I will change the concentration of the sodium thoisulpahe. I am going to do this to see what effect changing the concentration of the solution has on the rate of the reaction. I will use 10cm^3 of hydrochloric acid at…….. moles and 10cm^3 of sodium thiosulphate at varying concentrations, undiluted it is a 40g/dm^3. the first experiment will be conducted with 10cm^3 of sodium thiosulphate and 0cm^3 of water with 10cm^3 of hydrochloric acid. I will put the hydrochloric acid from the measuring cylinder in a clear glass beaker which is placed over a piece of paper that has a green dot on it 1cm in diameter. I will then add the sodium thiosulphate and watch the green dot until I can no longer see it.
I have undertaken a number of precautionary steps to ensure that the experimental conditions remain such that it will always be a fair test. After I have conducted one test I will thoroughly wash and dry the apparatus this will ensure that no residue of previous solutions is left to contaminate my experiment. I will use the same experimental apparatus for all the experiments, this is because one of the pieces of equipment may have an error on it, but as long as the error is maintained it will not effect my conclusion made from my results. I will stand a distance of 30cm away from the spot to minimise human error.
I am now going to do a preliminary experiment to find the data range and see if I need to make any alterations to my apparatus.
Preliminary experiment
For this test experiment I used a concentration of 10cm^3 of sodium thiosulphate and 0cm^3 of water, this makes the solution 40g/dm^3. I will then add 10cm^3 of hydrochloric acid. From this I can then predict what my results are going to be in the real experiment
Results
It took 59.3 seconds for the sodium thiosluphate to react with the hydrochloric acid so that I could not see the green dot anymore. The rate of the reaction was 0.01686
This experiment went well, I do not have any cause to drive me to change the equipment or alter the chemicals or their concentrations in any way. I can now move on to my real experiment. From this I can make a predication using the formula I have discovered from this preliminary experiment.
Mathematical prediction
I have discovered that the time for the reaction to happen and the concentration of the sodium thiosulphate are proportional to each other. This means that the rule for the time take for the reaction to happen will be:
Time for the reaction for happen at 100% / the amount of sodium thiosulphate in1cm^3 before it is added to the hydrochloric acid.
E.g. I am going to predict the time taken for the reaction to happen when I use 8cm^3 of sodium thiosulphate and 2cm^3 of water.
59.3 / 0.8 = 74.125
I have also discovered that the amount of sodium thiosluphate added is the concentration per cm^3 times 25.
Prediction for time when concentration of the sodium thiosulphate is 0.024g/cm^3:
59.3 / (0.024 * 25) = 98.3
I also predict that the rate of the reaction will be proportional to the amount of sodium thiosulphate. I predict that the rate of the reaction = the rate of the reaction when the concentration is a 100% / (1 / the amount of sodium thiosulphate per 1cm^3 before it is added to the hydrochloric acid)
E.g. Rate of reaction when using 8cm^3 of sodium thiosulphate with 2cm^3 of water
0.016863 / (1 / 0.8) = 0.01349
Prediction for rate of reaction when concentration of the solution is at 0.024g/cm^3
0.015863 / (1 / (0.024 * 25)) = 0.01012
I will now conduct the experiments to test my predictions. I am expecting there to be some small experimental and human error, but this has been kept to a minimum by the precautionary steps.
Experiment
Results
Graph 1
Graph 2
Analysis
I predicted that:
Time taken for the reaction to happen will be =
Time for the reaction for happen at 100% / the amount of sodium thiosulphate in 1cm^3 before it is added to the hydrochloric acid.
And
Rate of the reaction =
the rate of the reaction when the concentration is a 100% / (1 / the amount of sodium thiosulphate in 1cm^3 before it is added to the hydrochloric acid)
From my results I can make the conclusion that my formulae were correct. This also confirms my prediction that concentration is proportional to the rate of the reaction. My predictions were very close to the actual figures obtained by testing.
Evaluation
Taking into consideration experimental and human error I did not have an anomalous result. If I did they could have been caused by a number of things.
- Could have been caused by apparatus not being cleaned properly and leaving water or acid in the measuring cylinders, this would have caused the solution to be contaminated.
- I could have moved my head closer or further away from the experiment, causing the time I could see it to be reduced or increased.
- I could have measured the amount of one of chemicals incorrectly causing the concentrations to be wrong.
I could improve my experiment by using a set up the further reduces experimental and human error. I could do the experiment using a marble chip and hydrochloric acid at varying concentrations. This would improve the reliability of the experiment because the results would rest more on experimental data than human judgement.
