Lesson video

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

Today's lesson is on evaluating methods for measuring the rate of reaction.

What are the advantages and disadvantages for using different methods to measure the rate of reaction? We are going to be focusing on two methods today using an end-point method and measuring the volume of gas produced over time.

Today's lesson outcome is: I can compare different methods for measuring the effect of concentration on the rate of reaction and suggest improvements.

Some of the keywords we will be using in today's lesson include: accuracy, repeatability, parallax error, and systematic error.

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 evaluating methods for measuring the rate of reaction is split into three main parts.

First of all, we are going to be looking at methods we can use to measure the rate of reaction.

Then we are going to focus on the errors in measuring the volume of gas over time and we will finish the lesson by looking at the errors in measuring an end-point.

Let's get started on the first part of our lesson, looking at methods for measuring rate of reaction.

We can investigate the effect of changing a variable on the rate of reaction by measuring the amount of gas produced over time.

Now we can do this in different ways.

We can use a mass balance.

A mass balance can be used to measure the mass of product made.

As we produce a gas, it will escape to the surroundings and the mass of the reaction mixture will decrease over time.

We can also use an inverted or upturned measuring cylinder, and a measuring cylinder can be used to measure the volume of product made.

As our reaction produces a gas, it will travel through the delivery tube and it will be collected in our upturned measuring cylinder.

We could also use a gas syringe, and a gas syringe can be used to measure the volume of product made.

As a gas is produced, it will travel through the delivery tube and into the gas syringe where we can measure the volume of gas that is produced.

When measuring the production of gas over the course of a reaction, the results show us how the rate changes during the reaction, and we can see this on the graph below.

On the Y-axes we have the volume of gas in centimetres cubed and on the X-axes we have the time in seconds.

We can see that, initially, the volume of gas produced per second is initially very high, and that means that at the start of this reaction, the reaction has a high rate of reaction.

As the reaction progresses, the gradient becomes shallower, and this is because less gas is being produced per second, so the rate of reaction has decreased.

We have a lower rate of reaction.

And finally, we can see that the volume of gas produced stays constant, and this is because no more gas is being produced.

The reaction has ended.

We can calculate the gradient of tangents drawn at different points in the reaction to help us analyse how the rate of reaction changes as the reaction progresses.

Here we can see we have a graph where, on the Y-axes we have the volume of hydrogen gas produced in centimetres cubed, and along the X-axes we have the time in seconds.

We can calculate the gradient at different points, and here we have calculated the instantaneous rate of reaction at 20 seconds and at 80 seconds.

At 20 seconds, we have drawn a tangent line and then calculated the gradient of that line.

And the gradient at 20 seconds is 1.

11 centimetres cubed per second.

So that tells us, at 20 seconds, the instantaneous rate of reaction is 1.

11 centimetres cubed per second.

We can then take the gradient at another point, and here we've taken the gradient at 80 seconds, and the gradient at 80 seconds is 0.

36 centimetres cubed per second.

So the instantaneous rate of reaction has decreased from 20 seconds to 80 seconds.

Calculating the gradient of tangents at different points in the reaction shows how the rate of reaction changes as the reaction progresses.

Time for a check for understanding.

Is this statement true or false? Measuring the volume of gas produced throughout a reaction allows the changing rate of reaction to be calculated.

That statement is true.

So well done if you selected true and got that correct.

Now we all need to think about explaining why that statement is true.

Measuring the volume of gas produced throughout a reaction allows the changing rate of reaction to be calculated.

This is because tangents can be drawn at different points to calculate the gradient, and this enables us to show that the rate of reaction changes throughout a reaction.

We have looked at how we can measure the volume of gas produced over time to help us measure the rate of reaction.

An end-point method can also be used in a rate of reaction investigation.

An end-point method measures the progress of a reaction until it has reached completion or a desirable point such as a colour change.

For example, we can measure the time taken for an end-point to be reached in the disappearing cross experiment.

Here in the image we can see we have a conical flask, and underneath the conical flask is a card marked with an X, and at the start of this reaction, the X is visible.

At the end-point the X is no longer visible.

And on the left hand side we can see at the start of the reaction where the X is visible, and on the right hand side we can see that the X is no longer visible when the end-point of this reaction has been reached.

And reaching the end-point of a reaction does not necessarily mean that the reaction has reached completion.

The advantages of using an end-point method are that measuring an end-point in a reaction allows direct and simple comparison of reaction rates.

Here you can see in the results table we used two different concentrations of sodium thiosulfate when we were investigating the effect of concentration on the rate of reaction.

When we used the lower concentration, which was eight grammes per decimeter cubed of sodium thiosulfate, the time taken for the X to no longer be visible, in seconds, was 120.

However, when we used a higher concentration of 40 grammes per decimeters cubed, the time taken for the X to no longer be visible had reduced to 25 seconds.

These results allow us to make a simple and direct comparison, and these results clearly show that the higher the concentration, the shorter the time taken to reach the end-point.

Therefore, the higher the concentration, the higher the rate of reaction.

