Skip to 0 minutes and 10 secondsIn this activity we're going to look at the reaction between sodium thiosulphate and hydrochloric acid. When these two react together they produce colloidal sulphur, which makes the solution go cloudy. We use that cloudiness to actually act as a measure of the rate of reaction. This practical lends itself to changing both the concentration of the sodium thiosulphate and the temperature of the solutions. We're going to concentrate on temperature in this reaction and perform the classic experiment that's often found in textbooks and schemes of work. We're going to use a conical flask which is sitting on top of a white tile where we've drawn a black X. We're going to use 100 millilitres of 0.1 molar sodium thiosulphate.

Skip to 0 minutes and 55 secondsAnd we're going to use a syringe to dispense 5 millilitres of 1 molar hydrochloric acid. We're first of all going to add our sodium thiosulphate into the flask. And then using a syringe rather than measuring cylinder, because it's easier to get good mixing, we're going to suck up our 5 mil of hydrochloric acid. And then we're going to inject it into our conical flask and start our stop clock. Give it a swirl. And we're going to look down through at the cross. When working with students it's best to make sure that one student in a group does this measurement because different students will have different perceptions of depth as they're looking at the cross.

Skip to 1 minute and 36 secondsIt's Starting to go cloudy already, but I can still see the X in there. And it's now reached the stage where it's becoming more difficult to see the mark.

Skip to 1 minute and 49 secondsAnd we'll stop. And that's stopped at 31 seconds. Now, this practical is often done at different temperatures. And the solutions need to be brought up to the temperatures that you're going to measure. So that would often mean using a water bath to heat them up, or in some schools people would heat them directly on a Bunsen burner. You also may want to cool the solutions down as well, but you must take the accurate temperature of the solution, not what you are heating it in. There are some disadvantages with this practical. In fact, we're using a large amount of solutions. We're using 100 millilitres of thiosulphate at a time. And as you can see, the solution has gone very, very cloudy.

Skip to 2 minutes and 26 secondsAs well as producing colloidal sulphur, that's now starting to produce large amounts of sulphur dioxide as well. And as that enters the air it can cause respiratory damage, it can cause asthmatics to suffer, and it can be quite disturbing for students. What we would suggest is that students actually use a much more reduced scale to it. And also, they stop the reaction as soon as they see the cross disappear. And to do that we need to look at a safer way of doing this reaction that actually also gives better results. So now we're going to look at a different way of doing the reaction of sodium thiosulphate and hydrochloric acid.

Skip to 3 minutes and 1 secondWe're going to do it on a much smaller scale and we're going to reduce the output of sulphur dioxide. So we have, instead of using a large conical flask sitting on a white tile, we've got a plastic container which we've drawn two X's in the bottom and cut two holes in the top. And this is going act as our reference and our water bath as well. We're going to take some water from a kettle and top that up into the container. It doesn't need to be too hot because we don't want to go above around 55 degrees Celsius. We're then going to take a syringe to measure out our sodium thiosulphate.

Skip to 3 minutes and 40 secondsAnd we're going to use 10 millilitres of sodium thiosulphate and add that into our container. And here we can prep more than one at once. We can prep a second one. So this enables students to actually get much faster results as well.

Skip to 4 minutes and 2 secondsWe now stand those in the hot water and we take the temperatures of the solutions so that we know the actual temperature that we are reacting at. Normally we'd allow this to equilibrate for a few minutes. We can also, as it cools down, start adding some ice so we can take it below room temperature and actually get temperatures down to around 5 degrees or so. So if that's waited long enough to equilibrate. This is now at 38 degrees Celsius. So we'll use that as our starting point. And instead of using a large amount of acid, we're going to use a small syringe and we're going to draw up 1 millilitre of hydrochloric acid.

Skip to 4 minutes and 44 secondsWe're going to leave our thermometer inside as we add the acid so we can give it a quick stir at the same time as starting the stop clock. So as you add the acid, start the stop clock, stir, and remove my thermometer. And stop. And we can put that back in and take a final temperature, which is 46 degrees Celsius. Take that out. And we're now going to immediately pour this into a stock bath which is sodium carbonate solution, or sodium hydrogen carbonate solution where we've added some universal indicator to act as a warning when the stock bath has been exhausted.

Skip to 5 minutes and 26 secondsWe can also drop our container in there, which will stop the little glass vial from becoming cloudy with sulphur, which can be very hard to remove. That way we've stopped the production of sulphur dioxide, students get very rapid results, and it means they can use a lot less reagents. We're doing 10 experiments using the same volume of liquid as we did for one of the classic experiments. And that's something we want students to start appreciating, is that we want to learn how to reduce the waste from experiments and reduce the waste from industry in general.

Using one reaction for progression

The sodium thiosulfate and hydrochloric acid reaction is one that can be built in steps for students. Here is an example of how this one experiment can be used at different points in a progression plan.

Starting point

Students need to be able to carry out the basic reaction with given concentrations of reagents, so they can identify what they are looking for, as well as the manipulative requirements, plus disposal routes (the safety side). This should follow on from earlier rates experiments with simpler equipment.

Changing one factor

Students can then move to changing one of the factors and concentration is easiest. If using syringes or measuring cylinders, students should be able to work out the concentration (using their science and maths knowledge) to produce a fixed volume of a certain concentration, starting with just 1moldm-3 sodium thiosulfate solution.

Less able students may need a framework to help them before tackling the practical.

Students could also then try to see the effect of changing the concentration of the acid and relate this to the reaction. This is best done with syringes, given the small volumes involved.

Enquiry through practical

The most challenging version of this activity requires students to alter the temperature of the reagents (both acid and thiosulfate solutions) as well as the temperature of the reacting vessel. Students should be able to choose an initial set of concentrations that, at room temperature, are neither too fast or too slow, so a degree of preliminary experimentation by students might be required. This becomes more of an enquiry activity, as students will have to apply their knowledge of both the science and the practical procedures, to be able to produce a result that can be shared and discussed with the rest of the class.

Extending the practical

As an extension, students could look at other ways to investigate this practical, and could be introduced to colorimetry as a way the time is measured. A colorimeter allows a consistent measurement to be made of the reaction. There are a number of home-made devices that can be built, as well as more expensive commercial colorimeters. This could be a research task for students, or questions set around other procedures to evaluate their application of skills to other contexts.

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Teaching Practical Science: Chemistry

National STEM Learning Centre