Method:
Apparatus list: Sodium Thiosulphate (Na2S2O3)
Hydrochloric Acid (HCl)
Water (H2O)
1 x Conical Flask
2 x Measuring Cylinder
White cardboard marked with black ‘X’
Stop clock
Goggles
For this experiment I will use 5 different concentrations of Hydrochloric Acid. They are as follows:
1. Set up apparatus as above.
2. Measure 25cm³ of Sodium Thiosulphate and pour into conical flask.
3 Measure Xcm³ of Hydrochloric Acid and Ycm³ of Water and mix together. Pour into Conical Flask.
4. Start timer. End timer when the ‘X’ cannot be seen through the solution.
5. Record results.
6. Repeat steps 2 – 5 only changing the amount of Hydrochloric acid and Water as listed above.
7. Repeat steps 2 – 6 to get a second set of results and make it a fair test.
8. Record the Average results.
Results:
GRAPHS
GRAPHS
Conclusion:
From the results of this experiment I can conclude that my prediction was partly correct; with increasing concentration the rate of reaction increases. This can be clearly seen in the results table and the graph, which both show that with increasing concentration the time taken for the solution to become opaque decreases.
However, according to these results, I was not correct in saying that the concentration and rate of reaction are proportional. If you look at concentrations 2 and 4 in the results table, 10cm³ and 20cm³, the time taken for concentration 4 to turn the solution opaque is not half the time taken for concentration 2. If my prediction was correct then it should be half the time.
The reason that the rate of reaction increases as the concentration increases is Kinetic theory. Kinetic theory states that as you increase the concentration the number of particles of that substance increases. A chemical reaction can only occur between particles when they collide with a certain amount of force (Activation energy). As there are more particles to collide in a higher concentration the chance of them colliding is increased and so the reaction takes place quicker. In this reaction one of the products is Sulphur. It is the Sulphur that turns the colourless substance cloudy. So as more particles collide in a higher concentration, the sulphur is formed quicker and the time taken for the solution to become opaque is less. (Rate of reaction increases)
Evaluation:
Although the experiment was a fair test it was not as accurate as it could have been because there were a couple of anomalous results in both results which are shown on the graphs. There could be a number of reasons why there are these anomalous results.
It could be that the conical flask was not washed out properly each time and there was Hydrochloric acid or Sodium Thiosulphate left inside. Or the same measuring cylinder was used for different things. i.e. it was used for Hydrochloric acid and then Sodium Thiosulphate which meant that, if it was not washed properly, there may have some left inside which changed the balance of the solution (too much HCl or Na2S2O3).
It could also be that the measurements of the reactants were not that accurate. If it was just 1cm³ out than it could have affected the results.
Another possibility is that my eyesight was not that great at times and I may have missed the ‘X’, when I should have seen it, and stopped the timer. Or if I just didn’t check to see the ‘X’ accurately.
I believe that if I were to do this experiment again with more accurate equipment (such as a light meter rather than a cardboard marked with a ‘X’ or more accurate measuring equipment), or if I spent more time being accurate, then the results would be different.
I think that they would all be closer to the line of best fit. I also think that the results would show that the rate of reaction and concentration are proportional like I originally predicted.
If I were to do this again then I would like to investigate the rate of formation of Sulphur in a similar way to the rate of reaction of this solution.
However, despite the imperfections of this experiment, I believe that my results were clear enough to support at least part of my conclusion, namely the relation between rate of reaction and concentration. It was not good enough though, to prove that the rate of reaction and concentration are proportional as I predicted.
