- Room temperature
- Concentration of hydrochloric acid
- Volume of hydrochloric acid
- Concentration of sodium thiosulphate
- Volume of sodium thiosulphate
Fair Test
To make the experiment a fair test, I will change the concentration of sodium thiosulphate. I will keep the volume of sodium thiosulphate, the room temperature, and the concentration of hydrochloric acid all the same.
Plan
For my experiment I will use the same concentration of hydrochloric acid (Which is 2M) and I will change the concentration of sodium thiosulphate using the following table:
I will use the following apparatus:
Pipette 5, 10, 20, 25, -ml
Test-tube
Conical Flask
Stop-clock
Paper with a
cross on it
Method
- Clean all apparatus.
- Then add 5ml of Hydrochloric Acid to a test-tube.
- Next add the required amount of sodium thiosulphate and water to the conical flask.
- Then pour the test-tube of Hydrochloric Acid into the conical flask and then start the stop clock.
- When the reaction has stopped, record the time.
- Then repeat for all concentrations.
- Then repeat the whole experiment.
Precision
The pipettes that I will use have a very high tolerance level of ±0.06ml. This gives very precise results as you get a more accurate volume. When the volume of the pipette is between 24.94ml + 25.00ml it is still accurate.
Treatment of results
To calculate the concentrations of sodium thiosulphate I will use the equation.
Concentration = Volume of sodium thiosulphate
of sodium thiosulphate total volume
Example
Concentration = volume of sodium thiosulphate – ml
of sodium thiosulphate total volume - ml
= 15
25
= 0.6M
To convert time to rate, I will use the equation:
Rate = ______1____
Time (seconds)
Example
Rate = ____1_____
Time
= __1__ = 0.05Sˉ¹
20
I will use the following results table to record my results in:
I will then plot my results on the following graph:
Prediction
I predict that concentration will be directly proportional to rate.
I also predict that my graph will be a straight line and that as rate increases, concentration will increase too.
Safety
Before the experiment, I will make sure that my tie is tucked in, my blazer is off and my seat is pushed in.
Hydrochloric acid is an irritant, if it is spilled, then it should be flooded with water.
Sulphar dioxide is toxic, so all windows and doors should be open, so as to ventilate the room. Although only a small amount is made.
Results
When I used the concentration of 0.08M I found that my results (shown in red) were not accurate enough, so I repeated them (shown in green).
Conclusion
I can see by looking at my graph that there is a straight line, which tells me that the rate is directly proportional to the concentration.
As the graph goes through the origin it shows that when the concentration is zero, then the reaction will not take place, so the rate will be zero as well.
Scientific Background
The reaction that we are looking at can be shown as an ionic equation, as shown below:
S2O3²ˉ + 2Hˉ -> H2O + SO2 + S
Thiosulphate Hydrogen
Ion Ion
When the concentration of sodium thiosulphate is increased then you get more thiosulphate ions. More thiosulphate ions will give more collisions with hydrogen ions and a faster rate of reaction will occur.
When you double the concentration you will get twice the number of thiosulphate ions, which will give twice the collisions with hydrogen ions and when you double the rate i.e. Concentration is directly proportional to rate.
Do my results support my prediction?
I predicted that concentration is directly proportional to rate. In my graph I got a straight line which proves that concentration is directly proportional to rate. As the graph goes through (0,0) it shows that when the concentration is at zero, then the reaction will not take place, so the rate will be at zero as well.
Are my results reliable?
As the error of my results is less than 10%, then I know that they are reliable.
In my graph the points are close to the line, which also shows that they are reliable.
The straight line of my graph goes through (0,0) which proves that my results are also reliable. This means that as there is no concentration, then the reaction is not taking place.
Is my method accurate?
When measuring volume the pipettes/burettes have an accuracy of ±0.1ml which is better than if I was using a measuring cylinder, which has an accuracy of ±0.5ml. This means that using a pipette gives a 2% chance of error.
When measuring time it is only possible to stop the clock to the nearest second. So the total error for stopping the stop clock is 5.6%
The reaction involves the making of a sulphur suspension. This obscures the cross on the piece of paper, which is placed under the conical flask at the start of the experiment. When the cross is obscured, it is taken as the end point of the experiment. This is a potentially large error as the exact end point is subjective; this depends on one person’s viewpoint. This is then improved by the use of repeats and analysis of pairs of results.
The total error for measuring time is 8.3%.
Anomaly
When I used a concentration of 0.08M I found that my first time was 112seconds and my second was 122seconds. I then repeated it and found the second time was 114seconds, so I discarded 122seconds for the second time, as it was an anomaly. This was probably an error on my behalf as I might have added the wrong volume.
Have I Generated Enough Results To Support My Conclusion?
Yes, I have generated enough results to support my conclusion. As I used 0 to 0.2M for concentration. This gave me a wide variety of points, so I had a good graph.
I also used six different concentrations including zero which gave me enough points, to identify a straight line.
Improvements
To improve the experiment I could:
1) Add more concentrations, which will give me more points on the graph and it will make it easier to identify a trend.
2) I could use more repeats as this will improve the reliability and will address the problem of the subjective end-point.
3) Also, it is possible that the temperature might vary during the experiment. If I did the experiment in a water bath, it would control the temperature.
4) During the experiment the total time for the reaction to finish is measured, which only gives a measure of the average rate for the whole reaction. It would be better to measure the rate at the start. To do this I would use a P.C. connected to a light sensor and a light. This will also stop the problem of the subjective end point.
