Skip to 0 minutes and 9 seconds One of the easiest ways to get students to start investigating the factors that affect the rates of reaction is to start altering the concentration of one reagent whilst keeping the other either in excess or constant. What we’re going to do in this practical is react magnesium ribbon with hydrochloric acid. It’s a reaction that students are familiar with from lower down in science because it’s used to produce hydrogen gas, which they know to test with the squeaky pop test using a lit splint. So for this experiment we need a glass trough filled with water. And this is where we’re going to put an inverted measuring cylinder filled with water to collect the gas that we’re producing.
Skip to 0 minutes and 46 seconds You can use a glass trough for demonstration purposes to show students the technique, but for them to do it it’s often best to use a plastic tub because that reduces the risk of breakage. We’re going to mark on our measuring cylinder at the point that we want to stop our reaction at for our measuring purposes. And we’re going to choose to use 30 centimetres cubed on our measuring cylinder. We’ve also got a conical flask with a bung and delivery tube. We’ve got one molar hydrochloric acid and a measuring cylinder to measure it. We’ve also got our piece of magnesium ribbon, and we need to clean that quite often because it has a layer of oxide on the outside.
Skip to 1 minute and 23 seconds It’s often worth cleaning this before it’s given to the students, because if they rub very vigorously there is a very small, but significant risk that they could actually set fire to the magnesium. So if we’re using at least 4 centimetres of magnesium and so long as everyone in the group uses the same amount, then we can compare our results throughout the practical. So I’m going to first of all mark on my measuring cylinder where I want to measure at. And I want to measure to 30 centimetres cubed. So now I’ve marked my measuring cylinder, I’m going to fill it up with water. And then I’m going to invert it and clamp it.
Skip to 1 minute and 57 seconds Clamping is most useful because it allows you to keep the measuring cylinder steady and it reduces the chance that the measuring cylinder will fall over and break. Now, I’m going to measure out my acid. So I’m going to bring my eye level down and pour my hydrochloric acid into the measuring cylinder. I’m going to measure it to the 50 centimetre cubed mark.
Skip to 2 minutes and 23 seconds And then I’m going to pour that into my conical flask.
Skip to 2 minutes and 32 seconds I’m going to tuck my tube inside the measuring cylinder to collect the gas. And then once I’m ready to go I’m going to start the stopwatch at the same time as I drop my magnesium ribbon in. It’s much easier if students work in pairs to do this because it can be a little tricky.
Skip to 2 minutes and 53 seconds And I’m going to bring my eye level down to the mark. So as soon as it reaches that mark I will then stop my stopwatch.
Skip to 3 minutes and 12 seconds Once the reaction has gone past the mark on the measuring cylinder it will carry on reacting, but that’s fine. Once it is finished reacting we can discard the contents in the conical flask down the sink. And then we can refill it and set it up with a different concentration. So if students use the same length of magnesium ribbon and they use the same volume on the measuring cylinder, they can all use different concentrations which the more able students will be able to work out by serial dilution. For less able students you may wish to give them different concentration hydrochloric acid for them to change the concentration. They use the same volume every time.
Skip to 3 minutes and 46 seconds They will only be changing the concentration of the hydrochloric acid. That way students should start to be able to see the effect of concentration. And if you take a class set of results with averages and plot a graph of time taken against concentration, then you should see the relationship between concentration. Which roughly means that if you double the concentration you double the rate of reaction.
Skip to 4 minutes and 11 seconds The activity we’re going to look at now investigates the effects of different catalysts on the decomposition of hydrogen peroxide into oxygen and water. This reaction is something that we use in the laboratories to chemically generate oxygen, but it’s also important inside our bodies as hydrogen peroxide is often produced during reactions inside our body. And because it’s reactive, it can damage cells. So inside our body there are enzymes that break down hydrogen peroxide rapidly. We’re going to look at some commonly available lab chemicals to use, but you could also investigate using chopped liver, you could use potatoes, or try other fruits and vegetables to see if they help break down hydrogen peroxide.
Skip to 4 minutes and 52 seconds Enzymes work very similar to catalysts in the fact that they cause a reaction, generally, to speed up. The reasons for that is something that students then need to start pulling apart more theoretically. But it’s good if they can start and observe what happens in a reaction. So we’ve got two boiling tubes, which are held in place inside a beaker with two boiling tube clamps. And this is to help us make sure that if any spillage comes out it’s contained. We don’t want students handling generally or wiping up any hydrogen peroxide spills because it can be damaging to the skin. So what we’re going to do first is we’ll add a little bit of washing up liquid to each boiling tube.
