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Investigating the effects of concentration on the rate of a chemical reaction, hydrochloric acid + sodium thiosulphate

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Investigating the effects of concentration on the rate of a chemical reaction, hydrochloric acid + sodium thiosulphate Plan Introduction: I am going to investigate the effects of concentration on the rate of a reaction, hydrochloric acid + sodium thiosulphate. Chemical reactions normally fit into three categories, these are chemical reactions that take weeks or years to occur, chemical reactions that happens so fast that it can not be seen (spontaneous reactions), or, and there are very few in this category, reactions that take minutes or seconds to occur. The chemical reaction hydrochloric acid + sodium thiosulphate fits into the latter of these categories. It is because of this property that this will be the chemical reaction that will be observed in this investigation. When the two chemicals are added together there is no immediate reaction but over a course of a few seconds sulphur is slowly precipitated into the water and suspended in the water forming a slightly yellow cloud. As the reaction progresses this cloud becomes so thick that it is impossible to view through the water (the water becomes opaque). This property can be used to investigate the effects of concentration on the rate of the chemical reaction. If a cross, drawn on a piece of paper, is placed beneath the beaker, or conical flask, the time it takes for the cross to become obscured can be timed. ...read more.


and one sulphur molecule (which is precipitated into the water). Fair test This will be a fair test by all other variables, within my control, being kept constant. The temperature for this investigation will be room temperature, approximately 23?C. The pressure for this investigation will be 1 atmosphere. Measuring cylinders and conical flasks will be thoroughly washed to avoid contamination. The same conical flask will be used because of slight differences in the shape of different conical flasks, making the depth of the water/solution different and therefore affecting the thickness of the cloud and altering the time it takes the cross to become obscured. The same person will be watching for the point where the cross becomes obscured because of slightly different vision from person to person. The same piece of paper and cross will be used because of different intensities of black ink. The same bottle of hydrochloric acid will be used as the schools technicians have a tendency to produce hydrochloric acid with slightly different moralities with each bottle they produce. Prediction I predict that increasing the concentration of sodium thiosulphate will directly and proportionately increase the rate of the reaction. Because increasing the concentration means that there are more molecules to collide and course reactions (collision theory) in the same space. The range for this investigation is the concentration of sodium thiosulphate from 0.15 M to 0.03M. ...read more.


The gradient for the graph to 3 decimal places is: m=?Y/?X m=0.01-0.0035/0.03-0.0552 m=0.0065/0.0252 m=0.258 The results do support my prediction, but I would like to conduct the experiment several more times with a greater range for improved reliability. I would also like to refer to a secondary source to be completely sure. Evaluation The experiment in theory is sound, but in practise, it could offer a lot of errors. The reason is that the time for the cross to become obscured by the precipitated sulphur is down to human judgment and is not very trustworthy for accurate scientific investigations. So, the results do not have a very good reliability factor to start with. To improve this, light sensitive computer controlled sensors could be used where instead of the human eye judging if a cross is still visible or not, the cross is replaced with a light source and the person is replaced with a light sensor. The computer could then be programmed to have a timer and to stop it the moment the light drops below a certain intensity and to record this time on a results table. By doing this, human error is removed almost completely (we still got to hit the start button). I could not find any anomalous results but that could be an error on my behalf. Until I have done more experiments or I can compare with a secondary set of result, I cannot say with complete certainty that my evidence is 100% reliable. Keith Thompson 30/04/07 1 ...read more.

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