Investigating the effects of concentration on the rate of a reaction Na2S2O3(aq) + 2HCl(aq) 2NaCl(aq) + S(s) + SO2(g) + H2O(aq)

I will start with 50cm3 of 2.0M sodium thiosulfate, then in 5cm3 intervals remove some of the original sodium thiosulfate and substitute water.

E.g. my second experiment (after my retests to find an average) will involve 45cm3 sodium thiosulfate and 5cm3 H2O

Hypothesis:

From previous experience I think we will find that the higher concentrations react faster. Preliminary tests have already shown this trend and it is logical really. Particles must collide to react. If there are more reactant particles in a given area they will be closer together, collide more and therefore the substances will react faster.

Results:

H2O                Na2S2O3        Concentration        Time                Average time

(cm3)                (cm3)                (M)                        (mins)                        (mins)

0                50                2.0                        0.26.24

                                                        0.29.15        0.28

                                                        0.28.78

5                45                1.8                        0.30.35

                                                        0.31.30        0.30

                                                        0.29.97

10                40                1.6                        0.35.54

                                                        0.38.57        0.36

                                                        0.34.44

15                35                1.4                        0.41.52

                                                        0.44.43        0.43

                                                        0.43.17

20                30                1.2                        0.45.65

                                                        0.43.43        0.45

                                                        0.45.74

25                25                1.0                        0.55.26

                                                        0.53.44        0.53

                                                        0.50.99

30                20                0.8                        1.04.04

                                                        1.03.32        1.04

                                                        1.04.10

35                15                0.6                        1.35.24

                                                        1.29.88

2.16.55        ← I realised this was anomalous and repeated it for accuracy

1.28.62        1.31 (not inc. anomalous)

40                10                0.4                        3.23.13

                                                        2.25.00        2.59

                                                        2.30.60

45                5                0.2                        6.07.16

                                                        6.09.05        6.09

                                                        6.10.07

The hundredths of a second are not really necessary when working at this level of accuracy. Also the anomalous result occurred at the start of the second practical lesson, which means new factors could have been brought in e.g. temperature. I will not include the anomalous result in my graph.

Conclusion:

The graph above proves my hypothesis. As the concentration gets higher, the time taken gets smaller. As the same thing is happening is happening each time, the rate must therefore be increasing. So as concentration increases, so does the rate of the reaction. We can see a curve on the graph and must conclude that under ideal situations it would be a perfect curve.

Evaluation:

I found an anomalous result as I did the experiment, noticing that it did not match the other two times for the same experiment. There could be several explanations, ranging from a mistake in the prep room leading to me having the wrong concentration of sodium thiosulfate, to using a different sized conical flask and looking through more liquid than in the earlier experiments. However these are unlikely explanations. What is most likely is that, it being a colder day, the temperature affected the rate by reducing the number of particle collisions with sufficient energy to react, thereby reducing the speed with which the solution reacted and turned cloudy and opaque. AS the room heated up from our body heat and the heat of several Bunsen burners, the rates returned to roughly the same as in the previous practical session.

Apart from the one badly wrong result, my times are fairly accurate, as evidenced by the smooth curve on my graph. If I had been particularly inaccurate the graph would not have been as obviously a curve.