I think this will be the simplest way of gathering the results I want. By choosing this method I fell I have minimized the difficulty of the experiment and so there should be fewer human errors (which are inevitable), and the results will be more accurate.
Prediction
I predict that as concentration decreases the time taken to react will increase. This will be shown by negative correlation on a line graph. I predict this because is solutions of reacting particles are made more concentrated there are more particles per unit volume. Collisions between reacting particles are more likely to occur, therefore speeding up the rate of reaction.
Method
I will add the correct amount of thiosulphate into a beaker and 5cm³ of hydrochloric acid into a conical flask, and add the correct amount of water for each experiment. Whilst wearing goggles, I will then pour the thiosulphate, hydrochloric acid and water into a conical flask, which will be placed on a piece of paper with a cross on it. I will immediately start the timer, and stop it when the cross is no longer visible. I will repeat this for each experiment doing each one 3 times for accuracy and take an average.
I will make this experiment safe by using goggles to protect my eyes and by making sure I have a clear workspace.
Safety
Caution must be taken in all areas of the experiment, especially when handling the hydrochloric acid. Goggles must be worn at all times and hair / loose articles should be tied back. Bags and other obstructions will be placed under desks so people can walk around safely during the experiment.
For this experiment I will need:
- Stopwatch
- Goggles
- Conical flask
- Beaker
- Measuring cylinder
- Paper marked with ‘X’
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Sodium thiosulphate - 50gdm3
- Hydrochloric acid (HCl) - 2M
Fair Test
In order for my findings to be valid the experiment must be a fair one.
Criteria for ensuring a fair test:
- Same standard each time for judging when the ‘X’ has disappeared. (Same person making the judgement)
- Measuring cylinders for the HCl and thiosulphate will not be mixed up.
- The amount of HCl
- The temperature in which the experiment is taking place must be kept the same. (Room temperature)
- We will add water to make the total volume constant.
All of these precautions will make my final results more reliable and keep anomalies at a minimum so thus making my entire investigation more successful.
Preliminary Experiment
I conducted a preliminary experiment, to help me decide the amounts of thiosulphate, water and hydrochloric acid I will use. I also used this as a practice run, so I could get used to the experiment.
Preliminary Results
After doing the preliminary experiment I am going to change two things; I will use a burette instead of a measuring cylinder, as it will be more accurate, and I have decided to not use as little as 5cm3 as it took far too long for the action to happen. Instead my smallest amount of thiosulphate will be 10cm3.
The preliminary results also tell me that generally, the more thiosulphate and less water, the quicker the rate of reaction. I can now use this piece of information to check if I go wrong in my main experiment.
The Data
The inversely proportional graph supports my prediction that as the concentration of the thiosulphate increases the time taken to react would decrease.
The Reasons Behind My Findings
My results show that the higher the concentration of sodium thiosulphate, the greater the rate of reaction. This is because there are more molecules in a given space which could potentially react and so the overall reaction would be sped up. There would be a greater number of collisions per second in a beaker which has highly concentrated solution of sodium thiosulphate than a beaker which has a more diluted solution. In a highly concentrated solution there are more particles per unit volume. Collisions between reacting particles are therefore more likely to occur.
Evaluation.
The success of the experiment:
The experiment went very well, there were no injuries and the results appear to be accurate.
It was easy to se the chemical reaction had occurred, and so evidence was easy to obtain.
The graphs show the relationships between concentration of solution and reaction time, and they were easy to plot and interpret.
My original prediction has been proven to be true, and I feel I have explained why I came to the conclusions I did.
The Quality Of My Evidence:
I have tried to minimize anomalous results by calculating averages in the section regarding time ( for each concentration, three measurements were taken) as this was the area which was most likely to contain anomalous results, probably due to human error.
Changes I Would Make:
If I had to repeat this experiment, I would alter the volume of thiosulphate used. I would use the volumes of thiosulphate in the ranges of 25cm 3 to 65cm3 rather than what I did, which was between 10 cm3 and 50cm3 . I would do this because between 10 cm3 and 25cm 3 of thiosulphate, it took quite a long time for the cross to disappear, and as it was disappearing slowly it was harder to determine when the cross was fully gone. I think by using more thiosulphate it would decrease the chance of anomalies. Although I would have to be careful with this, because if I use too much thiosulphate, the reaction would happen to quickly which would increase the chance of anomalies, so I would have to get the right balance.
If I had enough time I would like to further my findings by repeating the experiment again, except making the thiosulphate the independent variable, keeping it at 5cm3 and make the HCl the manipulated independent variable. I would use the same amounts of HCl as I did thiosulphate and check to see if the results were similar.
