Lesson video

Hello, my name is Mrs. Collins and I'm going to be taking you through the learning today.

This lesson forms part of the "Energy changes in reactions" topic and is about displacement reactions, so let's get on with the lesson.

So during today's lesson we'll learn how to complete a practical to measure the temperature change in a displacement reaction, record the results, analyse the data, draw a conclusion, and decide if the reaction is exothermic or endothermic.

Here are the keywords for today's lesson.

Displacement, exothermic, hypothesis, conclusion, and bar chart.

Pause the video at this stage, read through the definitions, and make any notes if you feel you need to.

Today's lesson is going to be divided into three separate parts.

We're going to look at writing a hypothesis, investigating displacement reaction energy changes, and carry out data analysis and draw a conclusion.

So let's start by writing a hypothesis.

So displacement reactions happen where a more reactive element replaces a less reactive element in a compound.

And these reactions are exothermic, which means the temperature of the surroundings increases, and in this case, that's the solution.

So the example in the image is silver nitrate with copper forming silver and copper nitrate.

So the copper, remember, will displace the silver from the compound because copper is more reactive than silver, and you can see silver there forming on the surface of the copper in the image.

So here's a question based on that piece of learning.

What are the surroundings when silver is displaced by copper from silver nitrate solution? So pause the video here and answer the question.

So hopefully you've recognised that it's actually the solution.

Some people may think it's the air, but the energy dissipates to the air afterwards.

Which metal can displace tin? So this time we're going back to our learning about the reactivity series.

Have a look at the reactivity series and decide which of those four metals can actually displace tin from its compound.

Pause the video here and then answer the question.

So to answer this question we need to look at tin in the reactivity series, and anything that is more reactive than tin will displace tin from its compound.

So sodium is more reactive than tin and aluminium is more reactive than tin, so they'll both displace tin.

Silver and lead are lower in the reactivity series than tin, so they cannot displace tin from its compound.

So well done if you've got that correct.

Now we've got a true or false question.

So lead can displace aluminium from aluminium nitrate solution.

Is that true or is it false? Now what I'm going to do is flick back briefly to the previous slide so that you can see the reactivity series.

You need to see where aluminium and lead is in the reactivity series.

So aluminium and lead we're looking for.

Give you a moment to look at that.

And then back to the question.

So pause the video here, answer the question.

So the answer to the question is false, and the reason is that aluminium is higher in the reactivity series than lead, so lead cannot displace aluminium from aluminium nitrate solution.

Well done if got that correct.

So the energy changes taking place during a displacement reaction can be measured using temperature.

And if you remember, this is called calorimetry.

And what we need to do is measure the initial temperature, and because this is an exothermic reaction, the highest temperature that's reached, and we find the difference between those two values.

So here's an example, we've got the initial temperature here, the highest temperature later because it's an exothermic reaction, and we can find the difference between the two to find the temperature change.

And it's always the final, or the highest in this case, temperature minus the initial temperature.

So we've got an example calculation there.

Let's have a go at this true or false question.

The final temperature is the same as the highest temperature for a displacement reaction.

Is this true or false? So answer the question and use the statements to justify your answer.

Pause the video here and I'll see you when you're finished.

Welcome back, so let's just go through the answer quickly.

So the answer is false and this is because the temperature of the surroundings will decrease once the reaction is complete.

So well done if you got that correct.

Before conducting a practical, we need to consider the variables.

And if you'll remember, there are three different types of variables we need to consider.

The independent variable, which is the one we're in control of.

We choose the values for that before the practical starts.

The dependent variable, which is the one we measure during the practical.

And then the controls, the one we need to keep the same to ensure it's a fair test.

The practical is going to investigate the temperature changes which occur when different metals are added to a metal salt solution.

So which of the following is the independent variable in that practical? So pause the video here and identify the independent variable.

Welcome back, so the independent variable is the type of metal.

We're changing the metal during the practical.

Now we're going to identify the dependent variable in that same practical.

So pause the video here and identify the dependent variable.

During this investigation, the dependent variable is the temperature change, so well done if you got correct.

Now we need to consider control variables, so have a look at this list.

Which variables are control variables for this investigation? Pause the video and I'll see you when you're finished.

In this investigation, the control variables are volume of metal salt solution and the mass of the metal.

Remember, the type of the metal is the independent variable and the temperature change is the dependent variable, so well done for getting that correct.

Before conducting a practical, we need to develop a hypothesis, and a hypothesis is a proposal of what we think could happen during the practical and a suggestion of why this would happen.

And it's normally based on some limited evidence, so we've got an idea and we're kind of developing that.

So now we're going to have a go at writing a hypothesis.

I'm gonna do one with you and then you are going to do one by yourself afterwards.

So Lucas has different types of balls and wants to know which one will bounce the highest and we need to make a hypothesis for the practical before he carries it out.

