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# Experiment to find the effect of change of concentration on the rate of reaction

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Introduction

George Maund Experiment to find the effect of change of concentration on the rate of reaction Introduction Na2S2O3 + 2HCl ---> 2NaCl + S + H20 + S02 I am going to be studying the rate of reaction between hydrochloric acid (HCl) and sodium thiosulphate (Na2S2O3). By varying the concentration of sodium thiosulphate (using water to dilute it) I will time how long it takes for the solution to turn opaque. As I mix the hydrochloric acid and the sodium thiosulphate, the solution will turn from a transparent liquid to a murky, yellow-coloured mixture. I will judge when the solution has gone opaque by looking at a cross on a piece of paper beneath the beaker containing the solution. Plan I predict that the rate of the reaction will be inversely proportional to the percent concentration of sodium thiosulphate in the solution. That is to say, as I decrease the concentration the time taken for the solution to turn murky will increase. This prediction can be explained using the particle collisions theory, which states that the more particles there are in a solution, the more likely they are to collide. If the number of sodium thiosulphate particles is increased then they are more likely to collide with the hydrochloric acid molecules, thus speeding up the reaction. If there is a more dilute solution of sodium thiosulphate, for instance the second reading I intend to take will be using 5cm� and 45cm� (10% water, 90% sodium thiosulphate) ...read more.

Middle

Looking down upon the flask and the solution in it, I will stop the watch when the solution turns opaque to the point when the cross X is no longer visible. I will record the time taken for the solution to turn murky, and repeat the reading twice, being sure to clean out the flask container before the next measurement. I will then repeat the experiment, being sure to rid the flask of any trace of solution before pouring any sodium thiosulphate or hydrochloric acid into the flask, only this time reducing the amount of sodium thiosulphate by 5cm� each time. Again I will take 3 measurements at each different reading of concentration of sodium thiosulphate. The experiment will end when I take the third reading of 30cm� sodium thiosulphate solution, 20cm� water. The safety goggles are the most important aspect of the experiment, as it is crucial to maintain a high standard of safety when working with corrosive chemicals. Diagram: Graph to show predicted appearance of results Method and Results I carried the experimental investigation as stated in the Plan section, under Experimental Procedure. Having gained satisfactory results, I have arranged them in a table. Table of Results cm� of Na2S3O2 Time taken (seconds) Mean results solution 1 2 3 (average) 50 31'9 29'8 30'1 30'6 45 33'0 32'3 32'5 32'6 40 34'0 36'9 35'6 35'5 35 40'9 40'0 39'8 40'2 30 47'2 50'6 49'3 49'0 To draw ...read more.

Conclusion

Every time it is myself that judges when the solution has turned sufficiently opaque and I stop the watch systematically. Because my reflexes are not perfect it may seem like one measurement was longer than another, when it was only my slow reactions that distorted the result slightly. The particle collisions theory may also be to blame because if the region of the solution directly above the cross X had a larger amount of collisions, just by chance, then the reaction time would be quicker than average. I would suggest that in further experiments, the variable of heat be included, and the pair who carries out the investigation work as a team and play different roles in the experiment. One of them should time the reaction as I did, and the other should keep the temperature as constant as possible. That way anomalous results will be almost entirely eradicated. The other two issues of human reflexes and of chance of areas of collision are unfortunately uncontrollable. Some kind of computer will surely be able to judge when a solution has turned opaque better than a human being. To test my conclusion, I would suggest doing a similar investigation, but with magnesium strips instead of sodium thiosulphate. It will be easier to judge reaction time because one would only have to stop the watch upon seeing the magnesium completely dissolve. Also using magnesium in an experiment is a lot simpler and cheaper than using sodium thiosulphate. ...read more.

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