Plan
After the preliminary experiments, I now have results which can influence my decisions on how to perform the main experiment.
I will draw a thick black cross on a piece of paper. This will act as a visual marker to let us know when the reaction has occurred. Next, I will get the equipment needed to perform the experiment. I will get certain amounts of Sodium Thiosulphate, and certain amounts of Hydrochloric acid (amounts will vary according to the reaction table). I will place the two reactants into a beaker with a set amount of water, and place the beaker over the black cross. I shall then begin the stopwatch. The solution should become cloudy. When the cross is no longer visible through the mixture, the reaction has taken place. The time taken for this to happen is the measure of the rate of reaction. We must do this several times, and change the concentration of Sodium Thiosulphate.
It is agreed that we should reject results that take longer than 300 seconds or less than 15 – this is because if the experiment exceeds this limit, it is unlikely that the experiment will have been formed properly – either through human error, or a factor that affects the rate of reaction that would be out of our control (e.g. the temperature).
It is also agreed that ideally, time permitting, we should repeat the experiments at least 3 times and then take the averages, to ensure that our results are the most accurate they could possibly be.
The rate of reaction is a measure of the change, which happens during the reaction. The concentration of the reactants, along with the temperature at which the reaction is carried out, are all factors that affect the rate of reaction. After completing the preliminary experiment, I decided that it would be easiest to pursue the experiment involving altering the concentration of thiosulphate. This is because, using the equipment available, it would be too hard to accurately alter the temperature of the reactants efficiently. Also, it would be simpler to change the concentration of the reactants rather than the amounts, as it would be easy to keep the overall amount the same rather than increasing it, and using up more resources.
“For a chemical reaction to occur, the reactant particles must collide. Collisions with too little energy do not produce a reaction. The particles must have enough energy for the collision to be successful in producing a reaction. The rate of reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction.” http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway/rocks_metals/7_faster_slower1.shtml
To be systematic and to gain more accurate and practical results, we have also agreed that we would increase the thiosulphate amount in gaps of 5cm3, and increase the amount of Hydrochloric acid from 5cm3 per reaction to 10cm3.
Equipment
· 2 Measuring cylinders – to guarantee accurate measurements of reactants
· Beaker
· Stopwatch – for reliable and accurate timing of reactions
· Paper with black cross on it – for visual signal of reaction completion
· Sodium Thiosulphate (different concentrations)
· Hydrochloric acid (same concentration each time)
· Water (different amounts)
Making it a Fair Test
To make the experiments fair, I had to make sure that my method was accurately repeated for each experiment and to ensure that all the variables which might affect the rate of reaction, were held constant.
I took the following into account:
- The amount of solution, for a fair comparison of reaction times – A different depth of solution could make it harder (or easier) to see the cross, and this could affect the times.
- Measuring cylinders had to be the same size and shape to ensure same depth of solution.
- The concentration of each product in the solution
- Every product should be poured into the beaker at the same time
- Accuracy of timings - Stopwatch in sync with products reacting (start of experiment) and end of experiment (preventing time lapses between seeing the cross disappear and pressing the stopwatch button)
- The same cross “X” should be used
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Keeping the temperature the same as far as possible (Room temperature will vary but due to the thermostat in our lab, it should remain within ± 2oC during each session)
- Using the same observer for consistency of vision and human reaction times when using stopwatch etc
Method
My method followed my plan closely. I drew a thick black cross on a piece of paper. I then created a reaction table, seen below, which I could follow as I was performing the experiments.
I placed these amounts into a conical flask (which was placed on the cross) and used a stopwatch to record how long it took for the reaction to turn to a cloudy white colour. When I could no longer see the cross, I stopped the stopwatch and recorded how long it had taken for the reaction to take place. Once this had been done, I washed the beaker out and started again with the next amount.
It is possible that my measuring of the liquids could have been more accurate, and there may have been some time lapse problems using the stopwatch. Measuring the liquid using pipettes may have been more consistently accurate.
Results
Here is my table of results from the experiments. It involves the three experiments that we did, along with the average time and the average rate of reactions:
If we look at the average time (in seconds) column, we can see that each experiment’s reaction time gradually decreases. However, as we can see from the table, there is an evident outlier between the times gathered from the 25cm3 (of thiosulphate) experiment and the 30cm3 experiment (51.67 value). This can also be seen on my graph, as the 30cm3 experiment does not fit into the line of best fit.
On my graph (appendix 1) I have plotted the average rates of reaction and used the rates for the upper and lower time values to calculate other rates. These can be seen in the table below. I have used these calculations to build error bars into my graph.
To examine my outlier in more detail, I have decided to place a new column into my results table – The difference between each recorded time. This enabled me to see how my outlier was affecting the trend of my graph and tables. This spurious outlier affects the progression of the time difference between each experiment. With each experiment, the difference in time gets smaller. However, with the highlighted column, we can see that the time difference does not fit this progression. To fit the pattern I would suggest changing the average time of the 30cm3 experiment to 47 seconds and thus its rate of reaction would be 2.1 which is very close to the value of 2.2 suggested by the best-fit line on my graph. After doing this, the average time, time difference and the average rate of reaction all seem to fit the pattern, making my hypothesis clearer.
