a gas.

Your answer may be similar to this one where we have talked about using an upturned measuring cylinder to measure the volume of gas produced over a certain period of time.

We will then use the equation, rate of reaction equals the amount of product made or the volume of gas made divided by the time taken, and this will provide us with the rate of reaction and the units for that will be centimetres cubed per second.

Well done if you included those details in your answer and got that question correct.

We have looked at ways that we could measure the volume of gas produced if we were trying to determine the rate of a reaction.

Now we're going to move on to have a look at how we can measure the change in mass of a reaction mixture.

If a reaction in an open system produces a gas, then the gas can escape from the reaction mixture causing a decrease in mass throughout the reaction.

The decrease in mass can be measured in a given time to help us determine the rate of reaction.

Here we can see the equipment that we may use.

We have a mass balance, and on our mass balance we have a reaction mixture.

And our reaction mixture is going to produce a gas, and this gas can escape from the reaction mixture.

It can travel through the cotton wool and out into the surroundings.

And as our gas is produced and escapes to the surroundings, then the mass of our reaction mixture will decrease.

The results from measuring a decrease in mass of a reaction mixture in a given time may look similar to this graph.

We can see on the y-axis that we have the mass of the reaction mixture in grammes and on the x-axis, we have the time in seconds.

Hopefully you can notice that the mass is decreasing throughout this reaction, and initially, the mass decreases rapidly per second.

So at the beginning of our reaction, we have a high rate of reaction.

As the reaction progresses, the rate of reaction becomes lower and the decrease in mass per second is less.

Finally, the mass stays at a constant level, and this is because the reaction has ended.

Time for a question.

At what point A, B, or C is the reaction producing the greatest volume of gas per second? Is this at point A, point B or point C on that graph shown? The correct answer is A.

At point A, the reaction mixture is producing the greatest volume of gas per second.

And we can see this because we have the steepest gradient at this point.

Well done if you chose answer A.

Another question here for us to have a go at.

What is happening at point C? Is it A, that the reaction has ended? B, the reaction is continuing at a constant rate? Or C, the reaction is occurring at a faster rate than B? The correct answer is A, again, so the reaction has ended at point C.

The mass of the reaction mixture is staying constant and the rate of reaction at this point here is zero.

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

For this task, you have got two questions.

Question one, list the apparatus that would be used to measure the rate of reaction for a reaction that produces a gas in an open system.

And question two, describe what will happen to the mass of the reaction mixture throughout the reaction.

Pause the video now, answer these two questions to the best of your ability and then come back when you're ready to go over the answers.

Here, we've got a diagram that we are going to use to help us list the apparatus that we would use.

You may have included a label diagram in your answer.

The apparatus that we would use to measure the rate of reaction for a reaction that produces a gas in an open system would be a mass balance, we would have the reaction vessel, so we might use a conical flask, a stopwatch, and you might have mentioned that we would use some cotton wool too.

Well done if you got that question correct.

Question two, describe what will happen to the mass of the reaction mixture throughout the reaction.

Initially, the rate of reaction is high and therefore, there is a large decrease in the mass of the reaction mixture per second.

As the reaction progresses, the rate of reaction decreases and therefore, there is less decrease in mass per second.

And then finally, when the reaction has ended, there is no change in the mass.

Hopefully, you've talked about how initially, there's a large decrease in mass and then this decrease becomes less as the reaction progresses, and then finally, there's no change in mass.

For the third question, we have a reaction that had the following masses at the start of the reaction and after 20 seconds.

Question A is what mass of gas was produced in the first 20 seconds? So you need to have a look there at the mass when the time is zero seconds and the mass when the time is 20 seconds.

And part B, what is the mean rate of reaction for this reaction in the first 20 seconds? So see if you can remember that equation that we use to work out the rate of reaction and then apply that here to work out the mean rate of reaction for this reaction.

Pause the video now have a go answering this question and then when you come back, we'll go over the answer.

Let's see how you got on.

Part A, what mass of gas was produced in the first 20 seconds? The mass of gas produced is 15.

85 minus 15.

83, and this gives us a mass of gas produced of 0.

02 grammes.

If anyone's unsure how we worked that out, then we've looked at the mass of the reaction vessel at zero seconds and that was 15.

85.

And then we've taken away the mass of the reaction vessel at 20 seconds.

And the reason that reaction vessel has decreased is because a gas has been produced and that gas has escaped to the surroundings.

So the reaction vessel has decreased by 0.

02 grammes.

So we know that 0.

02 grammes of gas has been produced and escaped to the surroundings.

Well done if you got that correct.

