As you can see in this diagram there are less water particles to collide with the sodium thiosulphate so the reaction will be quicker.
Method
- Draw an ‘X’ in ink on a piece of paper and place it underneath a conical flask.
- Fill the conical flask with 50cm of sodium thiosulphate solution.
- Measure 50cm of hydrochloric acid and add it to the conical flask.
- Time how long it takes for the ‘X’ to disappear through the solution.
Every time we add the hydrochloric acid we have to change the concentration. The five concentrations we will be using are 0.5, 1.0, 1.5, 2.0 and 2.5.
Apparatus
The investigation looked like this:
Scientific Knowledge
As hydrochloric acid is added to sodium thiosulphate solution, sulphur is formed. The equation for this reaction is:
The reaction between hydrochloric acid and sodium thiosulphate is shown in the equation. The ‘X’ can be seen through the clear solution at the beginning of the experiment but not through the cloudy solution as the reaction takes place.
As the reaction takes place the solution becomes cloudy and will slowly cover the ‘X’ drawn on the piece of paper. The experiment is conducted at different concentration of hydrochloric acid, and by measuring the time it takes for the ‘X’ to disappear, the rate of reaction can be found.
Safety
It is very important that I made this investigation safe, if we did not make it safe there could be very dangerous consequences, I made this investigation safe by wearing goggles to protect my eyes from any accidental splashes of chemicals. Also when the hydrochloric acid and sodium thiosulphate mix together, they give of an awful smell which could cause you to feel unwell so we did not always stand right next to the experiment so we could have fresh air. We also made sure all our bags were under the desks so no one would be able to trip over and maybe cause a spillage.
Fair Testing
A ‘fair’ test is a test in which only one input is allowed to change. If you decide to change two inputs e.g. the concentration of hydrochloric acid and temperature of the solution, then there is no way that you would be able to tell which input changed the rate of reaction. The input that we have decided to change is the concentration of hydrochloric acid, we decided to choose this input because we thought it would have more affect on the rate of reaction, and also then we would not have to worry about setting up a Bunsen burner and taking other safety precautions.
Results
My prediction was correct, the higher the concentration the quicker the reaction will take place, this also proves that the collision theory is correct.
Conclusion
When looking at my results I can tell that the lower concentration the slower the rate of reaction would be. This happened because when the concentration of hydrochloric acid is higher there are more particles so when the sodium thiosulphate was added there were more successful collisions. When it was at low concentration there were less hydrochloric acid particles and more water particles, so when the sodium thiosulphate was added there was less successful collision because it was colliding with more water particles than in high concentration. The thiosulphate was still colliding with the water particles but they don’t react together. This is why it would have taken longer in the 0.5 concentration of hydrochloric acid, than the 2 concentration of hydrochloric acid.
Evaluation
The results in my investigation show that I was correct in my prediction, the higher the concentration of hydrochloric acid the quicker the reaction takes place, this also proves that the collision theory is correct, this is where if there is a higher concentration of hydrochloric acid then the sodium thiosulphate particles could collide with the hydrochloric acid particles quicker because there are less water particles, but with a weaker concentration there is more water so it would be harder for sodium thiosulphate particles to collide with the hydrochloric acid therefore make the reaction slower, there are no results that I have recorded which are totally out of place. I don’t think that I could have done anything to improve my results for my sodium thiosulphate investigation, if I could then it would probably have been to try and get my amount of hydrochloric acid and sodium thiosulphate as accurate as possible so my results are more accurate, but I think I did it well enough already as there are no anomalies.
