Find out how changing the concentration of Sodium Thiosulphate in a sodium thiosulphate and hydrochloric acid solution effects the reaction rate.

Obtaining the evidence:

Before I started my experiment I realized that I would have to make a couple of changes to my planned method. This I the method that I actually carried out for the experiment:

Instead of using pipettes or measuring cylinders to measure the amounts, we used burettes. These made the amounts more accurate, and are more reliable than using pipettes. You could see exactly how much of the liquid you were using to 0.5 of a ml. We measured the amount of sodium thiosulphate and water using pipettes but measured hydrochloric acid in a measuring cylinder. I put 65cm3 of Sodium Thiosulphate in a conical flask. As I added 5cm3 of hydrochloric acid I started the stopclock. No water was added. The flask was already on top of the piece of paper with the cross on. I swirled the flask five times in an anti-clockwise direction each time to make sure it was mixed up but also to keep it a fair test. When the cross could no longer be seen I stopped the stopclock. I carried out the rest of the experiment using my planned concentrations. I repeated the experiment with my planned concentrations but instead of doing it three times I just did it twice because I didn't have enough time. Instead of finding the average time out of the two as planned, I found the reaction rate for each result and then found the average out of the two. This is a more accurate way of finding the reaction rate.

The constants in the experiment were the amount of hydrochloric acid and the temperature. The hydrochloric acid reacted with the sodium thiosulphate, but the reaction rate varied depending of the concentration of the solution. I obtained 7 values for my input variable and repeated all my results.

Results:

Sodium

Thiosulphate

(cm3)

Water

(cm3)

HCl

(cm3)

Concentration

Time(s)

Time(s)

2

Reaction rate

2 AV.

65

0

5

37.1

8:94

25:42

0.05

0.04

0.045

55

5

5

36.7

29:56

27:38

0.034

0.037

0.0355

45

5

5

30

33:10

44:79

0.030

0.022

0.026

35

25

5

23.3

53:43

54:35

0.019

0.018

0.0185

25

35

5

6.7

67:42

69:28

0.015

0.014

0.0145

5

45

5

0

42:51

22:63

0.007

0.0082

0.0076

5

55

5

3.3

767:83

612:68

0.001

0.0016

0.0013

Evaluation:

This experiment was overall a success. There are a couple of possible anomalous results-the last two results I gathered I think are anomalous because they have a much slower reaction rate than the rest of the results and this shows on the graph. They are it is only slightly different to the pattern of the rest of the results, but enough for me to tell that they are anomalous. These results could be anomalous because of many reasons. It could be because slightly less of HCl or too much of sodium thiosulphate was added. When I repeated the experiment the second the first result where sodium thiosulphate was 65 and water was 0, the different between the first and second results was about 7 seconds which is a big gap so shows that it was an anomalous result.

In the experiment we used burettes to measure the amount of sodium thiosulphate and water that was added to the solution, this is a more accurate method than using a measuring cylinder or pipette. The amount of hydrochloric acid was measured using a measuring cylinder, this is less accurate and could have affected the results because there could have been slightly more or less than 5cm3 added. This would have either sped up the reaction or slowed it down. If I did the experiment again I would have used a burette for the hydrochloric acid as well, as the hydrochloric acid is meant to be the constant in the experiment so it should be exact each time.

The timings of the experiment were measured from the start of the reaction to when the cross could no longer be seen through the solution. This is very inaccurate because different people would have different opinions of when the cross had disappeared. If I could do the experiment in another circumstance I would get three people to watch the cross and each say when they thought it had disappeared. I would then take the average time. This is not possible to do for this experiment so you just had to judge it by your own view and try to make it as accurate as possible. A better and more accurate may of finding when the cross can no longer be seen. The light gates will time from when the liquid is clear to when its not because no light can pass through the liquid so it will stop recording the time. This is the most accurate way of doing the experiment and if I could have I would have done my experiment that way.

The two variables in the experiment were sodium thiosulphate and water. These were both changed so that we made different concentrations and could find out the aim of the investigation-'whether concentration effects reaction rate.' I came across one problem with the variables-water. The burette we were using first broke and we had to get a new one, but the second burette got blocked up and it took a long time for the water to come out, this took up a lot of my time, and slowed down the experiment greatly. When using the burettes, you had to judge when the liquid was at the amount you wanted it at, and because you were looking at the figures on the glass from below, the exact values might not have been obtained. This is an inaccuracy and to solve it we could have used smaller burettes that weren't quite as high above you or lowered them of the tables.

I made the experiment more reliable by repeating it, to get a more accurate set of results. In an ideal world I would have done it three times to make it even more precise, but time didn't permit me to do that.

I feel that I obtained enough evidence to answer the aim of the experiment as I got two readings for each concentration and an average reaction rate. If I was to get additional evidence I would repeat the experiment but I would use different concentrations than the ones I used in my experiment to make sure that concentration really does effect reaction rate. I do have the results of the preliminary work that back up the evidence that I obtained in my experiment:

Sodium Thiosulphate

Water

Time

Concentration

Reaction Rate

50

0

32.7

32

0.028

40

0

41.54

32

0.024

30

20

45.48

24

0.022

20

30

65.12

6

0.015

0

40

74.34

8

0.013

These results show the same pattern as the set of results I obtained for my experiment and are further evidence towards my conclusion.

Conclusion:

After carrying out my experiment I can conclude that as the concentration of the solution increases, the reaction rate also increases, which means that they are directly proportional. The reaction rate is found by 1/time. The higher the concentration of the solution the closer the ions are. When the ions are closer they will collide more often and there will be a greater chance of bonds breaking. The more bonds that are broken in a shorter period of time, the faster the reaction rate will be. If you were to keep the concentration of Sodium Thiosulphate constant and alter the concentration of HCL, the reaction rate would be proportional to the concentration of the acid.

When Sodium Thiosulphate and Hydrochloric acid react they form a precipitate of sulphur, a liquid form of sodium chloride and a sulphur dioxide gas. The gas given off is what gives it such a strong smell. This is the word equation for the reaction:

Hydrochloric + Sodium ==> Sodium + Sulphur + Water

Acid Thiosulphate Chloride Dioxide

The symbolic equation for the experiment is:

Na S 0 +2HCl ==> S+ 2NaCl + SO