Investigating the effect of temperature on the reaction between Sodium Thiosulphate (Na2S2O3) and Hydrochloric Acid (HCl)
Investigating the effect of temperature on the reaction between Sodium Thiosulphate (Na2S2O3) and Hydrochloric Acid (HCl)
Planning and Prediction
Apparatus List
Conical Flask
3 measuring cylinders
Sodium Thiosulphate (Na2S2O3)
Hydrochloric Acid (HCl)
Thermometer
Stop watch
When sodium thiosulphate reacts with hydrochloric acid, the solution becomes opaque. I will investigate how the rate of this reaction is affected when the experiment is carried out at different temperatures. The reaction that occurs produces sulphur dioxide, water and sodium chloride. It can be shown by this equation:
Na2S2O3 (aq)
+
2HCl (aq)
-->
S (s)
+
SO2 (g)
+
H2O (l)
+
2NaCl (aq)
sodium thiosulphate
+
hydrochloric acid
-->
sulphur
+
sulphur dioxide
+
water
+
sodium chloride
The rate of chemical reactions can be affected by a number of things:
* changes in surface area
* changes in concentration of the reactants
* changes in temperature
* added substances called catalysts
* and changes in pressure if the reactants are gases.
In most chemical reactions the rate changes with time, normally slowing down.
Prediction
I will be investigating the effect that temperature has on the reaction between sodium thiosulphate and hydrochloric acid. I predict that the higher the temperature, the faster the reaction will occur. This should happen because when heat is applied to a solution the particles in the compounds move faster and therefore come into contact with the other substance more rapidly. More importantly, the collisions are more energetic. An increase in collisions provides more energy than there is in the activation energy, so the reaction is faster.
Preliminary Investigation
In order to discover what ratio of HCl to Na2S2O3 I needed to conduct a preliminary experiment, changing the concentration of the various reactants.
I will need a time of two or three minutes at 20°C, my lowest temperature, because I predict that the reaction will speed up as the temperature is higher, and I do not want the reaction to happen in just three or four seconds once I get to 60°C as it will be difficult to measure the time accurately.
Results Of Preliminary Experiments
ml of Na2S2O3
ml of HCl
Time (seconds)
25
5
30
5
5
44
20
0
51
0
5
60
It was discovered after the preliminary experiments that there had been some contamination so the results were inaccurate. With a larger amount of Na2S2O3 than HCl the experiment should have happened slower not faster.
After further trials I discovered that:
Water 40mls
Na2S2O3 10mls
HCl 5mls
gave me an adequate starting time of ...
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Results Of Preliminary Experiments
ml of Na2S2O3
ml of HCl
Time (seconds)
25
5
30
5
5
44
20
0
51
0
5
60
It was discovered after the preliminary experiments that there had been some contamination so the results were inaccurate. With a larger amount of Na2S2O3 than HCl the experiment should have happened slower not faster.
After further trials I discovered that:
Water 40mls
Na2S2O3 10mls
HCl 5mls
gave me an adequate starting time of approximately 3:00 - 3:30 minutes at 20°C.
As a result of my preliminary investigation I will add water to dilute the Na2S2O3, as otherwise it is too concentrated and the reaction happens too fast. This will also give more solution so that it will cover the bulb of the thermometer in the conical flask, to give an accurate measurement of the temperature.
The Main Experiment
Method
I will heat the water and the sodium thiosulphate over a Bunsen to the desired temperature. I will add the acid, and start the stop watch. An X will be placed beneath the conical flask containing the solution, and when this is no longer visible the stop watch will be stopped, and the time recorded. In order to make a fair experiment the X should always be the same size and drawn with the same pen; ideally always the same X would be used.
I will carry out the experiments at 5 different temperatures starting at 20°C, room temperature, and the 30°C, 40°C, 50°C and 60°C. I will not go above 60°C because the reaction would happen so quickly that it would be difficult to take an accurate time measurement. I will not go below room temperature because not only will it be difficult to get the solution to an accurate temperature, but the reaction would happen very slowly. It may be difficult to make the solution exactly the same as the temperatures I want, but I will use one as close as possible.
