Prediction
I predict that the greater the concentration of Sodium Thiosulphate solution the faster the chemical reaction will take place. Therefore, the cross will disappear more quickly due to the cloudiness of the solution.
I think that the concentration of a solution affects the rate of reaction because the rate of reaction depends on how frequently the molecules of the reacting substances collide. A more concentrated substance has more molecules for a given volume than a more dilute substance. Because there are more molecules about, the frequency of successful collisions is greater, and the reactions happen faster.
The equation for the reaction is:
Sodium Thiosulphate + Hydrochloric Acid → Sodium Chloride + Water + Sulphur + Sulphur dioxide
Safety
To conduct my experiment safely I will follow normal lab rules, which include:
- The wearing of safety goggles to protect my eyes from chemical splashes.
- Standing up to conduct the experiment with stools and bags under the desk, therefore reducing the risk of tripping and spilling chemicals.
- Taking care when handling chemicals, particularly Hydrochloric acid and Sodium Thiosulphate because they are irritants. I will not touch my eyes or mouth until I have thoroughly washed my hands.
- Taking care when using glassware to prevent injury.
Plan
I am going to pour 50cm of Sodium Thiosulphate or Na S O into 10cm of Hydrochloric Acid in a conical flask. I will then test 40cm of Sodium Thiosulphate with 10cm of water, 30cm of Sodium Thiosulphate with 20cm of water and so on until there is only 10cm of Sodium Thiosulphate with 40cm of water, as shown in the table below. The flask will be placed on a piece of paper with a cross drawn on it. I will start the stopwatch when I add the acid to the Na S O. I will stop the stopwatch when the solution is cloudy enough to prevent me seeing the cross. I will repeat this procedure with the measurements in the table below. Then to gain an average I will repeat all the tests three times.
Results
I have recorded my results in tables and then worked out the averages afterwards to gain a more accurate result.
1st test
2nd test
3rd test
Average results
Conclusion
I conclude that the more concentrated a reactant is, the quicker the rate of reaction time will be.
I have come to this conclusion because of several reasons. Firstly, my results give conclusive evidence that as the amount of Sodium Thiosulphate decreases and the amount of water in the solution there are less atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. In a more concentrated solution, there are more atoms to collide so the reaction time is quicker.
My results support the prediction I made because I said ‘the greater the concentration of Sodium Thiosulphate the faster the rate of reaction time.’ I believe I was correct and a secondary source states that the reaction time will be faster with a more concentrated solution because,’ the more molecules there are, the frequency of successful collisions is greater and therefore the reaction rate is speeded up.
Evaluation
My experiment went according to plan but there were flaws in it. For example I could have done more tests to gain a more reliable average.
I think there is also a human error factor involved when you are measuring liquids and looking for an end point in the reaction. Although the reaction I chose had a fairly definite end point it was still hard to tell whether the whole cross had disappeared or not. Instead of using a cross a light beam could be used and when the beam goes out that is the end point.
There were two results that were not really anomalies as they still followed the pattern but were slightly different to the other results of the other tests. This could be human error, which is the reason for doing more tests.