So Lucas' hypothesis could be, "I think the rubber ball will bounce the highest because it's made of rubber, which is a very elastic material." So there's limited evidence there being used within the prediction.

So because rubber is very elastic, it suggests that the ball might bounce higher.

So you have a go now.

So Sophia wants to find out how changing the mass hung on a spring can affect the stretch of the spring.

So pause the video here and write a hypothesis for that practical.

Sophia's hypothesis could be that "the greater the mass, the bigger the stretch of the spring, as it will be heavier." So well done if you got something similar to that.

We are now going to do Task A, write a hypothesis for the displacement practical to predict which metal will cause the greatest temperature change when added to a metal salt solution, and why? So pause the video here and write a hypothesis.

So let's go through a potential answer.

So your hypothesis could include the following.

So, "I think that the most reactive metal will cause the greatest temperature change when added to the metal salt solution.

I think this because it will react the fastest to displace the metal from the salt solution, releasing energy quickly." So well done if you've got something similar to that.

We're now going to do question two.

Complete the word and symbol equations for these following displacement reactions.

So pause the video here and then complete those equations.

Welcome back, so let's go through those answers, and we need to think about displacement here, which ones are more reactive and which ones are less reactive.

So zinc is more reactive than copper.

So it'll displace the copper from the copper sulphate, forming copper and zinc sulphate.

And we can write that as a balanced chemical equation as well.

And then again, magnesium is more reactive than copper, so it will replace the copper, it will displace it in this copper sulphate to form copper and magnesium sulphate.

And again, we can write a balanced symbol equation for that.

And you notice where the copper and the magnesium have exchanged places basically.

And then iron is more reactive than copper again, so we end up with copper and iron sulphate.

And once again we can write that as a balanced chemical equation.

So well done if you got that correct.

We're now moving on to the second part of the lesson, which is about the investigation itself, so we're gonna investigate displacement energy changes.

We're going to use the calorimetry method, remember, and here is the equipment used in that method and a suggested method for the practical, so set up the equipment as shown in the diagram.

Add 20 centimetre cubed to the metal salt solution into a polystyrene cup, measure and record the initial temperature, weigh and add 1 gramme of the metal and stir, measure and record the highest temperature reached.

So notice we measure the initial temperature before we add the metal and the reaction starts.

So here's a question based on what we've done so far.

How should the temperature change be measured? So pause the video here and answer the question.

So hopefully you have recognised that B is the correct answer, so it's the highest temperature minus the initial temperature, and that's because it's an exothermic reaction, so we're thinking about the highest temperature reached.

Before starting the practical, a results table has to be prepared to record the data.

So it should include columns for the independent and the dependent variables with very clear headings and units where necessary.

For example, Izzie wants to measure the height of everyone in her class at school, so she prepared this results table.

And as you can see we've got the pupil name in the first column and the height in the second column.

So the pupil name is the independent variable and the height is the dependent variable.

You can see the pupil name doesn't have a unit, but the height does have a unit.

Which of the following results table is correctly presented for the investigation of the effect of the mass of magnesium on the volume of hydrogen gas produced? So have a look through those three tables, pause the video, decide which one's correct, and I'll see you when you're finished.

Now hopefully you've recognised that it was C that was actually correct, and that's because the independent variable, the mass of the magnesium, is in the first column, and then the dependent variable is in the second column.

We need to be really careful here because we're looking for centimetre cubed and the volume in centimetre cubed, not the height here.

Here the variables are the wrong way round, so we need the independent variable in the first column and the dependent variable in the second column.

Let's have a go at Task B.

So the investigation will involve the reaction between three metals, e.

g.

zinc, magnesium, and iron, and the metal salt solution, e.

g.

copper sulphate.

The initial, highest, and temperature change will be measured.

Prepare a results table for the investigation.

Pause the video here and design a results table.

Welcome back, so let's have a look at a possible results table for this investigation.

It may well look like this.

So we've got the metals in the first column, we've got the initial temperature, the highest temperature measured, and the temperature change.

That's the highest takeaway, the initial.

And make sure you've got units in those columns as well, tops of the columns, well done.

So for question two we're going to carry out the investigation, so perform the investigation involving the reaction between the three metals and the salt solution, and record your results in a table as follows.

So pause the video here and carry out the investigation.

Let's have a look at a set of possible results, so your results may look something like this.

So for zinc, we've got an increase in temperature there of 14 degrees Celsius.

For magnesium, an increase of 22 degrees Celsius.

And for iron, an increase of 9 degrees Celsius.

Now if we look at those three increases in temperature and think about which one of those three metals is the most reactive and see whether our hypothesis is correct, so we can be thinking about that at this stage.

Now we're going to move on to the third part of the lesson where we're going to do some data analysis and draw a conclusion.

So once data from an investigation have been collected, they need to be analysed.

And this involves presenting them clearly in a graph, interpreting what the data is showing, and concluding whether or not the hypothesis was correct and why not, or why if it did agree.

