Variables
Independent – I will change the concentration of the sodium thiosulphate by adding water in each case to make it more dilute.
Dependent – Time: I will measure the time taken for the cross to disappear, as it will show whether the rate of reaction has increased or decreased
Controlled – the same volume of hydrochloric acid, the same size of the cross. The same size conical flask the same person will be timing the stopwatch. And the same person will be judging how long it takes for the cross to become invisible
Predictions
I would predict to find that the more water that is added to the sodium thiosulphate solution would mean a lower concentration, which would mean a slower reaction. This means the higher the concentration of Sodium thiosulphate would mean the reaction would be faster. This is because the more concentrated a solution, the more particles are colliding. I would also predict that the rate of reaction has increased if the time has decreased as the reaction has occurred a lot quicker so therefore the time taken is a lot less.
Results table
I carried out the experiment as in my method and the results I got are shown above in the table. I took two results for time as to make sure my results were more accurate. I originally was going to take 3 results however my first two results were close enough so there was no point. To see the results more clearly I will place the results in a graph showing the relation between the concentration and the average time.
Conclusion
From the graph I can see that the concentration affects the speed of a reaction. I can see that as the rate increases the time decreases. When there was a higher concentration of sodium thiosulphate the reaction happened much quicker. I believe that the graph and my results agree with my original prediction, which was that, the higher the concentration the quicker the reaction. The reason it speeds up the reaction is because a more concentrated soulution of Sodium Thiosulphate has more particles in the same volume and therefore there will be more collisions with the acid particles. I also predicted that the rate of reaction would increase if the time had decreased as the reaction had occurred a lot quicker so therefore the time taken is a lot less.
Evaluation
Overall my results were good, my points were all good except one which seemed to be anomalous. However the reason for this anomalous point could be because the beaker wasn’t cleaned out properly. It could also be because any access water hasn’t been dried out of it thus meaning the reaction could have contained a lower concentration than it should have. To improve your results you should rinse out the substance left and then dry the conical flask out. In the future I could investigate how temperature affects the rate of reaction between sodium thiosulphate and hydrochloric acid. Another improvement could be that when doing the results that took less time, it would have been more accurate to have two people so one person could put the substances together while the other person started timing.