Skip to 5 minutes and 34 seconds And then we’re going to add some hydrogen peroxide, approximately 10 vol hydrogen peroxide, and around the same depth into each tube.
Skip to 5 minutes and 51 seconds And then we’re going to add the catalysts. And we can see both which one reacts quicker, and we’re also then going to see the height of the foam, potentially, as a way of actually measuring the effectiveness of the catalyst. And we’re going to try two different solids. One is manganese IV oxide, and the other one is potassium iodide. So first of all I’m going to put some potassium iodide into this tube. And I’m going to add just a little bit from the end of a spatula.
Skip to 6 minutes and 21 seconds And I’m going to add some manganese IV oxide to the other.
Skip to 6 minutes and 27 seconds Sometimes you might need to give the tubes a little shake to mix the catalyst up.
Skip to 6 minutes and 37 seconds In a period of over three to five minutes you’ll be able to see the various heights of the foam and how long it took to reach that height. And that way we can compare the effectiveness of these catalysts. So now we see the height of our two foams. The one here has had the manganese oxide in, and that one produced the foam quicker. And the one on the left, our potassium iodide, is still actually increasing, but it’s taking a lot longer to get there. We could also start adding some numbers to that by measuring the height of the foam from the liquid using a ruler. And students can then compare their different heights depending on the catalyst they’ve used.
Skip to 7 minutes and 17 seconds So long as they’ve used the same initial volume, then they should be able to work out which is the best catalyst for this reaction. However, this isn’t as precise as we might like it to be. So what we can do is we can start to do this in measuring cylinders instead, where students can be a bit more quantitative about their results.
Skip to 7 minutes and 39 seconds We’re now going to do the same magnesium acid reaction again, but in a different way. This time we’re going to use a burette instead of a measuring cylinder as a more precise way of measuring the volume of gas produced. And this is starting to get students used to the idea of choosing the right equipment for the right job. It can be quite challenging filling burettes up if you fill them with water, put your finger over the end, and then invert them. So instead here we’ve attached a length of plastic tubing to the burette top, and we can use a syringe to suck the water up inside the burette. It allows students to do this quite quickly for repeat measurements.
Skip to 8 minutes and 17 seconds So if we open the valve, we can start pulling up the water inside.
Skip to 8 minutes and 28 seconds And then we can let it down to start the line, so we know what reading we’re starting at. We’ve got the same set up as the previous magnesium and hydrochloric acid experiment. We’ve got a conical flask with a bung and a delivery tube into the base of the burette. We’ve marked on the burette the level that we want to read, which is 30 centimetres cubed from the top to the bottom. And that’s the challenge for students being able to read scales upside down. We’ve also got 50 centimetres cubed of 1 molar hydrochloric acid set up. So we’re now going to run the same reaction again.
Skip to 9 minutes and 0 seconds Pour our acid in, and then get ready to introduce our magnesium ribbon and start the stop clock at the same time.
Practical skill progression
In this video, we are going to look at similar experiments that increase the level of practical demand for students, while helping students explore the factors that affect the rate of reaction. As you watch, consider what practical skills are being demanded of and developed in students.
Experiment 1: concentration
Our first reaction is between magnesium ribbon and hydrochloric acid. Most students will be able to remember the reaction from their earlier studies:
Metal + acid \(\rightarrow\) hydrogen + a salt
So here is:
Magnesium + hydrochloric acid \(\rightarrow\) hydrogen + magnesium chloride
Mg(s) + 2HCl(aq) \(\rightarrow\) H2(g) + MgCl2(aq)
This reaction just uses a fixed mark on a measuring cylinder
Experiment 2: catalysts
Here we introduce the idea of catalysts in reactions: that they can alter the rate of a reaction, usually to speed it up. Biological catalysts (enzymes) can also be used here, such as some crushed potato, or chopped liver.
However, it is hard to convince students that the catalyst is not used up in the reaction. Using manganese oxide, for example, it does not change colour and you could collect the catalyst after the experiment, dry and weigh it, to show you have the same mass, which can help with the explanation.
Experiment 3: increasing precision
This time we are using a more precise measurement, although we are using a fixed volume to measure, students could time how long it takes to reach 5, 10, 15cm3 etc. They will then be able to plot a graph of time against volume, rather than just “time to reach 30cm3 against volume”, which can start to extend students understanding.
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