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The reaction to be studied is as follows:
Na2S2O3 + 2HCl 2NaCl + H2O + SO2 + S
There are several possible ways of finding out the reaction rate, as there are four by-products formed in the above reaction.
The fact that sodium chloride is in solution accounts for why it is not suitable to be collected. Water is hard to measure, as there are too many moles of it in comparison to the salt, which is in aqueous solution. Sulphur dioxide gas would be a suitable factor for measuring reaction rate, and it would not have to separated from any other gases, as no other gases are given off.
However, it will react and dissolve into the water, and it is also fairly toxic, making it a safety hazard. This leaves us with sulphur, which is left as a solid.
Sulphur is the most suitable substance to measure reaction rate, as when the two colourless, aqueous solutions are mixed together, a cloudy precipitate is formed.
This is the sulphur, which is insoluble in water. Therefore, an experiment can be designed to measure the amount of time for the solution to completely precipitate.
There are four factors, which affect reaction rates: surface area, use of a catalyst, temperature and concentration.
It is impossible to vary surface area, as both reactants are liquids, and in order for this to work, they would both have to be solids. Use of catalysts is a hit-and-miss variable, as it involves putting in random chemicals and seeing if they react.
Temperature and concentration are both suitable for variation, but I have decided to choose varying the concentration, as varying temperature is often too quick, and
can therefore be inaccurate.
Na2S2O3 + 2HCl 2NaCl + H2O + SO2 + S
* As we are using Na2S2O3 as a variable, we need to ensure that the HCl is constantly in excess. Therefore, I need to use the following calculation to find out how much HCl I should use:
Moles = concentration x volume
= 0.23 x 0.05
Volume = (0.0115 x 2) = 11.5 cm3
Amount of precipitate formed
* Reaction Rate = time
Extent of reaction
* When Na2S2O3 reacts with HCl, a precipitate forms. The time taken for the precipitate to form is an indicator for the reaction rate.
* In order for a reaction to occur, reactant particles must collide with a certain minimum energy – activation energy
* Increasing reaction rate results from : – more collisions
– more energetic collisions
* When performing rates experiments, we must have a suitable way of measuring the extent of the reaction.
* It is essential to alter only one factor at any given time. This is called fair testing.
* Temperature affects reactions, so it must be ensured that the temperature remains constant throughout.
The more concentrated the sodium thiosulphate is, the faster the reaction rate.
Key: – HCl molecule – Na2S2O3
There is the same amount of acid in both container (i) and (ii), but container (ii) has a higher concentration of sodium thiosulphate in it. This means that, as shown above, there are many more Na2S2O3 molecules present, in the same volume. This means that the reaction will occur much quicker, as there are many more molecules to react with.
As the concentration increases, the time taken for the X to disappear from view decreases, meaning the reaction rate is faster. And as the values for reaction rate are directly proportional to 1/t , I have drawn up the above hypotheses graphs.
Some safety precautions were undertaken to ensure no serious accidents occurred:
– No running in the lab
– No bags were allowed near the practical area of the lab
– Lab coats were worn to ensure that if any spillages occurred, they were not on your clothes
– Safety goggles were also worn, in case any solutions splashed up towards the eyes.
A preliminary experiment was performed to see whether the reaction works, and what needs to be improved on before the main experiment.
To discover reaction rate, we need to know when the solutions have completely precipitated. In this case, we can put a piece of paper, with an X, under a conical flask and see how long it takes for the X to disappear from view, because of the precipitate formed.
The experiment was only a rough guide, so only the maximum and the minimum concentrations were tested, to ensure that the experiment worked as it was supposed to. This was then repeated to back up the original, and make sure that there were no extremes.
The results are as follows (all volumes are in cm3):
Time taken (s)
1 piece of paper
1 marker pen
2 large beakers (500ml)
1 measuring cylinder (100ml)
1 measuring cylinder (25ml)
1 conical flask
1 distilled water bottle
1 stop watch
Dilute Hydrochloric Acid
1) Take a piece of paper, and using the marker pen, draw a large X onto it
2) Put the thermometers into the two solutions, to ensure that the same temperature is maintained throughout the investigation.
3) Fill the beakers with the two solutions, and label appropriately.
4) Pour the sodium thiosulphate into the large measuring cylinder, up to 50ml.
5) Then pour into the conical flask.
6) Then, using a pipette put 15ml of HCl into the small measuring cylinder.
7) Finally, put this HCl into the conical flask and start the stopwatch.
8) Wait 5 seconds and then start shaking the flask over the X
9) Wait until the X can no longer be seen and then stop the clock, recording the time.
10) Repeat the experiment, but in step (3), vary concentrations – i.e. put 40ml Na2S2O3 and 10ml water …
11) Repeat the whole experiment again, to backup data collected.
There were several things that were done to ensure that the experiment was a fair test.
– Only one thing was changed throughout the experiment, which was the concentration of sodium thiosulphate, as that was the variable factor.
– So, a constant check was kept on the temperature, volumes of reactants, cleanliness of apparatus to ensure that these factors did not interfere with the experiment, and therefore, effect results.
– Both solutions were shaken consistently 5 seconds after the acid was introduced.
– I was constantly the person checking to see whether the X had disappeared, as I knew what it was like for each time and knew when the test was over.
– The conical flask was thoroughly washed after each reaction, to ensure no solutions were left over, and could affect results.
All volumes shown are measured in cm3.
Time taken (s)
1 2 3
33.41 32.81 33.93
41.19 43.06 45.00
67.07 63.15 59.72
99.75 97.97 96.48
163.44 173.25 165.78
Straight away, looking at these results, we can seen that, as I predicted, as the volume of sodium thiosulphate decreases, the time taken for the X to disappear from view, increases.
The concentrations are as follows:
From these results, I have drawn up some graphs, which are shown on the following page.
The higher the concentration, the quicker the reaction rate. This is due to the fact that there is more chance of particles colliding with each other, as there are more particles. This is because, as I predicted, where there is a higher concentration of Na2S2O3, there are more molecules present to react with the acid.
For lower concentration values, the times for the X to disappear are a little more spread apart than usual. I believe this is because, as there are fewer molecules to react with, the likeliness of collision between the two reacting particles is much less, and as a result of which, the overall reaction is not very great.
From the graph, we can see that the rate:concentration graph is almost a straight line, which is as predicted.
The set of results which I obtained was quite reliable, as it agrees with my hypothesis, which was made using detailed scientific knowledge. And also, there were three sets of results taken, all of which are relatively close to the average.
The stopwatch records milliseconds, but for a person to acknowledge the change of colour and stop the clock, makes results a little bit inaccurate. Also, the extent of the reaction, although seen by the same person and through the same eye(s), it is hard for the eye(s) to notice the exact point at which the colour changes on 15 separate occasions.
Therefore, as an improvement, I would use a colorimeter to measure the extent of the reaction. And, if possible, I would attach this to a stopwatch, so that the when the reaction was sufficiently complete, the clock would stop.
I am not surprised that my graph for rate against time is not a perfect straight line, as with any experiment, there is a certain amount of error, which cannot be abstained. The results for 14.8 g/dm3 and 22.2 g/dm3 are a little odd, but still show the growing trend. I would repeat these results again, if possible.
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