So, I expect the rate of reaction to go up as the concentration goes up. If the concentration doubles, I would expect the rate of reaction to double and if the concentration is zero, I would expect the rate of reaction to be zero.
Equipment
The following is a list of equipment that will be needed to carry out this experiment:
- Different concentration of Sodium Thiosulphate
Fair Testing
I will keep the following things constant throughout my experiment to ensure that my experiment is fair:
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The volume of the reactants must be same throughout my experiment. The volume that I will be using in my experiment will be 10cm3 from both reactants.
- Washing out the beaker with water before use and between measurements to stop the build up of solid sulphur on the inside of the beaker, which will make my experiment incorrect.
- Temperature and surface area should also be kept constant. If there is a higher temperature, then the time taken for the reaction to complete will be affected.
- The concentration of the Hydrochloric Acid should be kept constant. If the concentration is higher, then the speed of the reaction will increase.
- The piece of paper with the cross on it, to ensure that accurate readings can be taken.
- The same person will be taking the readings to make the results accurate.
In the reaction between Sodium Thiosulphate and dilute Hydrochloric Acid,
HCl + Sodium Thiosulphate Sodium Chloride + Sulphur Dioxide + Sulphur + Water
HCl(aq) + Na2S2O3(aq) NaCl(aq) + SO2(g) + S(s) + H2O(l)
Increasing the concentration of Sodium Thiosulphate in the experiment means that there will be less time before the cross can no longer be seen. This is because the sulphur that is being produced in the reaction will be produced even faster, due to the higher concentration of Sodium Thiosulphate.
When making different concentrations of Sodium Thiosulphate, I made sure to take care to use the same total volume (of Sodium Thiosulphate and Hydrochloric Acid) for the comparison to be correct. While the reaction is taking place, I tried to make sure that reactants aren’t stirred, because this will cause the reaction to be sped up and therefore making the results incorrect.
I repeated the results of my experiment to make sure that they could be as accurate as possible. I did this three times for each concentration of Sodium Thiosulphate and looked for any anomalous results in my reading.
The readings I obtained from my preliminary work are shown in the table below.
The table shows the concentration of Sodium Thiosulphate and the time it takes for it react with Hydrochloric Acid. This table does not coincide with what I originally thought would happen to the reaction time. I expected the reaction time to decrease as the concentration of the Sodium Thiosulphate solution increased, but obviously, this isn’t the case, because the reaction time also increased. It only does this until the concentration of Na2S2O3 reaches 1.25m, then it starts the initial task of decreasing in time.
There could be a number of factors that made my results incorrect. One of which is using a stronger concentration before a weaker one and not washing out the flask properly. This causes a stronger concentration of Na2S2O3 in the beaker, which may speed up the reactions. Another factor may be that, when I washed the beaker, there may still have been traces of water inside, that could dilute the solution and make the reaction time increase.
Obtaining Evidence
To obtain the evidence needed to do this coursework, I first needed to do the experiment. Firstly, we collected all the necessary equipment needed to carry out this investigation. We then measured 10cm3 of 0.25m Sodium Thiosulphate, and 10cm3 of Hydrochloric Acid. We poured the Hydrochloric Acid in the beaker first, then before the Sodium Thiosulphate was poured in, then we checked to see if the stopwatch was working correctly. On the ‘Start’ command, the stopwatch was started at the same time as the Sodium Thiosulphate hit the Hydrochloric Acid, and stopped when the person looking at the cross said, “Stop!” After the time was taken for the concentration, the beaker was then carefully washed with distilled water, and the process continued with the other concentrations of Sodium Thiosulphate.
The measurement was taken within the closest second, to ensure that the results were accurate. This was done because if it was measured to one, or even two decimal places, the results I would have obtained would be incorrect because, we, as humans aren’t perfect and don’t know the exact time that the Sodium Thiosulphate hit the HCl, so the timing would be off.
These concentrations of Sodium Thiosulphate were used because it is a sensible concentration to use. The time taken for the reaction would not take too long, or too short for the time to be recorded.
The following is a table of the results I obtained after completing my experiment.
I am quite happy to say that there aren’t any obvious anomalous readings in my experiment, although, I had to stick to my ‘Fair Testing’ as closely as possible.
Analysis
In this experiment, I have found that concentrations of a chemical affect how fast that chemical react with another reactant. I have learned that, as the concentration of the chemical increase, the time taken for the reaction to complete decreases.
These graphs show the rate of reaction fro each of the Sodium Thiosulphate concentration.
From these graphs, I can conclude that the reaction time decreased as the concentration of the Sodium Thiosulphate solution increased. Which means, that if the concentration of the Sodium Thiosulphate solution can be increased to be very high, there will be a point where humans will not be able to accurately plot results from the reaction time.
I have come to this conclusion because of several reasons. Firstly, my results give pretty conclusive evidence that as the concentration of Sodium Thiosulphate increases, the time taken for the reaction to complete, decreases.
My results support the prediction I made because I said ‘the greater the concentration of Sodium Thiosulphate the faster the rate of reaction time.’ I believe I was correct and a secondary source states that ‘the reaction time will be faster with a more concentrated solution because, the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up’. – Secondary source Britannia Interactive Encyclopaedia 2002.
Evaluation
My experiment went according to plan but there were flaws in it. I think there is a human error factor involved when you are measuring liquids and looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a cross a light beam could be used and when the beam goes out that is the end point. A better standard of measuring cylinders and pipettes could be used.
My results seemed reliable but you always have to consider more. Because I am basing my interpretation of their reliability on a hypothesis and my own personal view it is hard to tell. I see them as reliable but if my views and hypothesis are wrong then the results are not reliable. I would need more time to research this further in order to make a firm decision.
I think that our original idea of using a computerised light sensitive device was a good idea. I feel that with more time I could develop a way to let me use this. The results that I could get from this (if it worked) would be more reliable and ‘strong’.
To extend this work one could look into the affects of catalysts (and other variables) on reaction times or you could try the same experiment with different substances or by varying the amount of acid instead.
