When I did the experiment with hydrochloric acid and calcium carbonate, the same was apparent, but here I was timing for a certain amount of time for a gas to be formed.
PLAN
Prediction: -
I predict that as the concentration of sodium thiosulphate increases, the rate of reaction will also increase, because there are more particles to collide. 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 reactant particles of the reacting substances collide. A more concentrated substance has more reactant particles in a given volume than a more dilute substance. As there are more particles, the frequency of successful collisions is greater, and the reactions happen faster.'
When a reaction takes place, the particles of the reacting substance must collide with each other. For the reaction to commence, a fixed amount of energy known as the activation energy must be reached, for new products to be made, and the old bonds need to be broken. If the particles collide fast enough and in the right direction, sufficient energy to break these bonds will be produced (the activation energy), which will allow the reaction to take place. However, if this energy is not produced, a reaction will not take place, so not all collisions result in a reaction. The reaction can be speeded up if the number of collisions is increased.
I predict that the rate of reaction will increase (and get more cloudy, more quickly) when the Sodium Thiosulphate is most concentrated and there is no distilled water. To change the concentration of a solution, we can dilute it by adding water.
When the concentration of the sodium thiosulphate is doubled, the time taken for the solution to go cloudy will decrease, as the number of reactant particles will also double. This will therefore result in the rate of reaction increasing.
Safety: -
- Wear safety goggles to protect my eyes from any harmful 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 washed my hands with soap, after conducting the experiment.
- Taking care when using glassware to prevent injury.
-
The experiment in which the concentration of the sodium thiosulphate is 0.15 mol/dm3 will be done inside the fume cupboard.
-
0.15 mol/dm3 is the most concentrated solution I will use. This is because a solution with a higher concentration will produce a lot of acidic fumes.
Apparatus: -
Conical flask
10 cm3 measuring cylinder
50 cm3 measuring cylinder
Stopwatch
Sodium thiosulphate
Distilled water
Hydrochloric acid
Pipette
Safety goggles
Diagram: -
Method: -
(Improvements to the Preliminary)
I wanted to change the concentration of Sodium Thiosulphate and Hydrochloric Acid, but without changing the overall volume. To do this, the Sodium Thiosulphate and water were mixed at different ratios, with always a constant amount of acid and concentration. In the preliminary, I had not measured out the volume of distilled water and sodium thiosulphate whereas; I was going to measure the volume of the sodium thiosulphate, distilled water and hydrochloric acid using measuring cylinders for the experiment to get more accurate results.
I will also draw a cross on a piece of paper and place it under the conical flask. When the cross is no longer visible, then I will stop the stopwatch. This is a more accurate way of timing how long it takes for the sulphur precipitate to form. In the preliminary, I was timing how long the reaction took by just looking into the boiling tubes and estimating when the solution had gone cloudy. This method was not very accurate as the cloudiness of the solution could have been different each time.
I will do the experiment at intervals of 0.3 mol/dm3, repeating the experiment after each reading. This is so I can take an average of the results that I have collected. This also gives me more reliable results. To change the volume of sodium thiosulphate, I had to add distilled water. The following amounts of distilled water and sodium thiosulphate were added to each conical flask:
Results:
The graphs on the following pages show the results that are in the tables above.
Analysis: -
The results show, that as I had predicted, the more concentrated solutions acted more quickly that the less concentrated ones. As the concentration got weaker, the reaction got slower. The correlation of the graph shows that the results between the rates of reaction against concentration are related, and that the experiment worked well.
The graph showing the rate of reaction shows that that there is only one point that goes through the line of best fit. This is at 0.09 mol/dm3. The other four points on the graph are not far from the line of best fit, but they are not exactly on it either. This shows that my results were not entirely accurate. I think that the readings taken at 0.03 mol/dm3 were not accurate. They do not fit the trend of the other points.
The graph, which shows the average time for the solution to go cloudy, shows us that at 0.03 mol/ dm3, the solution takes 168 seconds to go cloudy. I would have therefore expected that at 0.06 mol/dm3 the solution would have taken half the time to go cloudy, as the concentration had been doubled. I found that instead, the solution took just under three times the amount the amount of time it should have done. This does not occur all through- out the experiment. At 0.06 mol/ dm3 the solution took 65.5 seconds to go cloudy. Therefore at 0.12
mol/ dm3 I would have expected the solution to go cloudy at about 32 seconds. Although my results were not a hundred percent accurate, the time taken came out to 27 seconds.
When I draw the graph with the rate of reaction, I can see that the rate of reaction is directly proportional to the concentration. I was expecting a proportional relationship between the concentration and rate of reaction because, according to the collision theory, by increasing the concentration, there will be more sodium thiosulphate particles in the solution, causing more collisions between the particles. As a result, it has a faster rate of reaction and takes a shorter amount of time.
I have come to this conclusion because of several reasons. Firstly, my results give conclusive evidence that as the amount of Sodium Thiosulphate decreases, there are fewer atoms to collide and therefore less successful collisions causing chemical change so the reaction rate is slower. My results support the prediction I made because I said, “that as the concentration of sodium thiosulphate increases, the rate of reaction will also increase”. I believe that I was correct.
Evaluation: -
I thought that I had carried out the experiment moderately well because I achieved a reasonable set of results. The experiment went according to plan but there were flaws in it. For example, I only obtained two sets of results and then worked out an average. I could have done more tests to gain a more reliable average.
Only one point out of five went through the line of best fit. The other four points were not too far off. At 0.12 mol/ dm3, the point was furthest away from the line of best fit. This was 1.4 cm away from the line, whereas none of the other points were more than 1 cm away. I do not think that the results were anomalies as such, but the result of inaccuracy and human errors.
I think there is also a human error factor involved when 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. Although I tried to judge how long it took for the cross to become obscure, I may not have been entirely accurate as the experiment took place over a long period of time. Instead of using a cross, a light beam could be used and when the beam goes out that is the end point. A light beam would be placed underneath the conical flask, and a sensor above it. The sensor is then connected to the computer. When the sensor can’t detect the light beam due to the amount of sulphur collected, the end point has been reached. The computer records the time taken. The sensor and light beam should be kept at the same distance away from the conical flask each time to make the experiment accurate. This is a more accurate way of distinguishing when the solution is cloudy. When the light beam goes out, this means that the same amount of sulphur has been collected in each experiment. Also, better standard of measuring cylinders and pipettes could have also been used.
Also, when pouring the acid into the conical flask, there may have been some acid left behind in the measuring cylinder, making the acid content not exactly 10 ml for each experiment. This could have made the experiment unfair. Instead, we could have used measuring burettes. These would have been more accurate as they have taps, which we can turn on and off, therefore increasing the reliability of the results.
Another factor that could have affected the results is the timing. The stopwatch could have been started and stopped at different times, which would also make the experiment unfair.
Bibliography: -
- Internet
- Chemistry exercise book
- Chemistry for you- Lawrie Ryan