Time for another question.

What does measuring an end-point in a reaction allow for? A: calculation of the reactant concentration, B: direct and simple comparison of reaction rates, C: measurement of gas colour, D: determination of product purity? Measuring an end-point in a reaction allows us to make direct and simple comparison of reaction rates.

So well done if you got that correct and you chose answer B.

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

Jun wants to investigate factors that affect the rate of reaction.

Describe to Jun a method he could use to measure rate of reaction and an advantage of using this method.

Pause the video now, have a go at answering this task, and then come back when you've finished.

Hopefully you have been able to identify a method that he could use to measure the rate of reaction and an advantage of using this method.

Your answer may include: Jun could use an upturned measuring cylinder or gas syringe to measure the volume of gas produced over time.

The data collected can be plotted on a graph where the rate of reaction at different points throughout the reaction can be calculated, and you may have mentioned that you could draw tangents and calculate the gradient of those tangents at different points to show how the rate of reaction changes as the reaction progresses.

An alternative answer to this question is: Jun could choose an end-point method to measure the time taken to reach a specific point in the reaction.

The data collected from using an end-point method allows for a simple and direct comparison of reaction rates.

Well done if you have correctly answered this question by mentioning that Jun could use either an end-point method or he could measure the volume of gas collected over time, and you've mentioned an advantage of using that method.

We have looked at different methods we could use to measure the rate of reaction.

We are now going to move on to look at the errors in measuring volume of gas over time.

In experiments involving measuring volume of gas over time, there are potential errors that can affect the accuracy and repeatability of results.

Accuracy refers to how close a measured value is to the true or standard value.

Repeatability refers to, if you use the same method, can you collect the same or similar results when you repeat the experiment? Errors in timing can affect the time at which readings are recorded, and we can see there in the image we have a picture of a stopwatch.

Timing errors can arise due to human reaction time.

Based on an individual's reaction time, we could get different results because it will affect when the timer is started and stopped.

Errors in measuring the volume of gas produced over time can arise from timing errors, but they can also arise from reading the volumes in real time.

Errors caused by reading the volume in real time include: parallax errors, and these are due to not reading the meniscus at eye level.

If you are measuring the volume of gas that has been produced in an upturned measuring cylinder, then you need to be at eye level.

If you stand from above or below and try to read the volume of gas produced, then you will not get a true reading.

Another source of error is difficulty reading a stable volume or mass, and this is due to uneven gas production.

Another source of error is that the reading may not represent the exact volume at a given point.

You may, at 10 seconds, for example, go to record the volume of gas that has been produced, but because the gas is being produced rapidly, it may be difficult for you to get an exact reading because the volume of gas is increasing when you are trying to record the volume that has been produced.

Which of the following are examples of errors caused by reading volumes in real time? A: a parallax error.

B: misreading the thermometer, C: the result not representing the exact volume due to slow reading speed, D: subjectivity in the observation? Errors can arise when we are reading volumes in real time and these can arise due to: A: a parallax error, this occurs if we do not measure at eye level.

And also C: the result may not represent the exact volume due to slow reading speed.

So if a gas is being produced at a rapid rate, we are unable to read that measurement quickly enough to get a true value.

Well done if you chose answers A and C.

Errors can also occur when we are measuring volume of gas produced over time.

if we use a bung to seal a container.

Now this is because if we use a bung to seal a container, then it will displace a small volume of gas.

Here we can see the experiment set up where we have a reaction occurring.

We've placed a bung in the top of the conical flask and this is attached to a delivery tube which is attached to a gas syringe, and we can use the gas syringe to measure the volume of gas that is produced.

As we place the bung on top of the conical flask, some gas will be displaced, and the displaced gas we can see will move into the gas syringe where it could be interpreted as gas that is produced by the reaction.

And we need to remember to take into account the volume of displaced gas.

This displaced gas needs to be accounted for to avoid systematic errors.

Systematic errors are errors caused by the equipment used.

Another source of error when we use bungs as part of our apparatus is that if they are not secured tightly, gas can escape to the surroundings.

The images show different apparatus setups where gas may escape to the surroundings if the bung is not tightly sealed.

We can see in the first image we have a reaction mixture and we have secured a bung to the top of that conical flask.

The gas will then be produced and travel through the delivery tube and into our gas syringe where we will measure the volume of gas produced.

However, if that bung is not tightly secured, then the arrows are representing where gas can escape to the surroundings.

We can see in the second image we have used an upturned measuring cylinder to measure the volume of gas produced, but again, if that bung is not tightly secure, then gas that is produced won't be collected and measured in the upturned measuring cylinder, but it can escape to the surroundings, and those arrows represent where gas can escape to the surroundings.

Gas that escapes to the surroundings will not be accounted for in the results, and this affects the accuracy of the results.

To improve the accuracy of the results and therefore to try and obtain results that are close to the true or standard value, we can take multiple readings and then we can use our multiple readings to identify any outliers, disregard these outliers and calculate a mean result.

We can also account for any initial gas displacement when we place the bung in place.

