Apparatus
- Conical flask
- Sodium thiosulphate (different concentrations)
- Hydrochloric acid (same concentration each time)
- Water (different concentrations)
- Stop watch
- Paper with black cross on it
- Small meniscus'
- A large meniscus
Method
Firstly, we collected the apparatus that is listed above. Then, we put 10 cm3 of hydrochloric acid in a conical flask (measuring the HCL with a small meniscus), and then put the conical flask on the black cross on the paper. We then added 50 cm3 of sodium thiosulphate to the hydrochloric acid, and at this time we do not need to add any water. Immediately, we started the stopwatch.
We then waited for the black cross to be completely obscured by the mixture. When this happened we stopped the stopwatch. By this time the mixture was completely yellow. Then we recorded what the time was.
We then emptied out the contents of the conical flask, and cleaned it out, and dried it as best we could. Next, we added 45 cm3 of sodium thiosulphate, we kept the HCL acid constantly at 10 cm3. Now, because we have to keep the volume the same for it to be a fair test, we added 5 cm3 of water. (We had to add the sodium thiosulphate and the water at the same time for this to be a fair test also). We then recorded the time for the cross to become completely obscured.
We repeated this many times, adding different amounts of sodium thiosulphate and water each time, and recording the time taken each time.
Diagram
Fair test
To make this experiment a fair test, we need to make sure we do a number of things.
In this experiment we are trying to find the rate of reaction using concentration as a factor, so there are a number of things we need to make sure we do to keep it a fair test.
Firstly, we need to keep a chemical at a constant concentration. So, in this experiment we have chosen to keep hydrochloric acid at a constant concentration. We could have, however, used sodium thiosulphate as a constant, but we had chosen to use hydrochloric acid.
Next we must make sure that the solution is kept at a constant volume throughout the experiment. If the volume is different, then it could give different results to if it was at a constant volume.
We must also make sure that we add both the water and the sodium thiosulphate at exactly the same time (into the conical flask with the hydrochloric acid in it), or it could effect the results of the experiment.
We must start the stop watch at the exact time as we put the water and the sodium thiosulphate into the conical flask. To do this it is a lot easier if there are two people doing the experiment, so one person can put the two substances in the HCL, and one person can start the stop watch.
Another thing we must do is to make sure that the conical flask is completely clean and free of any water or any other substance before we attempt to start the next experiment.
Safety
There are a lot of safety issues we must abide by in this experiment also.
We must remember that the substances which we use in this experiment can be very harmful if used the wrong way.
When we do this experiment, it may be necessary to wear safety goggles, as things are very unpredictable, and even though it is very unlikely that the solution would come out of the conical flask during the experiment, one must still be cautious of spills.
We must make sure that coats and bags are all out of the way while doing the experiment. Ties and hair should be tucked out of the way, so they do not make contact with any of the chemicals. It would also be preferable to wear a scientific apron, however this is not essential.
We should also try our best not to spill any chemicals, and we must not eat or drink in the lab while dealing with these harmful chemicals, as they can get on to our hands
Results
Table of results
1st try:-
2nd try:-
3rd try:-
I have also come up with a set of average results for all 3 experiments:-
Conclusion
In conclusion I have found that my prediction was correct. The rate of reaction was quicker when there was a higher concentration of sodium thiosulphate. This can easily be seen when we look at the results table.
This may have also have been the same if I had used the hydrochloric acid as a variable instead of the sodium thiosulphate. It may have been faster or slower at reacting than the sodium thiosulphate.
Evaluation
I think that most of the experiment I have done has been fine, with as little anomalous results as possible. However, there are still a few anomalous results which I will now point out.
This is the only anomalous result that I have recorded. The result may have turned out anomalous because of basic human error, or maybe because we measured the substances wrong. It may have even been because we did not clean the apparatus properly.
Apart from this, the accuracy of my experiment has been more or less accurate. Although there are a number of ways in which we could have made the results more reliable. For instance, we could have used better measuring equipment, because the apparatus we used was mainly basic equipment.
Another thing we could have done to bring more evidence is to have tried to use the hydrochloric acid as the variable substance, and used the sodium thiosulphate as the constant substance. This would have brought more evidence to support the idea that the higher concentration of a substance, the faster it will react.
I think that the evidence, which I have received, is enough to reach a suitable conclusion, but there are a few flaws to the experiment (which I have mentioned). Apart from them, the experiment is fine.
