This hypothesis is related to the collision theory which states that for a reaction to be successful “the particles (atom/molecule) in the reaction must collide into each other with the correct amount of energy, triggering a change, or reaction.”
Method:
Equipment
2 Measuring cylinders
Conical flask
Beaker
Stopwatch
Paper with cross marked on it
40g/1000cm³ Sodium Thiosulphate solution
Hydrochloric Acid
Water
Pipette
I am going to measure how changing the concentration of sodium thiosulphate affects the reaction time between it and hydrochloric acid. Each time I will add 5cm³ of 2M hydrochloric acid to the 50cm³ of the stated concentration of sodium thiosulphate. To vary the concentration of the sodium thiosulphate I will use the following formula:
New diluted concentration = volume of thiosulphate used x 40
50
By varying the amount of water in the 50 cm³ of sodium thiosulphate solution I will vary the concentration. I will first put the hydrochloric acid in a conical flask with a cross on the bottom. Then I will add the sodium thiosulphate and will start the stopwatch immediately. As soon as the cross disappears I will stop the timer.
In order to get a wide range of results I will use a variety of concentrations. The concentrations I will use are:
40g/1000cm³ = 50 cm³ thiosulphate + 0 cm³ water
30g/1000cm³ = 37.5 cm³ thiosulphate + 12.5 cm³ water
20g/1000cm³ = 25 cm³ thiosulphate + 25 cm³ water
15g/1000cm³ = 18.75 cm³ thiosulphate + 31.25 cm³ water
10g/1000cm³ = 12.5 cm³ thiosulphate + 37.5 cm³ water
5g/1000cm³ = 6.25 cm³ thiosulphate + 43.75 cm³ water
I will use other concentrations if I do not feel that my results are accurate and to help me define and 'see' anomalies.
Safety
To conduct my experiment safely I will follow normal laboratory rules, which include:
> The wearing of safety goggles to protect my eyes from chemical splashes.
> Standing up to conduct the experiment, therefore reducing the risk of tripping and spilling chemicals.
> Taking care when handling chemicals, particularly Hydrochloric acid and Sodium Thiosulphate because they are irritants. I will not touch my eyes or mouth until I have thoroughly washed my hands.
Variable Control
To make this experiment a fair test I will only vary one thing - the concentration of the Sodium Thiosulphate solution. I will conduct all the tests at room temperature, as said which will be considered constant even though there may be a slight effec on the results. The measures of Hydrochloric acid will all be the same (5 cm³). The person timing the experiment will look for the disappearance of the cross, otherwise there would be a time lapse between seeing the cross disappear and telling the other person to stop the clock and then eventually stopping the clock.
Results:
The results were taken to the nearest second due to the fact that the timing was being done by human recognition, so can never be definite.
When the results are first looked at there are no anomolies that are blatant, shown by the graphs.
Conclusion
I conclude that the more concentrated a reactant is, the quicker the rate of reaction time will be.
I have come to this conclusion because of several reasons. Firstly, my results give relatively conclusive evidence that as the amount of Sodium Thiosulphate decreases, and the amount of water in the solution increases there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.
Relating back to my hypothesis of “the greater the concentration of Sodium Thiosulphate in the solution the faster the chemical reaction will take place” it seems to have been proved correct, through the evidence that I have obtained.
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. That is the major reliability issue in this experiment. A better standard of measuring cylinders and pipettes could also have been used, because although the markings may be accurate the human decision of saying when a marking has been reached is never 100% accurate.
I feel that because my results have a certain amount of inconsistency between them it would be wise to repeat them again, even 4 times if I had sufficient resources.
My results are good in their accuracy and the points on the graph were plotted as accurately as possible, by computer in one case. Although, I did get some apparantly anomolous results but that is to be expected, because there are other variables involved in the experiment. I tested my results twice to try to ensure that I had not made any mistakes, but even that is not a substitue for the experiment being repeated many times.
The problem comes when using the overall average that any anomalous results that are included to produce this average make it less accurate and so less dependable. Increasing the chance of error and the need to do the experiment again.
The results seem reliable but as I have said there is a list of variables that have not been totally taken in to account. 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.
A potential answer to some of the problems would be to keep the solutions in a carefully controlled environment, and to use colormetric analysis to see when the solution has reached a certain colour therefore making the test fair.
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.
