Equipment:
The equipment I will be using in the experiment is as follows:
- Conical Flask
- Hydrochloric Acid
- Sodium Thiosulphate
- Paper with cross drawn on it
- Stopwatch
- Measuring Cylinder
- Ruler
- Safety Goggles
Diagram:
Preliminary Experiment:
Before starting these experiments we carried out preliminary experiments to find out which measurements would make the reaction between the two chemicals quicker. We basically did a smaller version of this experiment apart from using different amounts of hydrochloric acid and sodium thiosulphate equalling 50ml until we found the quickest way of doing this experiment.
Results from preliminary:
From our preliminary results you can see that when the amount of sodium thiosulphate is greater than the amount of hydrochloric acid the reaction takes place quicker. This is why for this main experiment we chose 10ml of Hydrochloric acid and 40ml of Sodium Thiosulphate. Also with the larger solution of Sodium Thiosulphate it would be easier to change the concentration, which is why we chose it as the variable we would change.
Method:
I plan to make sure this experiment is carried out safely by using safety goggles while carrying out the experiment and also taking care when handling chemicals and being wary of the safety precautions stated on the bottles.
In the experiment I plan to mix a solution of sodium thiosulphate and water measuring 40ml with hydrochloric measuring 10ml, this is because we are not allowed over a total volume of 50ml, to preserve chemicals. I will mix the two in a conical flask. I will then place the flask above the paper with an ‘x’ on it. The observer will watch from a distance of 30cm above the flask. Once the observer can no longer see the ‘x’ the reaction, in my view is complete. I will repeat each molarity three times to obtain conclusive results. The measurements I will use for the different concentrations are:
I am going to time the experiment for the time the two solutions are mixed together in the conical flask until the cross on the paper is no longer visible. The degree of accuracy I will use is split seconds fro the most accurate results.
Prediction:
I predict that the rate of reaction will increase as the concentration increases. I think this because have predicted this due to the knowledge I have gained through my research. I found out that if I want to predict he rates of reaction, the collision theory would be useful to know about. The collision theory states:
‘A chemical reaction can only occur when two chemicals collide with each other with sufficient force and at the right angle’
When there is a stronger concentration of Sodium Thiosulphate there are more Sodium Thiosulphate molecules to react with Hydrochloric molecules so the reaction happens quicker.
Results:
To further my experiment I am able to work out the speed of the reaction. The formula for this is 1/time. The number given as the answer shows the speed per second of the experiment.
Conclusion:
From our results I can immediately tell that my prediction was correct. By looking at my graph I can see that when the concentration of the Sodium Thiosulphate is increased, the time taken for the cross to disappear decreases. This means that the concentration is inversely proportional to the time taken for the cross to disappear. The time taken for the cross to disappear relies on the concentration of the Sodium Thiosulphate. This means that Sodium Thiosulphate acid to water ratio is bigger. If there is more Sodium Thiosulphate to water in the solution then this means that there are more Sodium Thiosulphate molecules in it. The fact that there are more Sodium Thiosulphate molecules knocking about with water molecules means that collisions with the HCL molecules are more likely to occur. The reaction that takes place in the experiment is:
Na2S2O3 + 2HCL -> 2NaCL + H2O + SO2 + S
The Sodium Thiosulphate reacts with the hydrochloric acid to produce a fine deposit of sulphur, which is responsible for the cloudiness of the solution. The deposit of sulphur soon makes it impossible to see through the solution, when this happened we stopped timing.
Evaluation:
The method used was good and produced reliable results but it could have been improved. We used the most accurate method of timing as possible, to a hundredth of a second, so if there was a error with that there was not much that could improve it. The results were mainly good, there are no anomalous results, which did not match with the general trend of the results on the graph and everything came out as expected. Although all the average times and rates of reaction all conform to a pattern, they are not all evenly spaced and therefore are probably not perfect. The experiments were carried out fairly but it could have been even fairer. One factor that could have contributed to the slight imperfections in the results could be the varying temperatures. As temperature can also affect the rate of reaction this may have altered our results slightly, if we did this experiment again this could be avoided by doing the experiment in a temperature-controlled environment. Another factor could be as the reaction was finished when the red cross on the paper disappeared and this is only seen by the naked eye. With the whole class doing this with different eye types, there is no exact fairness in the experiment. Using devices such as a light sensor can solve this problem. The light sensor will pass through the flask were a reaction is taking place to a receiver. When the light sensor cannot reach the receiver, the timing will be stopped automatically. This will bring fairness to experiments to test all similar to rate of reaction.
