Experiment
I will use 10ml hydrochloric acid as my constant variable and I will test the rate of reaction with 10ml sodium thiosulphate, then 9ml sodium thiosulphate and 1ml water, then 8ml sodium thiosulphate and 2 ml water and so on until I get to 5ml sodium thiosulphate and 5ml water. I will test each different concentration 3 times so I can get a more accurate average.
Prediction
I predict that the more water there is the slower the experiment will be as there is less sodium thiosulphate in the mixture to react with the hydrochloric acid. Therefore when there is 10ml of sodium thiosulphate I predict the reaction will take place fastest, and where there is 5ml of sodium thiosulphate and 5ml of water I predict the reaction will be slowest.
Keeping it a fair test
I will keep the test fair by:
- Keeping the temperature constant. The higher the temperature the more active the particles become and the more often they collide. Therefore, a higher temperature would mean a faster reaction, which would make the test unfair.
- Keeping the person doing the timing the same as people with different eyesight’s will loose sight of the cross at different times and this could make the test unfair.
- The concentration of hydrochloric acid and the volume will also be kept constant throughout.
- Keeping the beakers, measuring cylinders and conical flask clean between experiments.
Variables
Constant variables:
- Temperature
- Volume of hydrochloric acid (10ml)
- Concentration of hydrochloric acid
- Source of the water
- Person doing timing
The only variable that I will change is the concentration of sodium thiosulphate.
Chemical Reaction
Hydrochloric acid + Sodium Thiosulphate + Water = Sulphur dioxide + sulphur + water
2HCL(aq) + NA2S2O3 (aq) + H2O (aq) = 2NACL (aq) + H2O (aq) + SO2 (aq) + S (s)
Sulphur is the substance which causes the cloudiness that obscures the cross.
Method
I firstly measured 10ml of hydrochloric acid using a small measuring cylinder and then added it to an empty conical flask. I then measured out the 10/9/8/7/6/5ml of sodium thiosulphate and the 1/2/3/4/5ml of water in 2 separate measuring cylinders. I then added them at the same time to the hydrochloric acid and started the timer. I stopped the timer at the point at which I could not make out that the shape was a cross, which is where I could not see it at all. I did the same 3 times for each different amount of sodium thiosulphate.
To make the experiment a fair test I did not mix the sodium thiosulphate and water until they were both added to the hydrochloric acid. I also tried my best to keep the beakers and measuring cylinders clean after each experiment. I will keep the temperature the same (room temperature) and the amount of hydrochloric acid the same.
In the experiment I will take six sets of readings that I will repeat 3 times each. I will repeat them 3 times so that an accurate average can be obtained and so that I can spot any anomalies.
Apparatus
- Laminated cardboard with bold black cross on it.
- Beakers
- Conical Flask
- Water
- Stop watch
- Measuring cylinders
- Sodium Thiosulphate
- Hydrochloric acid
Risk Assessment
I will wear a lab coat and safety goggles at all times whilst conducting the experiment. I will be careful not to spill any of the substances as they can be hazardous and I will be careful to keep all books and other equipment out of the way of the experiment so in the case of a spillage they wont get damaged. Hydrochloric acid and sodium thiosulphate can both be harmful to your skin and are irritants so it is important these are not spilt.
Results
Conclusion
As we can see from the graphs, the higher the concentration of sodium thiosulphate, the quicker the reaction takes place. This is because of the collision theory, which says that the more particles the more collisions take place meaning a faster reaction. The more sodium thiosulphate particles the more collide with the hydrochloric acid causing sulphur to form faster. This means the speed of reaction and the concentration of sodium thiosulphate are directly proportional.
The rate graph also shows that the reaction is fastest when there is the most sodium thiosulphate. In the rate graph an anomaly is also clear where there is 8ml sodium thiosulphate and 2ml water.
The graphs show what I explained in my prediction was correct.
Evaluation
My experiment went well. I got the results to all of the readings I wanted and I got enough information to produce good accurate tables and graphs. There weren’t many anomalies in my results except for in the rate graph there is a clear anomaly for 8ml of sodium thiosulphate and 2ml water. This anomaly may have occurred because of a couple of different reasons. I obtained my results on a number of different days so the temperature may have differed and therefore affected the reaction. There is also the possibility that after washing the conical flask, beakers and measuring cylinders between experiments that there was drops of water left in them. This water may have affected the experiment as it would dilute the mixture further and therefore slow it down due to the collision theory, which I have explained. If I were to do the experiment again I would try to obtain all of my results on the same day so that the temperature would be the same. I would also dry out the equipment after washing it between experiments so that there would be no water left.