And then we need to think about how to evaluate the investigation to see if there's any improvements we could make to gather more reliable data.

The type of graph depends on the type of data that's collected, and discrete data are categories if we think about colour, type of dog, or countries of the world, or in our case, for our investigation, type of metal.

And if we've got this type of discrete data, we always plot it on a bar chart with the independent variable on the X axis and the dependent variable on the Y axis.

Consider the following data and decide which ones are discrete.

So we've got football teams, distances of the planets, and blood groups.

So pause the video here and have a think about your answer.

So the following data are discrete.

Football teams and blood groups.

The distances of the planets is continuous, so that would be shown on a line graph.

When we're writing a conclusion, we need to consider the following.

A summary of the results observed, a decision as to whether the hypothesis was correct or not, and we need to try and explain the results that we've observed.

So what needs to be included in a conclusion? Think about the following statements, pause the video, and I'll see you when you're finished.

Welcome back, so a conclusion needs to include a summary of the results and why you think you've got those results.

What you think will happen appears in the hypothesis.

Finally, we need to carry out an evaluation.

We need to think about how we could improve the experiment.

Do we need to use different equipment? Do we need to change the method? We need to think about how to improve the reliability of the data.

Did we get any anomalous results? Did we need to repeat the experiment and take a mean of the results? We're now going to do Task C.

The investigation involved the reaction between three metals and a metal salt solution.

We're going to draw a graph of your data.

You could use the data below, or the data you gathered during your investigation, or the data which is included in the following video.

Once the experiment has finished, pause the video and complete your graph.

Firstly, we're going to measure 20 centimetre cubed of copper sulphate solution and pour it into the polystyrene cup.

Next we need to find the initial temperature.

We're now gonna add two spatulas of magnesium powder to the copper sulphate solution and stir it to ensure the two reactants are well mixed.

We then need to watch until we see the highest temperature on the thermometer.

That part of the video I'm going to speed up for you.

Record the final temperature of the reacting mixture.

We're now going to repeat the experiment with zinc powder.

The initial starting temperature is again 22 degrees Celsius.

I'm going to speed up the reaction and you need to record the highest temperature.

Finally, we're going to repeat the experiment again with iron powder, so record the initial temperature and the highest temperature.

So let's go through the answers.

Your graph should look something like this.

So it should be a bar chart, it should have the metal on the X axis and the temperature change on the Y axis.

So depending which data you use depends on what your graph will look like.

Make sure you've got spaces between the bars, that's really important for a bar chart.

We're now going to write a conclusion from your findings.

So A, did the metals react with the metal salt solution, and how did you know? B, did your findings match your hypothesis? C, which metal was the most and which metal was the least reactive, and how do you know? Pause the video now and answer those questions.

Welcome back, let's go through the answers one at a time.

So for 2A, did the metals react with the metal salt solution, and how do you know? All three metals reacted with copper sulphate as a temperature increase was measured for each.

You might also mention that there was effervescence if you saw that as well.

Did your findings match your hypothesis? Yes, the most reactive metal, magnesium, caused the greatest temperature change when added to the copper sulphate solution.

Then finally, C, which metal was the most reactive, which metal was the least reactive, and how do you know? So the largest temperature change occurred when magnesium reacted with copper sulphate as magnesium is the most reactive metal and released energy very quickly.

The smallest temperature change occurred when iron reacted with copper sulphate as it's the least reactive metal and released energy very slowly.

So well done if you got that correct.

We're now going to do question three and write an evaluation for the experiment.

So for A, how could you have improved your equipment? B, how could you have improved your method? C, did you obtain any anomalous results? D, how could you have improved the reliability of your results? Pause the video here and I'll see you when you're finished.

So let's go through the answers to that question, we're going to deal with A and B first.

So how could you have improved your equipment? You could have insulated the cup a little bit better to make sure there was no heat loss.

You could have used a temperature probe to prevent misreading of the thermometer.

How could you have improved your method? So you could have measured the temperature at regular time intervals for a set period of time.

This would have ensured that the highest temperature was recorded.

And you could have thoroughly stirred each reaction mixture for the same number of times.

Then onto part C, did you obtain any anomalous results? No, the temperature changes obtained were as expected due to the reactivity of the metals.

Now if you carried out the experiment yourself, you may have had an anomalous result, in which case your answers to that will be different.

How could you have improved the reliability of your results? You could have repeated each experiment measuring the temperature changes and taken a mean for each metal.

We're now going on to summary.

So displacement reactions involve a more reactive element replacing another in a compound.

Exothermic reactions release energy to the surroundings, often through heating.

The initial and highest temperature of the reaction mixture need to be recorded to calculate the energy change in an exothermic reaction.

Hypotheses guide predictions about the effect of variables in experiments, and bar charts can visually represent collected results to help analyse data.

Thank you for joining me for this lesson.

I hope you learned a lot.