Reasons why controlled variables would have an effect on the rate of reaction if they weren’t controlled:
Apparatus: two measuring cylinders, two 150ml beakers, a piece of paper, felt pen, Sodium thiosulphate solution, 1M sulphuric acid, 2 separate pipettes ,a beaker and a stopwatch.
Method for data collection:
- Mark a piece of paper with a felt tip pen
- Put an empty beaker over the marked spot
- Adjust the concentration according to the selected amount between 0 -1M.
- Fill up the test tube with distilled water until the 10ml line according to the ratio of concentrations of sulphuric acid (e.g. if a concentration of 0.2M should be attained, then the measuring cylinder should measure 0.2ml of sulphuric acid and 0.8ml of distilled water is then added). If 10ml of acid is present then no water should be added. For this experiment every interval of 0.2M concentrations of sulphuric acid are measured.
- Use a different measuring cylinder to measure 10ml of excess sodium thiosulphate with a pipette.
- Add the two mixtures into the beaker, simultaneously.
- Start timing immediately
- Observe the marked spot under the beaker
- When the reaction is finished, the cloudy mixture is present and the cross should not be able to be visible. That’s when the stopwatch should be stopped
- Record the time taken for this reaction to happen.
- Do three trials for each set of concentrations
- Repeat this whole process for 5 different concentrations of sulphuric acid.
Method for effective controlling of variables:
- Do this experiment under laboratory conditions
- At room temperature (not fluctuating)
- 1 atm pressure
- Make sure no catalyst is added
- Make sure the size of the beaker is the same because of keeping the same surface area of liquids during the reaction.
Raw data of results
Qualitative data: When products are formed then the solution changes from colorless to cloudy yellow/white and the marked cross disappears.
Processed Data
Figure 1:
Figure 2:
Uncertainties and errors
Total uncertainty calculation for this experiment: ±0.2 (cm3) + ±0.5 (s) + ±0.2 (cm3) = ±0.9
±0.4 (cm3) from measuring cylinder and pipette
±0.5 (s) from stopwatch
Errors:
- Human error in not being able to determine when reaction has fully stopped.
- The stopwatch hasn’t started on time when the two solutions were mixed.
Conclusion
The aim of this investigation was to show relationships between the rate of reaction and one aspect that would affect it. In this experiment the concentration of sulphuric acid was used to determine relationships with the rate of reaction with sodium thiosulphate. A direct relationship could be seen from the results attained. It showed us that as the concentration of the sulphuric acid was increased, the time taken for marked spot to disappear was shorter for every occasion. The average set of results helped to determine this relationship more clearly. Figure 1 was a graph plotted to show a relationship between concentration and time. The reaction showed a general trend that the time taken was shorter as the concentration increased because the gradient was negative. Unfortunately a curve had not been represented in this first order reaction. However, Graph 2 had shown a more precise trend that there was a direct relationship for the collected data. A first order reaction occurs linearly (as seen below) when the rate is plotted against the concentration. An estimation of a linear graph was shown as an approximate rough representation but generally the graph had given an indication that this reaction is determined as a first order reaction.
Evaluation
The main limitation of this experiment was that it was difficult to determine when the reaction had stopped because it was assumed that the reaction had stopped as the marked cross had disappeared.
The results weren’t accurate enough to show a significant graphical representation of a first order reaction and not enough trials were taken to prove that there was a direct relationship because of anomalous results.
Improving the investigation
In order to avoid these limitations and weaknesses, the experiment could have been improved. Knowing whether or not the reaction has stopped could be determined by light. If a light is shined through the beaker during a reaction and once the reaction changes color, the wavelengths could be measured through a device that would measure wavelength of light as they would change when the reaction has stopped from the colorless solution into a cloudy yellow/whitish. This is to be done so the reaction can be stopped without human perception.
The number of trials that is repeated for this experiment should also be increased so that the raw data of results can give a more significant set of results that can show a relationship between rate and concentration to determine the order of the reaction.
The concentrations of should be evaluated in smaller intervals such as concentrations between every 0.1M of sulphuric acid to make the results more valid.
All other factors should be kept under control and measured before the experiment is conducted if this experiment was done. The assumptions that were made could have altered the rate of reaction. Temperature, pressure and surface are to be measured before hand and of course with no presence of a catalyst.
