(Insert predicted graph)
Equipment list:
▪ Conical flask (100ml)
▪ 2 Measuring cylinders (15 and 25ml)
▪ Paper with cross drawn on it (10 x 10cm)
▪ Stopwatch
▪ Sodium Thiosulphate (0.1M)
▪ Hydrochloric Acid (1M)
▪ Water
Diagram:
(Insert diagram here)
Method:
1. Collect the apparatus and arrange as shown in diagram
2. Collect your desired volume of Sodium Thiosulphate water and Hydrochloric acid, using the conical flasks (see number and range)
3. Draw cross on paper and place under conical flask
4. Pour all Hydrochloric acid, water and Sodium Thiosulphate into conical flask, in that order.
5. Start timer
6. Stop timer when the cross is no longer visible.
7. Record Results
8. Repeat again with different concentrations of Sodium Thiosulphate.
Reliability
Fair testing
Fair testing is very important to achieve reliable results. Fair testing includes controlling variables. These include my independent variable and my controlled variables. My dependent variable is the time it takes for the cross to disappear. My independent variable is the concentration of the Sodium Thiosulphate. My controlled variables are the concentration of the Hydrochloric acid, the person who judges when the cross disappears, the temperature of the room, the degree of agitation, the volumes of the Water and Hydrochloric acid and finally the initial concentration of the Sodium Thiosulphate and Hydrochloric acid.
Accuracy
Accuracy is very important to get reliable results. To make it more accurate I will be measuring the time to whole seconds. I have decided not to time to a 10th of a second even though the stopwatch measures to a 10th of a second because of the reaction time taken to stop the timer and because it is hard to judge when the cross has ‘disappeared’. I will be using a measuring cylinder to measure the volume of Sodium Thiosulphate, Hydrochloric acid and water. The measuring cylinder is accurate to +/- 1ml, which should be accurate enough for this experiment.
Number and range
I will be using 5 different concentrations of Sodium Thiosulphate. There will always be 30 ml of liquid. The volumes will be as below:
It is important to have a wide range of results to determine whether the concentration of the Sodium Thiosulphate effect the rate of reaction.
Repeating
Repeating is very important to filter out anomalous results thus increasing reliability.
I will repeat my results once. Then if there are any large differences (more than 10%) then I will repeat again.
Results:
Graph:
(Insert graph here)
Analysis
This graph shows that as the concentration of the Sodium Thiosulphate increases so does the rate at which the cross disappears. I can also see that the concentration of the Sodium Thiosulphate double as the rate does.
Conclusion
The rate of reaction is manipulated by the temperature, concentration, agitation, surface area and catalysts. In this experiment I changed the concentration whilst keeping the temperature, agitation, surface area and use of catalysts the same. I determined that if you increase the concentration of the Sodium Thiosulphate, the frequency of collisions increased so the rate of reaction also increased. This is because the higher the concentration of Sodium Thiosulphate the more frequent the collisions will be between the Sodium Thiosulphate and Hydrochloric acid particles. Therefore bonds will break quicker and new bonds will be formed which will make new products quicker. Therefore the rate of reaction will increase.
(Concentration diagram here)
If you double the concentration then you will double the frequency of collisions between the Hydrochloric acid and Sodium Thiosulphate particles thus doubling the rate of reaction.
Evaluation
I did not have any anomalous results. I did each concentration twice and they were always within 10% of each other. Also when I plotted my results onto the graph all the points were near my line of best fit.
The reason the lack of anomalous was that my experiment was reliable. This was because I carried out fair testing - controlling the variables. I used a good number range - 5 different concentrations. I repeated my experiment once - if they differed more than 10% I would have repeated it again. Finally I made sure the timing was as correct as it could be. All of this was important to obtain reliable results.
Even so I had reliable results there are still improvements to be made. My main problems were timing and volumes.
To improve the timing I would use a light sensor connected to a computer this would help the experiment because it could judge when the cross-had ‘disappeared’. If I did this then I would time to one tenth of a second instead of timing to whole seconds.
Another thing I could do to improve the experiment would be to change the measuring equipment. I was using a measuring cylinder to measure the volumes of Hydrochloric acid, Sodium Thiosulphate and water. A measuring cylinder is only accurate to +/- 1ml. Instead I would use a Burette, which is accurate to +/- 0.1ml. This would make my results even more reliable. Also instead of using the same conical flask each time and washing it out I could use a different conical flask, if I was to use a Burette I would use different Burettes also. As each time it was washed there was still a small volume of water left in the conical flask, which would effect the concentration of the reactants thus slowing the rate of reaction and making my results less reliable.
Further experiments I could do to extend this enquiry would be whether the concentration of Hydrochloric acid affects the rate of reaction. It would be similar to this experiment apart from the fact that I would be changing the concentration of Hydrochloric acid instead of the Sodium Thiosulphate. The method would be the same apart from using the volume below. I predict the results would be very similar to this experiment.
