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Investigating How the Concentration of Sodium Thiosulphate Affects the Rate of Reaction, Between Sodium Thiosulphate and Hydrochloric Acid

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Investigating How the Concentration of Sodium Thiosulphate Affects the Rate of Reaction, Between Sodium Thiosulphate and Hydrochloric Acid Background We had been experimenting with sodium thiosulphate and hydrochloric acid. We found that when we added hydrochloric acid to sodium thiosulphate it went cloudy, and eventually opaque. We decided to investigate how the concentration of the sodium thiosulphate would affect the rate of reaction. The experiment will be carried out as shown in 'fig 1', hydrochloric acid will be added to the sodium thiosulphate, and we will time how long it will take for the 'X' to be obscured. Fig 1 Planning Safety We must follow certain safety guidelines to avoid injury to ourselves or others; these safety measures are listed below: * Goggles must be worn, as hydrochloric acid is caustic and could cause damage to eyes. * Make sure all bags, stools etc are out of the way, so as not to be tripped over. * Remove blazer and tuck in ties or any other loose ends * All spillages must be reported to the teacher immediately. Variables Independent Variable The independent variable is the concentration of the sodium thiosulphate. We will vary the concentration of sodium thiosulphate from 80g/dm3 to 5g/dm3. We hope to see a trend of change in the reaction time as the concentration is changed. Dependant Variable The dependant variable is the time taken for the 'X' to be obscured; every time the 'X' is obscured, the same amount of sulphur is released. ...read more.


As these will not be altered in any way throughout the experiment, this ensures a fair test every time we perform the experiment for different concentrations of Sodium Thiosulphate. Also, I will take two readings for each concentration of Sodium Thiosulphate, and will then take the average of each. This will ensure accurate results. Results Range of Readings We performed a preliminary experiment, in order to decide upon a realistic range of results. On below (tab 1) is a table showing the range of readings we will use during the experiments. Tab 1 Concentration of Na2S2O3 Volume of Na2S2O3 Volume of H2O Volume of HCl (g/dm3) (cm3) (cm3) (cm3) 80 50.000 0.000 10 40 25.000 25.000 10 20 12.500 37.500 10 10 6.250 43.750 10 5 3.125 46.875 10 Repeat Readings As I have already mentioned, to ensure accurate results, I will take two separate readings for each concentration of Sodium Thiosulphate, and then, I will find the mean. If the two readings vary by a substantial amount of time, I will take a third reading and then drop the most inaccurate result, and calculate the average of the other two. Processing Results Below, is a blank table tab. 2, which I will record my results on? Tab 2 Concentration of Sodium Thiosulphate (g/dm3) Time 1 (s) Time 2 (s) 80 40 20 10 5 Tab 3 Concentration of Sodium Thiosulphate (g/dm3) Average Time (s) 1/ Time 80 40 20 10 5 Tab 3 will contain my processed results. ...read more.


All the other results are reasonably close to the trendline. I have however deduced that graph 2 is very accurate, as the trendline touches the x-axis very close to the origin. We can use this distance from the origin to measure accuracy, because the reciprocal of infinite has to be 0; therefore the closer the trendline is to the origin when it cuts the x-axis, the more accurate the graph is. Future Improvements Experimental Procedure During the experiment, we found it extremely difficult to measure the small volumes of the liquids. This meant that the measurements were not entirely accurate, which meant that the final results were not flawless either. To improve the experiment, we could have used, for example a syringe, or other more sophisticated apparatus to measure the tiny volumes. Another improvement, which could be made, is the use of opto-electronic systems to measure when the solution becomes opaque, for example, an optoswitch could be used. As the switch would never change, then the stage it detects that the water is opaque will be the same for all of the experiments, eliminating human error. Execution of Experimental Procedure Although the equipment we had to work with was somewhat basic, the experiment was successful, giving the expected outcomes, and reasonable correlation on the graphs. We always made up much more of the solution than required, so that we did not have to measure out tiny quantities of water and sodium thiosulphate, this improved the reliability of the experiment, although the very small quantities were still difficult to measure. I expected to see a much less accurate graph, and I was very pleased that there was only one anomaly. ...read more.

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