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I will not use a catalyst in my experiment, as I feel that the experiment will be fast enough without one. Prediction I predict that if the concentration of thio is increased, the rate of reaction is also increased. This is because in dilute acid, there are not many acid particles. This means there is a more remote chance of an acid particle, hitting a sodium thiosulphate particle. In a more concentrated acid, there are more acid particles. There is now more chance of an acid particle colliding with a sodium thiosulphate particle, and so more chance of a successful collision.

As doubling the concentration will double the number of thio molecules per cubic centimetre, the rate of reaction is also likely to double. The graph is likely to look something like this: Apparatus Glass Beaker – This will be used for containing the reactants, a glass beaker will be used, as it is transparent and the cross can easily be seen through the base. Range of measuring cylinders, 10cm3, 25cm3, and 50cm3 – These must be used to measure out the required amounts of reactants. It is good to use different sizes, so that smaller ones can be used for smaller amounts out liquids, to ensure that the measurements are more reliable

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Sodium thiosulphate – One of the reactants used in the experiment Hydrochloric acid – The Other reactant used in the experiment Cross, marked clearly on a small piece of paper – The cross must be clearly marked, so that it is easy to see from the start of the experiment, and will not be obscured from view too quickly and easily. Safety Goggles – These must be worn to prevent to the eyes from any of the dangerous chemicals being used. Lab Coat – The substances being used can damage clothes, so a lab coat should be worn to protect them. Preliminary Results

I felt it was necessary to do some preliminary tests, to ensure that I had the correct range of results for the final experiment, and also to verify the reliability of my method. Concentration of Na2s203 (%) Time (sec) 100 25 80 28 60 37 40 59 20 105 In terms of procedure, I believe my method worked fine. I did, however notice that after a few results. The sulphur precipitate began to form in the inside of the beaker. In my main experiment, I will ensure to clean the beaker thoroughly after each result is given, to minimise the risk of this affecting the results adversely.

I also notice that there is a very large gap of 22 seconds between the 40% concentration, and the 60% concentration. The next highest increase is only 9. I will therefore add a 50% concentration to my range of results, so I can get a closer view on this large gap in results. I have chosen to use a range of 6 results, as I feel that the range used in my preliminary tests worked well, with the addition of the 50% concentration. The results showed a clear trend, and this is a good indication that I have a wide enough range of results.

6 results would also be fine to add onto a graph. Precision and Reliability I will repeat my experiment twice and compare all the results obtained. If each set of results shows the same trend, or are similar, I will assume that they are reliable. If I have any anomalous results, I will repeat that section of the experiment to see if I got the same again. I will measure the volume of the sodium thiosulphate and the hydrochloric acid carefully using a measuring cylinder in cmi?? using the markings on the cylinder, and measure the units of time in seconds.

It will not be necessary to take the time using any decimal places, as it would be almost impossible to measure this accurately using the stopwatch, and to see the exact moment that the cross was obscured from view. I will use different measuring cylinders for each the hydrochloric acid and the sodium thiosulphate, so that they aren’t mixed and cannot react outside of the experiment. I will also try and use smaller measuring cylinders for the smaller values of liquid, in order to increase the accuracy, as it would be easier to measure using a more specialised cylinder for that particular amount.

Results Concentration of Na2s203 (%) Time 1 (secs) Time 2 (secs) Average time (secs) Rate  Conclusion It is clear to me that by looking at my results, and the graphs I have drawn, changing the concentration of sodium thiosulphate affects the rate of reaction. As the concentration increases, so does the rate of reaction. This is directly proportional, as the graph displays a straight line. I can explain my results as this:

In dilute acid, there are not many acid particles. This means there is a more remote chance of an acid particle, hitting a sodium thiosulphate particle. In a more concentrated acid, there are more acid particles. This means there was a greater chance of an acid particle colliding with a sodium thiosulphate particle, and so more chance of a successful collision. As doubling the concentration will double the number of thio molecules per cubic centimetre, the rate of reaction is also doubled. Evaluation I feel that my experiment was fairly successful, and relatively easy to perform.

The graphs do show an accurate trend and they are all fairly close to the line of best fit. This implies that they were as a whole, accurate. This also shows that there are no anomalous results. Although the results were fairly accurate, the accuracy of the experiment could have been improved. It was difficult to see clearly exactly when the cross was obscured from view. The method could be modified so that a light sensor is placed through the beaker, and when the light becomes broken, a beep is sounded and the stopwatch can be stopped.

It was also difficult to measure exactly the right amounts of liquids. Syringes could be used for smaller amounts, or an electronic measuring machine. I was not able to complete the entire experiment on the same day, so the room temperatures would have differed slightly. However, this did not affect my results. I am able to tell this, as if the temperature had increased on the second day, there would be a slight jump in the rate of reaction, and this would be noticeable on the graph. Doing the entire experiment on the same day though, would have improved the reliability slightly.

I noticed that as the experiment went on, some of the precipitate remained on the sides of the beaker. I’m sure that this hampered the visibility of the cross and no doubt affected the outcome. If I were to do this experiment again I would be sure to thoroughly wash out the beaker after each test. I believe that the evidence I was able to collect is sufficiently good and wide-ranging to support a firm conclusion. I managed to collect six different sets of results, and with this I was able to create a good graph.

This number I feel was enough, as the line of best fit on my graph was a straight one and showed no sign of altering. Collecting results using concentrations in between the ones I have used would have been difficult to measure, and the variations in time would have been too short, and of no real significance. Show preview only The above preview is unformatted text This student written piece of work is one of many that can be found in our GCSE Patterns of Behaviour section.

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