As the results show, any concentration above 0.1M reacts too quickly at 60°C. Therefore, I decided to that I would use that concentration in the actual experiment.
Method:
For the investigation I used various apparatus which I did not use in the preliminary test. Instead of a cross on a paper I used a colorimeter, as it is more reliable. I also used a cuvette and a pipette. To start with I made a stock solution of sodium thiosulphate and water. This, as well as an equal amount of hydrochloric acid, was heated in a separate test tube using a water bath. I started off with the lowest temperature, 20°C. I measured 1.5cm3 of both the acid and the diluted solution using pipettes. In the colorimeter I placed the cuvette, and in that I added the chemicals. As soon as they were both added, the time for the reaction to complete (when the absorbency reached 2) was measured, and the constant absorbency readings were recorded. I then repeated this experiment for each temperature twice. These are the results:
It is apparent that for the first two temperatures the experiment was repeated for a 3rd time. The reason for this was because after doing the experiment twice, the results were not very similar. This suggested that there was a mistake in either the first or second attempts.
Whilst doing the experiment I had to make sure that both solutions had reached the desired temperature. Also, before using the colorimeter I had to set the absorbency to zero when the cuvette was filled with water. I even used the same stock solution of sodium thiosulphate and water each time.
I am going to plot the results in the graph, but first I will need to find the average time for each temperature. For 20°C and 30°C the 1st attempts will not be included as they are anomalies.
Conclusion:
In conclusion I found that there was a pattern with the rate of reaction and the temperature. Graph 1 shows that as the temperature increases, the time taken for the reaction to take place decreases. Graph 2 also shows that as the temperature increases, so does the rate of reaction.
Here is a balanced equation for the reaction:
This is the balanced ionic equation:
2H+(aq)+ 2Cl-(aq)+ 2Na+(aq)+ S2O32-(aq) 2Na(aq)+ 2Cl-(aq)+ H2O(l)+ SO2(g)+ S(s)
The reason why the temperature increases with the rate of chemical reactions is because of the Collision Theory. This theory states that in order for the particles in a chemical to react, they must collide with each other. The rate of the reaction depends on four things: the concentration, surface areas, the use of a catalyst, and the temperature. If the temperature is increased the particles have more energy, more frequent, and more faster. Because of this there will be more collisions, therefore increasing the rate of reaction, and decreasing the amount of time the reaction takes.
A Rate of Reaction vs. Temperature graph should be directly proportional, like my graph is.
Evaluation:
One of the most important pieces of equipment was the colorimeter. It was important because it measured the absorbency of the solution. The reason I used this was because it was more reliable than using a cross on a paper. The colorimeter would notify us when the absorbency reached 2, whereas when using the other method there may have been disagreements of when the solution became opaque.
The colorimeter gave out the absorbency readings every 1-2 seconds, therefore we could not use the readings to measure accurately the time taken for the reaction to take place. We decided that a stopwatch would be the solution.
To warm the chemicals a water bath was used. The test tubes of hydrochloric acid and diluted sodium thiosulphate were warmed at the same time whilst in a test tube. I also used a thermometer as a precaution to measure the temperature, in case there was a fault with the bath. Using a pipette I extracted 1.5cm3 of each chemical and put them in a cuvette. Whilst doing the experiment I realised that it was difficult to change the temperature of the chemicals to 20 C using a water bath, as the room temperature was higher. I solved this problem by keeping a thermometer in the test tubes and putting the bottom of the test tube under cold running water from the tap. When the temperature reached 20 C, I quickly and carefully poured them into the cuvette, before the temperature rose again.
There were two anomalous results from the investigation. Because of this, I repeated the experiment, and ignored the incorrect results.
One aspect of the investigation which was not very reliable was making sure the temperature was correct when starting the reaction. It was a bigger problem with low temperatures, as the chemicals were more prone to a change in temperature over a short amount of time, i.e. the time taken to transfer the chemicals from the test tubes to the cuvette. I did not realise this when doing the experiment, so did not start the reaction as soon as the desired temperature was reached. This may have been the reason for the anomalous results. In hindsight I should have started the reaction between the two chemicals as fast as possible from the moment when I removed them from the water bath.
Overall, I am fairly confident with my conclusion. However, I would be more confident if I had the opportunity to try the experiment with different chemicals.
