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I chose to use hydrochloric acid at 100% for the main experiment. Main Experiment For the main experiment I chose to change the concentration of the Sodium Thiosulphate. I will be testing the concentrations 100%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20% and 10% and recording the results. I will then plot two graphs, a time graph, and a rate of reaction graph. Results: HCL (cm) Na2S2O3 (cm) Water (cm) First Experiment (s) Final Experiment (s) Increase in Time.
On the next page I have drawn a graph to show the time taken for the reaction. By looking at my graph I can see that some points are slightly off of the line of best fit. To make sure that my line of best fit wasn’t wrong, I decided to plot a reaction graph. To plot the reaction graph I had to work out 1/time for each result and then plot it on a graph. Instead of the time on the y axis of the graph, I had to do a scale for my 1/time and so the numbers were not too small I had to time the 1/time by 100, so on the axis I had to label it, 1/time x 10. The reaction graph is on the next page.
By looking at the reaction graph I could see that the results for the sodium thiosulphate at 70%, 50%, 30% and 20% were quite away from the line of best fit. I retested these and worked out the 1/time for them, I then plotted them in blue on the reaction graph and found they fitted much better. These are the retest results: HCL (cm) Na2S2O3 (cm) Water (cm) Time Taken (Seconds) 5 35 15 60 5 25 25 80 5 15 35 167 5 10 40 251 Some results were significantly different. The original results may have not been as accurate as there may have been contamination, inaccurate measurements or different lighting making the image seem to disappear quicker.
Analysis By looking at my results I can see that my prediction was correct: The more concentrated the Sodium Thiosulphate, the faster it will react. This is because there are more molecules of Sodium Thiosulphate, so they are more likely to collide making the reaction faster. When the concentration is lower, the reaction takes place slower. The following diagram, used in my prediction, seems to be correct. In the low concentration, 50% concentration on the Sodium Thiosulphate, you can see that although there are the same amount of molecules, there is water which the Hydrochloric acid will also ‘bump’ into.
This means that the Hydrochloric acid is less likely to ‘bump’ the Sodium Thiosulphate molecules and thus, make the reaction slower. I have also found out that the higher the temperature, the faster the reaction takes place, this proves my theory correct in which the higher the temperature, the faster the reaction. However, this does not prove that the particles are moving faster as I would need an electron microscope to see if this was correct. However, when the concentration of the Sodium Thiosulphate and Hydrochloric acid is changed it also has an effect on the reaction rate.
This is because there are more molecules of the Sodium Thiosulphate and Hydrochloric acid and thus, they ‘bump’ into each other more often. Evaluation Overall this reaction was successful, but I think it could have been made more fair. I carried out the experiments over several days, and this may have made it unfair as the temperature and weather was different. I could have had more accuracy by carrying out the experiments in a temperature controlled environment. I also feel that the reaction was unfair due to inaccurate measurement, all measurements relied on the naked eye and may have not been the measurement we wanted, but appeared to be.
I think that a way to make them fairer would be to use scales and weigh the chemicals, or use a pipette. Both of these methods are more accurate, but are much slower. In the main experiment I found some odd results, but retested them and they ‘fitted in’. The original results may have not been as accurate as there may have been contamination, inaccurate measurements or different lighting making the image seem to disappear quicker. I also feel that the reaction is an unfair one to test as it relies on the naked eye to tell when the image has been obscured completely.
This can be difficult as different lighting levels can make it easier to see and sometimes the human brain may fool you into thinking the image is still there. I could change my method to make it more fair I could use lasers instead of the eye to see when the image has ‘gone’. I would place lasers on each side of the beaker, when the laser beam path gets fully obscured by the build up of sulphur, the beams would get ‘cut’ and the computer will accurately record when this has happened.