We also need to ensure all connections are airtight to prevent any gas from escaping to the surroundings and not being accounted for when we measure the volume produced.

Let's have a go at this question.

50 centimetres cubed of gas has been collected in the measuring cylinder.

When the bung was placed on the conical flask, some gas was displaced.

What volume of gas has been produced in the reaction? Is it A: less than 50 centimetres cubed, B: 50 centimetres cubed, or C: more than 50 centimetres cubed? The correct answer is A: less than 50 centimetres cubed.

Well done if you got this question correct.

50 centimetres cubed of gas was collected in the measuring cylinder.

However, when the bung was placed on the conical flask, some gas was displaced and this would've travelled into the measuring cylinder, so some of that collected gas will be gas that was displaced by the bung.

So when the reaction occurred, the volume of gas that has been produced would actually be less than 50 centimetres cubed.

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

Jun measures the effect of concentration on the rate of reaction using the following apparatus: We can see he has a reaction mixture in a conical flask.

He has placed a bung on top of the conical flask and then has a delivery tube to an upturned measuring cylinder where he's measuring the volume of gas produced.

Jun repeats the experiment multiple times and calculates a mean.

He notices his results are different to those collected by other students in the class.

How can Jun improve the accuracy of his results? Read back over that information you have been given and think about sources of error in the method that he has used, and use these to help you think about how he can improve the accuracy of his results.

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

How can Jun improve the accuracy of his results? Your answer may include: To improve the accuracy of his results, Jun should identify and disregard any outliers before calculating a mean.

It says that Jun repeats the experiment multiple times and calculates a mean, but it does not mention in the information that we've been given that he identified and disregarded any outliers.

It is important that Jun identifies those outliers and disregards those outliers before he calculates a mean.

Jun can also improve the accuracy of his results by accounting for any initial gas displacement when placing the bung in place and also ensuring all connections are airtight to prevent any gas escaping to the surroundings.

Well done if you identified all three of those points as ways that Jun could improve the accuracy of his results.

We are going to move on to the final part of our lesson on errors in measuring an end-point.

Errors in measuring end-points can arise due to the subjectivity of observations and the difficulty of visually determining an exact end-point.

Subjectivity means that this is based on someone's opinions or perspective.

To reduce errors, it is important that the same person determines the end-point each time because different people may have different opinions as to when the end-point has been reached.

A light sensor connected to a data logger can be used to determine the end-point.

This may be used when we are using the disappearing cross experiment.

In this experiment, we measure the time taken until the X is no longer visible underneath the conical flask.

But it's hard to determine that exact end-point each time.

Instead, we could use a light sensor to measure the amount of light that is transmitted through the reaction mixture.

What is a common source of error when identifying the end-point? A: inaccurate thermometer reading, B: incorrectly setting the timer, C: using incorrect concentrations, D: personal judgement of colour change? The correct answer is D, so well done if you've got this correct.

A common source of error when identifying the end-point is personal judgement as to when the colour change has occurred.

Other sources of error when measuring an end-point include: timing errors associated with starting and stopping the timer, measuring errors such as parallax errors, and we can see there how we can get an incorrect measurement if we do not measure at eye level, and also contamination, such as residue from a previous experiment.

So timing errors, parallax errors, and contamination are all sources of error when measuring an end-point.

How can we minimise these errors? To minimise errors, we can use automated methods to determine the end-points for us, such as using a light sensor connected to a data logger.

We can minimise the errors by measuring at eye level to reduce parallax errors, and also prevent contamination.

We can do this by using clean glassware each time we repeat an investigation.

Addressing these factors will make the method more repeatable and improve the accuracy of end-point measurements.

Time for another question.

What can cause errors in measuring times and volumes at the end-point? A: incorrectly calibrated thermometer, B: human reaction time, C: adding too much catalyst? The correct answer is B: human reaction time.

When we are measuring times and volumes at the end-point, human reaction time can cause errors.

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

Describe to Jun potential sources of error he should be aware of if he performs an end-point method to investigate the rate of reaction.

Include at least three sources of errors in your answer.

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

We are looking for three sources of error when using an end-point method to investigate the rate of reaction.

Your answer may include: Errors may arise due to subjectivity of observations, making it difficult to determine the end-point, reaction time affecting times recorded, or the time at which volumes are recorded.

Measurements not being taken at eye level, so parallax errors, and also contamination and this can affect the rate of the main reaction.

Well done if you were able to correctly identify three sources of error.

We have reached the end of today's lesson on evaluating methods for measuring the rate of reaction.

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

Measuring volume of gas produced throughout a reaction allows the changing rate of reaction to be calculated.

We can draw tangents at different points on the graph to allow us to calculate the gradient at different points throughout the reaction.

Errors in measuring volumes of gas over time include timing errors, reading volumes in real time, and gas being displaced by the bung.

Measuring an end-point in a reaction allows direct and simple comparison of reaction rates.

Errors in measuring end-points include subjectivity of observations, measuring times and volumes, and also contamination.

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

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