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Sodium Thiosulphate Reaction.

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PLAN: The aim of this investigation is to observe how varying the concentration of a solution affects its rate of reaction. I will conduct the same experiment five times, but each time changing the amount of sodium thiosulphate and water that I use. The amount of hydrochloric acid that I use will remain constant throughout the experiments. The amounts of acid, sodium thiosulphate and water that I will use are as follows: Sodium Thiosulphate Water Hydrochloric acid 50cm3 0cm3 10cm3 40cm3 10cm3 10cm3 30cm3 20cm3 10cm3 20cm3 30cm3 10cm3 10cm3 40cm3 10cm3 METHOD: For the first experiment I will measure out 50cm3 of sodium thiosulphate and put it into a beaker. During this experiment, I will use no water. I will then place this beaker on top of a piece of paper with a cross drawn in the centre of it. I will then add 10cm3 of hydrochloric acid to the beaker, and immediately start timing the reaction. When the cross can no longer be seen, the reaction is complete. I will repeat this four more times, each time increasing the amount of water used by 10cm3, and decreasing the amount of sodium thiosulphate used by 10cm3. ...read more.


When the particles size decreases, the particles have a larger surface area. This means that more of the solution can come into contact with the particles - hence, a faster reaction. (Scientific information obtained from Letts textbook and Encarta '98) PRELIMINARY WORK: I have conducted this investigation previously, but instead of changing the concentration to alter the reaction rate, I changed the temperature of the water that I used. My results were as follows: Temperature Reaction time (seconds) 200C 81 400C 40 600C 16 These results show that altering the temperature also changes the rate of reaction - as the temperature increases, the rate of reaction decreases. This shows that other things apart from the concentration can be used to change the time taken for a reaction to happen. OBTAINING EVIDENCE: Whilst conducting these experiments, I timed the rate of reaction each time, until the cross could no longer be seen. To make this fair, the same person each time judged when the cross could no longer be seen. These are my results: Sodium Thiosulphate Water Hydrochloric acid Rate of reaction (seconds) 50cm3 0cm3 10cm3 21.06 40cm3 10cm3 10cm3 24.00 30cm3 20cm3 10cm3 29.86 20cm3 30cm3 10cm3 43.20 10cm3 40cm3 10cm3 118.59 After conducting my experiments and recording the results, I took results from two other groups, Sophie's group and Laura's group. ...read more.


For example, the beaker may not have been washed out thoroughly from the previous experiment, or a different person may have judged when the cross could no longer be seen. Because not all the results are completely accurate, they are not sufficient enough to support a firm conclusion. IMPROVMENTS ON FURTHER WORK: I could improve on the accuracy of this investigation in a few different ways. For example, instead of decreasing the sodium thiosulphate by 10cm3 each time I could instead decrease it by 5cm3. This would give me a wider, and possibly more accurate range of results. I could also do this for the amount of water that I use. Although the experiment was handled fairly, it could have been even fairer. This is because different members of the class judged when the cross on the paper could no longer be seen, and as all the members of the class have different levels of eyesight, this is not exactly accurate. We judged that the reaction was complete when the cross on the pad could no longer be seen by the naked eye. This could have been made 100% accurate by using a device such as a light sensor. The light sensor would pass through the beaker, and when it could no longer reach the cross, the timing would be stopped automatically. This would make the experiment both accurate and exactly fair. ...read more.

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