What is the rate of reaction for this reaction in the first 20 seconds? To work out the rate of reaction, we can use the equation amount of product made divided by time taken.

And the amount of product made was 0.

02 grammes of gas divided by the time, which was 20 seconds.

And that gives us a rate of reaction of 0.

001 grammes per second.

Hopefully you got that one correct too.

It's time for us to move on to the final part of our lesson and we're going to move on to have a look at measuring the light passing through a solution.

If a chemical reaction involves a precipitate being formed, we can measure this change over a period of time.

Here, we can see a reaction that produces a precipitate making the clear, colourless solution turn cloudy.

At the start, we have a clear colourless solution with an X visible underneath the conical flask.

So we have our clear colourless solution in a conical flask, and underneath that conical flask is a piece of paper or card with an X on it, and we can see that X because the solution is colourless and clear.

This reaction produces a precipitate and as that precipitate is formed, the solution becomes cloudy and therefore, the X is no longer visible underneath the conical flask.

We can time how long it takes for this X to no longer be visible as a precipitate is formed.

We can determine the rate of reaction by timing how long it takes for the X to no longer be visible under the conical flask.

And the rate equals 1 divided by the time taken for the X to no longer be visible.

The X is visible underneath the conical flask at the start of the reaction, and then the X is no longer visible underneath the conical flask at the end point of this reaction.

We could use a light sensor to measure the amount of light that is transmitted through the reaction mixture over the course of the reaction.

The disadvantages of using a light sensor include light being detected from the surroundings and this can interfere with the results.

And also using a light sensor is more expensive than us just visually determining when that endpoint has been breached.

The advantages of using a light sensor include, we get more accurate results.

It is difficult for us to determine the exact same endpoint each time that we repeat this experiment.

And also rather than us just recording the time taken for an endpoint to be reached, we can collect data throughout the reaction as we record the amount of light that is transmitted through the reaction mixture.

Which apparatus could be used to determine the rate of reaction for a reaction that produces a precipitate? A, a mass balance.

B, a stopwatch.

C, a light sensor.

Or D, a gas syringe.

There are two answers here.

We would need B, a stopwatch, and C, a light sensor.

We could use the light sensor to measure the amount of light that is being transmitted through the reaction mixture.

We would want to use a stopwatch so that we can either record the amount of light that is being transmitted at set intervals, or we can time how long it takes for there to be no light being transmitted through the reaction mixture.

Well done if you chose both of those answers.

Plotting the data collected from a light sensor to measure the amount of light transmitted through the reaction mixture over the course of the reaction may give us a graph similar to this.

We have the percentage of light transmitted through the reaction mixture on the y-axis, and we have the time in seconds on the x-axis.

The amount of light transmitted through the reaction mixture decreases as our precipitate forms until finally, no light is transmitted through the mixture as we have formed our cloudy mixture.

Let's have a go at this question.

Why may the amount of light transmitted through a reaction mixture change over the course of a reaction? A, the mass of the reaction mixture decreases.

B, a precipitate is being formed, or C, the temperature increases.

The correct answer is B.

A precipitate is being formed, and this is a reason why the amount of light that is transmitted through a reaction mixture may change over the course of a reaction.

Time for a final practise task of the lesson.

For this task, we have two questions.

Question one, describe the advantages and disadvantages of using a light sensor to measure the light transmitted through a reaction mixture and try to give as much detail as you can in your answer.

And question two, describe what the graph shows giving reasons for this trend.

Question one, describe the advantages and disadvantages of using a light sensor to measure the light transmitted through a reaction mixture.

Hopefully your answer includes, the advantages of using a light sensor are that more accurate results can be obtained and the data can be collected throughout the reaction.

The disadvantages of using a light sensor are that they are more expensive than just observing with the human eye when an endpoint has been reached and the results can be impacted by light from the surroundings.

Question two, describe what the graph shows giving reasons for this trend.

The graph shows that the percentage of light transmitted decreased over time until eventually, no light was transmitted through the reaction mixture.

And this is because as more precipitate was formed during the reaction, less light was able to pass through the reaction mixture.

Well done if you got that question correct.

We have reached the end of today's lesson on appropriate equipment for measuring the rate of a reaction.

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

Before you go, let's just summarise some of the key points that we have covered in the lesson.

Choosing the correct equipment is crucial for obtaining accurate and reliable results.

An upturned measuring cylinder is suitable for reactions producing large volumes of gas at moderate rates.

Gas syringes are ideal for measuring small volumes of gas with high precision, and the size of the measuring equipment should be close to the expected volume of gas produced.

Light sensors can be used to measure the percentage of light transmitted through a reaction that produces a precipitate.

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