The variable I have decided to investigate is
The concentration of the sodium thiosulphate.
Prediction
I will be changing the concentration of the sodium thiosulphate used in each reaction. To do this I will change the amount of water and sodium thiosulphate in each solution used in the experiment. The concentration of a solution is how much there is in the solution in comparison to how much water. The more sodium thiosulphate there is in the solution the higher the concentration. Changing the concentration of the sodium thiosulphate will change the number of particles of sodium thiosulphate in the reaction. If you make the concentration stronger there will be more particles of sodium thiosulphate in the reaction. This will increase the likelihood of more collisions. Increasing the number of collisions will cause the rate of reaction to increase because it is the collisions that are the basis of the reaction. I predict that doubling the concentration of the sodium thiosulphate will roughly double the rate of reaction and that the higher I make the concentration of the sodium thiosulphate the higher the rate of reaction will rise.
Pilot study
Before the main experiment I did a pilot study to determine a suitable range of values to use. The purpose of the pilot study is to test my predictions on how well the experiment will work, this way, any mistakes that I make in the pilot study can be improved upon in the final experiment.
Method:
- Measure out 10ml of sodium thiosulphate in a 10millilitre measuring cylinder
- Measure out 40ml of water in a 50ml measuring cylinder.
- Fill a 100ml conical flask with 10ml sodium thiosulphate and 40ml of water
- Measure 10ml of hydrochloric acid in a 10ml measuring cylinder.
- Draw a cross on a piece of paper and place it under the flask so that it is visible from directly above.
- Pour the dilute hydrochloric acid into the solution of sodium thiosulphate, simultaneously start the stopwatch
- Time how long it takes for the cross to disappear.
- Repeat the experiment, each time changing the concentration of the sodium thiosulphate.
Accuracy of the pilot study
During the pilot study I used varying concentrations and volumes of sodium thiosulphate. My first reading was taken using a twenty percent concentration of sodium thiosulphate in a 25ml solution. This reaction was not very successful as it took over four minutes for the cross to be obscured. I decided that 529 seconds was too long and tried the same concentration in a 50ml solution. I found that this reaction only took 151 seconds. I then used a volume of 50ml to test a suitable range of concentrations and my results were as follows:
During the pilot study I took the readings twice and then took the average of the two. During the real investigation I have decided to take the readings three times in order to gain a more accurate average.
Safety Precautions:
Safety glasses are the only safety precaution that I will be taking.
Table
Conclusion
As you can see from the above graph the rate of reaction increases consistently with the concentration of sodium thiosulphate. This means that the reaction got faster as the concentration increased. The results that I have obtained satisfy the prediction that I made in my plan. It also supports the idea of collision theory. Collision theory states that reactions occur when two reactant molecules collide with one another. If there is enough energy in the collision then the bonds in the molecules will be broken and new ones will be formed. In this reaction molecules of Sodium thiosulphate collide with molecules of dilute hydrochloric acid. The bonds between the sodium and the sulphur break and solid sulphur is formed. Sulphur oxide is given off and water is also formed. The rate of reaction gets faster as the concentration increases because there are more molecules of sodium thiosulphate in the reaction when the solution is more concentrated. If there are more molecules in the reaction it makes collisions more likely, thus making the reaction faster.
The graph is a straight line, this means that the rate of reaction increased with the concentration, showing that concentration is directly proportional to rate of reaction. In my prediction I predicted that the rate of reaction would double if I doubled the concentration of the sodium thiosulphate. From the graph you can see that the prediction was correct. The rate of reaction when the concentration was 30% was 0.001. When the concentration doubled to 60% the rate of reaction doubled to just over 0.002. My results support collision theory because the rate of reaction increases as the concentration increases. The rate of reaction increasing indicates that more collisions occurred.
Evaluation:
I believe that the results that I obtained in this investigation were very reliable. This is because many of the points on my graph are on the line of best fit and all are very close to it. There is only one point that is slightly further away from the line, (80,0.027) this error may have been caused either by inaccurate measuring or by inaccurate timing. The results shown in the table all appear to be reliable as the rate of reaction increased at the same rate as the concentration. The fact that there are no obvious anomalous points is significant as it shows that the readings taken were very accurate.
Accuracy and improvements
The main problems that I encountered during the investigation were accurately measuring the sodium thiosulphate solution and the dilute hydrochloric acid and accurately timing the reaction. It was quite difficult to time the reaction because it was hard to judge when you could see the cross any more. During the preliminary investigation I used a cross, drawn on a piece of paper, this can easily become smudged, impairing your vision and affecting the results. I improved on that in the main investigation by using a bold, laminated cross, this meant that it could always be seen clearly. Another possible problem is the temperature at which the reaction happened. It is impossible to keep the temperature of the room completely constant as it can easily be affected by factors such as central heating and a window being closed or opened.
The investigation could be extended by increasing the number of variables that you change, such as the concentration of the dilute hydrochloric acid or the temperature. You would always compare the variables to the rate of reaction to see if there is a correlation.
