Add the acid to the flask and start the stopwatch as you do this. Swirl the flask to mix the solutions and place it on a piece of paper marked with a cross.
Look down at the cross from above. When the cross disappears, stop the clock immediately and record the time.
Repeat these steps using different concentrations of sodium thiosulphate solution. Make up 50cm³of each solution by mixing different volumes of the original sodium thiosulphate solution with water.
Equation for the experiment:
Na2S2O3 (aq) + 2HCl (aq) >2NaCl (aq) + S (s) + H2SO3 (aq)
Prediction:
I believe that the higher the concentration of sodium thiosulphate there is, the higher the rate of reaction will be. This is because if there are a higher number of particles of sodium thiosulphate then it is more likely that there will be collisions with the hydrochloric acid particles. Therefore, as there are a higher number of particles colliding then there are a higher chance of a reaction.
Results:
Modifications:
A modification that could be made to improve the accuracy of the experiment is to use clean, sterile burette in order to get and accurate measure of volume and to make sure that there are no traces of other substances that may compromise the experiment.
Also, each of the readings that were taken could be done more than once, preferably three times to ensure that the results were accurate and to identify any anomalies.
Thirdly, a clean conical flask could also have been used to make sure that there were no other substances that could have affected the investigation. However as this was not possible, the conical flask was rinsed and then dried each time it was used to stop contamination.
My partner and I agreed that the same person would swirl the flask and decide when the cross-disappeared, while the other timed how long the cross took to disappear. This way there was no difference in opinion on whether the cross had disappeared or not.
Another modification was that we decided that it was not worth doing the experiment with 5cm³ of sodium thiosuplhate as it would take a long time and it is not a reproducible result. However the readings interval was 5cm³, not 10 cm³ during the preliminary investigation.
Finally, a potential problem was the temperature, as it cannot be controlled; however we monitored the temperature instead to see if any problems arose, such as overheating.
