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Investigating the rate of reaction between sodium thiosulphate and hydrochloric acid

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Introduction

Investigating the rate of reaction between sodium thiosulphate and hydrochloric acid. I am going to measure the rate of the reaction when hydrochloric acid is added to sodium thiosulphate. I am going to investigate what effect the concentration of the sodium thiosulphate has on the reaction rate. When sodium thiosulphate and hydrochloric acid are mixed, a yellow precipitate of sulphur is produced. The solution becomes increasingly difficult to see through as more and more sulphur is formed. This is how I plan to measure the rate of the reaction. I will place a laminated cross underneath the solution and will stop the stop clock when the cross can no longer be seen. I aim to conduct a fair and safe investigation to determine what affects the rate of the reaction. Variables. Possible independent variables are temperature, amount of hydrochloric acid, concentration of Hydrochloric acid, amount or concentration of sodium thiosulphate and whether or not a catalyst is added. My independent variable is going to be the concentration of sodium thiosulphate. My dependant variable will be the rate of the reaction, or time taken for the reaction to take place. Watching the cross disappear as precipitation occurs and sulphur is formed as a solid will be the method used to realise the rate. My controlled variables will be the temperature, the concentration of hydrochloric acid, the amount of sodium thiosulphate and the amount of hydrochloric acid. I will try and control these as far as possible, and be as accurate as I can when measuring out the chemicals. ...read more.

Middle

It was decided that 16 minutes was too long and the test was abandoned. Therefore, it was decided that 20cm� hydrochloric acid would be used in the actual experiment. This will hopefully make the experiment more time efficient. With this in mind, we will need to add another 10cm� measuring cylinder to our apparatus which has the same level of accuracy; i.e. �0.1cm�. Revised method. 1) I will firstly measure out 10cm� sodium thiosulphate using the burette and a funnel into a conical flask. 2) I will then add 40cm� water to the sodium thiosulphate from a second burette, which I filled using a small beaker and a funnel, making sure the tap was closed before proceeding. 3) Next I will measure out 20cm� of hydrochloric acid into a small measuring cylinder. 4) I will place the conical flask containing the sodium thiosulphate and water onto a laminated diagram of a cross. 5) My partner will then pour in the hydrochloric acid whilst I activate the stop clock. 6) My partner will watch the solution to see when the cross is no longer visible, and deactivate the stop clock. 7) When the reaction is complete, the results must be recorded in a table. 8) Whilst they are observing, I will prepare the other water and sodium thiosulphate solutions so as to make best use of the time available. I will then repeat the points 1 to 7 three times for each concentration of sodium thiosulphate to ensure accuracy, using the same amount of hydrochloric acid. Any anomalous results will be repeated. I will then take the average of each set of three results to plot on a graph. ...read more.

Conclusion

Each set of results was fairly similar, with the exception of the results taken at 30.0cm� of sodium thiosulphate, which is discussed in due course. More readings may have given more accurate and reliable results, but would not have been very time efficient. That was the one downside to the burette. It was very accurate, but it took up more time then, perhaps, a measuring cylinder would have done. Anomalous Results: - There is but one example of an anomalous reading within my results. This s circled in green on my graph, with a dotted purple line showing why it is an anomaly, i.e. that it does not fit on the curve. I think that the honest possible explanation for this result is that the person measuring out the sodium thiosulphate and water failed to mark the burettes so as to distinguish what they contained. Subsequently, I believe it was at this point when she mixed the burettes 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 burette, even after rinsing. If there had been sufficient time, this result would have most certainly have been taken again. The small red marks protruding from the points on my graph are attempted error bars. However, with the exception of the anomalous result, these were so absurdly small they are all but indiscernible. Stability of conclusion. Obviously, with more time, more readings would have been taken to form a more accurate curve on the graph. However, I believe that sufficient information was recorded to form a firm conclusion and to prove the aforementioned theory to be correct. ...read more.

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