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OCR Coursework Investigation

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OCR GCSE Coursework Investigation An investigation into the effect of sodium thiosulphate concentration on rate of reaction Aim The aim of the investigation is to see how changing the concentration of sodium thiosulphate affects the rate of reaction. I will use the 'disappearing cross' method which shows us how long it takes for the mixture to become cloudy and cover the thick black cross that is placed underneath the beaker. When the black cross has completely disappeared and we can no longer see it through the cloudy mixture, we know that the reaction has finished. Scientific Knowledge I am carrying out this experiment to investigate how long it will take for a mixture of sodium thiosulphate, water and hydrochloric acid to react and turn cloudy, hiding the black cross (X) underneath the beaker. The mixture will turn cloudy because the particles of sodium thiosulphate, water and hydrochloric acid will chemically bond together. When the mixture has turned cloudy enough for us not to see the black cross, we know that the reaction has finished. In order for the particles to make a successful collision, each particle has to cross the activation barrier. The collision theory states that the particles have to collide with sufficient energy to make a successful collision and chemically bond together. ...read more.


When we did our experiment, we found we had one outlier - an anomalous result, which is a result that doesn't fit the pattern. See more about anomalous results in the evaluation. Evaluation I feel confident with my results. I had only one outlier in the first attempt for the 70% concentration, 35cm3 of sodium thiosulphate. I did not include this anomalous result in the average time because then it would make that one average result unreliable, which would then also make the graph of results unreliable. This outlier could have happened because there could have been a mistake with the amount of sodium thiosulphate, water or hydrochloric acid when I did the main investigation, but this could not be true because I did repeat the 70% concentration three times, all with similar answers. To get this outlier, it could have been because I did this experiment in two different rooms on two different days, and the heat and weather conditions could have changed which probably affected the experiment. Also, the timing may not have been correct because I were watching it with my own eyes. The stopwatch may have been started and stopped at the wrong time - even one second out each time could make the result wrong. ...read more.


The improved method would be: * Set up the correct apparatus. * Ensure that the apparatus is set up correctly, this time using a colorimeter. This will make sure the black cross cannot be seen. * Using the burettes, fill a measuring cylinder with the correct amount of hydrochloric acid. * Pour it into the conical flask. * Using the burettes, fill a second measuring cylinder with the first concentration of sodium thiosulphate and water. * Pour into the same conical flask and at exactly the same time, start the stopwatch. This is because it will start to react straight away. * Stop the stopwatch when the black cross has completely disappeared. * Repeat this concentration three times to make the results reliable. * When this concentration has been repeated three times, wash the apparatus and repeat the experiment again using the next concentration. * Make sure all concentrations are repeated three times. * Record all results in a table. I think that using a colorimeter and burettes will make the experiment more accurate and the results more reliable. If we plotted another graph after doing this improved experiment in the future, the range bars would be smaller because the results would be a little more accurate. ...read more.

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