However, the values seen below also did not fit the pattern fully. The time difference was an increase from the previous value, instead of a decrease. In my first table the three times taken for this concentration were 62, 63 and 85. This last figure is an obvious anomaly and is highlighted on my graph as very wide error bars. Because the average time did fit the pattern, and due to time constraints, I decided not to pursue this any further.
I have tried to identify some possible reasons for these outlying values in my evaluation.
Conclusion
From the data above, we can see that as we increase the amount of thiosulphate in the solution, the average rate of reaction increases – doubling it however, does not double the rate of reaction. With an increased amount of thiosulphate, there are more molecules in the solution and therefore, there will be more molecular collisions with the hydrochloric acid molecules, as stated in the ‘collision theory’ in my plan.
Evaluation
- Procedures
I have tried to look at each aspect of the experiment and analyse its strengths and weaknesses below:
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Human error – It is possible that I did not pay enough attention to detail when setting up the experiments. Example of this might include, wrong measurements, failure to wash the equipment properly (possible presence of a catalyst in dirty beaker or in the water supply to speed up the reaction). Any one of these could alter the reaction times or the mean that the test was not fair.
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More accurate measuring equipment - Whilst the stopwatch was accurate, the time I actually pressed stop and start could have been varied, creating time lapse problems. This is particularly true at the finishing time when the cross under the beaker was very dim. Instead of measuring cylinders, it would have been more accurate to have used burettes or pipettes. Perhaps a more scientific reading could have been taken using something like a luxmeter under the beaker to give an accurate light reading to indicate completion of reaction. We could have set an agreed threshold to show when the reaction was complete.
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Every product into the beaker at the same time – This was relatively easy to achieve.
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Same cross – Using the same cross each time meant no inconsistencies in the image.
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Temperature the same as far as possible – I knew that it was possible that the room temperature could vary between sessions but due to thermostat in our lab, I assumed that the temperature should be within ± 2oC at each session. It is, however, quite possible that the temperature fell outside these parameters. A warmer mixture will raise the energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster which will increase the speed of the reaction. To avoid this possibility I should try to do all the experiments in one day and take more careful measurements of the temperature during the work sessions.
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Scale measurements - Using the same beakers with pre-marked units on them should have ensured some consistency of measuring but is possible that we may have misread a scale when adding the elements of the solution. It is also possible that each measurement could have been ± 0.5 ml due to the angle of reading the scale.
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Safety – Great care must be taken when handling hydrochloric acid. If pouring this liquid there can be splashes, and so protective eyewear should be worn. It is better to use a pipette.
- Reliability of evidence
The presence of the two pieces of outlying data in the tables above suggests that my procedures must have contained errors or omissions. This is seen again on the graph where the 30cm3 value is well away from the best fit line and the 20cm3 value has a very large spread of error bars. The 85 second measurement at 20cm3 thiosulphate seems to be spurious. If the second timing (55s) at 30cm3 were actually 45s, then the average time would have been 48s and this would have fitted the pattern well (and in keeping with my later estimate of a 47s average). This is a possible recording error. However, from the figures and from the graph a trend or a pattern seems to have emerged.
A further check on my data can be made using the graph. The majority of my readings are close to the line of best fit and the line passes within the error bars of most. From my graph, I can see that where the volume of thiosulphate is 20cm3 the rate of reaction is 1.8. Therefore, I might expect that at the 40cm3 mark (20 x 2), the rate of reaction to be twice this i.e. 3.6. In reality, the rate of reaction at this point was just over 3.00. Again, this suggests that my data is not perfect but close enough to demonstrate a positive correlation between volume and rate of reaction.
- Reliability of Conclusion
I think that my conclusion was very reliable. There are several reasons for this;
A pattern was clear from my graph (an upward sloping straight line suggesting a strong positive correlation between volume and rate of reaction) and from the table of figures even with outlying values. We would need to repeat the experiments again several more times to improve reliability but if my theories in section (B) above are correct, then the overall conclusion would seem to be sound. With more time I should also like to have done an experiment where I varied the volumes of acid and kept the thiosulphate constant to cross-check the results and ensure reliability.
The graph suggests that a fair degree of reliability in my results. My line of best fit passes within the error bars of most of the points plotted. According to the table in my results section, the average reaction time falls steadily with an increase of thiosulphate and the average rate of reaction increases with increased levels of thiosulphate. This suggests that the results have an acceptable level of accuracy and thus the conclusion is reliable.
I have tried to investigate my conclusion in more detail by calculating the gradient of my line of best fit. Using my knowledge that the equation of a straight line graph can be expressed as ‘y=mx+c’, I can show that the equation of my line is ‘y=0.088 x – 0.26’. Therefore, the expectation that rates of reaction would double with twice the volume of thiosulphate was inaccurate. However, this equation does give me the ability to forecast the reaction for any given volume of thiosulphate. To improve the reliability of my conclusion I could redo the experiments and make predictions about the rates of reaction using my graph. If my predictions were correct, this would show that my conclusions were correct.