The temperature will be taken at the start and end of each experiment and an average taken. This is because by the end of each experiment the temperature of the solution will have cooled and the temperature taken at the beginning would be inaccurate. Each experiment will be repeated at least twice and an average will be taken. This will ensure that results that are inaccurate will be noticed and the time will be as accurate as possible.
The only variable in the experiment will be the temperature; the amounts of HCl, Na2S2O3, water and everything else will be kept constant. The apparatus must be carefully cleaned out after each experiment because just one drop of HCl in Na2S2O3 can contaminate it and make all the results inaccurate, as I found out in my preliminary tests.
Results
Desired Temperature (°C)
Temperature at start
(°C)
Temperature at finish
(°C)
Time
(seconds)
20
24
24
211
24
24
82
23
23
99
30
32
31
06
30
28
08
40
42
40
51
43
41
52
50
50
48
40
52
50
35
60
64
64
22
64
62
24
Averages
Desired Temperature (°C)
Average Temperature (°C)
Average Time
(seconds)
Rate of reaction (1/time)
20
24
97
0.0051
30
30
07
0.0093
40
42
52
0.019
50
50
38
0.026
60
64
23
0.043
Conclusion
As the temperature increases, the time for the cross to disappear decreases, so the reaction becomes faster. The graphs both show this. The first graph is a curve and although I cannot find a formula from it, it would be a 1/x graph. The values of 1/time are proportional to the rates of reaction, and produce a straight line graph with gradient 0.0008. From the straight line I can calculate an equation: y = 0.0008 x - 0.014. From 20°C a rise of 10°C approximately doubled the rate of reaction. From 40°C, however, a rise of 22°C is needed before the rate of reaction is doubled. This shows an increasing rate of reaction.
The reaction is a precipitation reaction, meaning that the product of the reaction is a precipitate which clouds the solution. This is exactly what happens in the reaction between sodium thiosulphate and hydrochloric acid.
The reason that the higher temperature decreases the time taken for the reaction to occur is shown in the COLLISION THEORY. This theory is as follows: The rate of a reaction depends on how often the reacting particles collide with each other. They must collide in order to react. An increase in temperature increases the number of collisions. Temperature gives energy to the particles. This allows them to move faster which results in more collisions, and gives the collisions greater energy. Temperature also provides the particles with the energy needed to start reacting. This is called the activation energy.
I predicted that the increase in temperature would increase the rate of reaction, and my results agree with my prediction.
Evaluation
I think that my experiment was accurately carried out and that the results were firm enough to base a strong conclusion on. The times taken for each experiment seemed to be similar enough, except for room temperature, 20°C. Here the results differed by almost half a minute over the range. This could be because the apparatus was not cleaned properly after the previous experiment. However, the average time was as I expected, and so I counted all of the times in my results.
For our experiments, we had to wash the apparatus out after each one. This is were inaccuracy could have occurred, if the flask was not washed completely. Just a trace of the solution left behind would have been enough to alter or affect the results. A solution to this would be to have freshly cleaned new apparatus for each experiment. However, for the purposes of my investigation this would not really be necessary and would have been impractical. My results came out how I had expected, anyway.
It was sometimes hard to judge exactly when the cross could no longer be seen. This may have resulted in some inaccuracy. I am not sure what to suggest to remedy this, except to take more readings at each temperature to make the results as accurate as possible. The same person had to judge this each time because different people have different levels of sight.
The first graph produced a good curve which did not seem to have any inaccuracies in the points. However, on the straight line graph the point for 60°C did not fit the straight line like the other points. It could be that the graph should curve up at this point. To see if this is the case I would have to take more readings, possibly at a higher temperature. The other, more likely, possibility is that the readings I took for 60°C were slightly inaccurate and although this did not show up on the first graph it showed on the second graph because the second graph used a much smaller scale. The rate of reaction appears to be too fast. It could be that the stopwatch was stopped a few degrees of a second out, or that the person watching the cross judged that the cross had disappeared too soon, or there could have been inaccuracies in the exact measurements of the sodium thiosulphate and the hydrochloric acid. Either way I would have to do further experiments to sort out the inaccuracy.
To provide a more detailed pattern of results the experiments could be done at more temperatures, for instance every five degrees instead of every ten degrees like I did.
My experiments showed quite successfully that temperature speeds up a chemical reaction at an increasing rate.
