The factors I will control are:
- Temperature - all my experiments are going to be done at room temperature.
- Volumes of the reactants – I will always use 10ml³ of hydrochloric acid and when I am changing the concentration of the sodium thiosulphate, it will always have a volume of 20ml³.
- Equipment used – I will use all the same equipment i.e. stopwatch in order to keep this experiment as fair as possible and I will do this by just not changing the equipment.
The pieces of equipment I will use are:
- Conical flask - this is to put the reactants in for them to react.
- Stopwatch – to time how long it takes for the reactants to react.
- Five test tubes – to put the 5 different measurements of sodium thiosulphate in.
- Test tube rack – to hold the test tubes.
- Three measuring cylinders – one to measure the sodium thiosulphate, one to measure the water and one to measure the hydrochloric acid.
- A piece of paper/card with a thick black X on it – so we know when the reactants have finished reacting.
Diagram
Method
First we collected our equipment and set it up as above. We measured out all our amounts of sodium thiosulphate and put each one in a different test tube. We also measured 10ml³ of hydrochloric acid. We took the test tube which had the sodium thiosulphate with 20ml³ in. e poured this into the conical flask which was on top of the black X. We then poured in the hydrochloric acid and started to time straight away. We watched the black X and as soon as we could no longer see it we stopped timing. We recorded this time. We poured the solution back into the test tube and then placed the test tube back in the test tube rack. We also rinse out the conical flask. We then took the next measurement of sodium thiosulphate (16ml³) and poured this into the conical flask which had now been placed back on the X. We then measured out another 10ml³ of hydrochloric acid but this time we measured 4ml³ of water as well. We put the hydrochloric acid and water into the conical flask and then started to time again and stopped when we could no longer see the X. We recorded this time. Again we poured the solution back into the test tube and rinsed out the conical flask. We did this again but using the different amounts of concentration (as seen in the factors I will vary).
We repeated the whole experiment another two times so we could take an average and make the experiment as fair as possible, also to see if we got any erroneous data.
My results will not be totally precise for reasons like when we rinse the conical flask out, some water may be left in it and this could affect the concentration and give us some erroneous data and if the room temperature was to suddenly drop or increase this could have an effect on the rate of reaction as well.
How to work out my levels of concentration
The concentration for just 20ml³ of sodium thiosulphate was 0.3 so:
20/20 x 0.3 = 0.3
16/20 x 0.3 = 0.24
12/20 x 0.3 = 0.18
8/20 x 0.3 = 0.12
4/20 x 0.3 = 0.06
Obtaining
My Results
Analysis
My averages are pretty good, there doesn’t appear to be any erroneous data from looking at the results table either.
My graph shows that as concentration gets more dilute, the time it takes for the reaction to occur increases.
The trend in my data is that the more dilute the concentration gets, it takes even longer for the reaction to finish.
e.g.
0.3 = 25.69
0.24 = 32.85
0.18 = 44.49
0.12 = 81.98
0.06 = 132.39
My results back up my prediction. I predicted that as concentration decreased the rate of reaction would increase. Looking at my graph and results table, this is what happened.
Evaluation
My repeat experiments were pretty close, only usually one or two seconds out from each other. This would mean that my evidence was of good quality.
In the future I could do something like using 40ml³ of sodium thiosulphate instead of 20ml³ to see if the volume I used had anything to do with what results I got.