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Investigating the rate of reaction between sodium thiosulphate and hydrochloric acid.

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Brigitte York Investigating the rate of reaction between sodium thiosulphate and hydrochloric acid. Aim To investigate the rate of the reaction when a fixed amount of hydrochloric acid is added to varying concentrations of sodium thiosulphate. The reaction that will be used is: Sodium Thiosulphate + Hydrochloric Acid --> Sodium Chloride + Water + Sulphur Dioxide + Sulphur Na2S2O3 (aq) + 2HCl (aq) --> 2NaCl (aq) + H2O (l) + SO2 (g) + S (s) When the two substances, hydrochloric acid and soduim thiosulphate, are mixed, a yellow preciptate of sulpur is produced. Throughout the reaction, the solution becomes increasingly opaque as more and more sulphur is produced. To measure when the reaction has reached a certain set stage, a sticker with a large visible cross drawn on it shall be placed on the side of the beaker, as the reaction occurs the cross with become increasingly obscure when viewed from the opposite side of the cross, and will eventually end up becoming completely hidden. This is when I will stop the stop clock. Obviously, it would be impossible to tell when the reaction has completely finished as the remaining reaction would be invisible, so the reaction cannot be timed exactly, but only to a certain set stage. I aim to conduct a fair and safe investigation to determine what affects the rate of the reaction. The rate of reaction is the rate of loss of a reactant or the rate of formation of a product during a chemical reaction. ...read more.


All apparatus will be washed in cold water to ensure that they are not subjected to additional heat throughout the experiment. Method The following appartus will be used in my investigation: * A 10cm� measuring cylinder which measures to 0.1cm�. * A conical flask. * A laminated cross. * A small beaker. * 6 test tubes of appropiate size. * A stop clock. * A thermometer. * 2 bulbpepettes, which both measure to �0.1cm�. * thermometer. * Hydrochloric acid of original concentration 1mol/dm�. * Sodium thiosulphate of original concentration 20g/dm�. * 1 pair of goggles. I plan to use the following method: 1) I will firstly measure out 10cm�/0.1M sodium thiosulphate using the burette and a funnel into a conical flask. 2) I will then add 50cm� of water to the sodium thiosulphate from a second bulbpepette to vary the concentrations. This amount of water will decrease by 10cm� for each following experiment. For the final experiment water shall not be added. 3) Next I will measure out 10cm� of hydrochloric acid into a small measuring cylinder. 4) Then I will tape the diagram of the laminated cross to the back of each test tube containing the sodium thiosulphate. 5) My partner will then pour in the hydrochloric acid whilst I activate the stop clock. 6) I will watch the solution to see when the cross is no longer visible, and deactivate the stop clock. ...read more.


Another improvement in accuracy would be a more effective way of judging when the cross is no longer visible. This could be done with the use of light sensors and a data logger attached to a computer. This would give more accurate results than the human eye it would replace. It would act not only as an eye, but also a stop clock, measuring until there is zero light intensity getting through the solution. Although I feel enough results were found to prove my prediction, further sets of results such as 15,25,35% concentration, could have been gained to ensure my prediction was indeed correct if time had allowed it. Anomalous Results Throughout my investigation I only encountered one anomalous result, in the 70% concentration experiment. The possible cause for this result is that the person measuring out the sodium thiosulphate and water failed to mark the bulbpepette so as to distinguish what they contained. Subsequently, I believe it was at this point when she mixed the bulbpepette up and had to rinse them out before starting again. I think that this anomaly is the result of this mix up, as maybe some residue remained in the bulbpepette, even after rinsing. Consquently this experiment was repeated, as stated in the plan, and a more credible result was gained. Stability of conclusion. As previously mentioned, with added time, more readings would have been taken to form a more accurate curve on the graph. However, I feel enough information was gained from each experiment to prove my the previously mentioned theory to be correct. ...read more.

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