Measuring cylinder (10cm3)
Conical flask
“X” on paper
Distilled water
Sodium thiosulphate 0.4M
Hydrochloric acid 2M
Timer
Method:
When varying the concentration of sodium thiosulphate (HCl is in excess):
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Using the volumes in the table, make up the sodium thiosulphate solutions using the correct pipettes to make up the relative volumes for each attempt, e.g. use a 25cm3 and 5cm3 pipette to make up 30cm3 of thiosulphate and having washed out the 5cm3 pipette complete the solution by pipetting 5cm3 of water. The pipettes should be washed out with distilled water several times after each use, ensuring that none of the solutions are contaminated, this is very important in order to gain accurate results.
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Ensure that HCl and Na2S2O3 is poured from the main source into separate beakers for the individual’s use, this prevents contamination of the source.
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With the solution of thiosulphate made up, in a 10cm3 measuring cylinder, fill with 2M HCl making sure the bottom of the meniscus touches the 10cm3 line. A measuring cylinder is used as the acid can be poured in quickly, as opposed to using a pipette, which is slow and leaves margin for error, although more accurate.
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Then place the conical flask with the thiosulphate solution on top of the “X” and with the timer ready on the one hand, pour the 10cm3 of HCL into the flask and press the time as the acid hits the solution.
- Swirl the flask, a colour change will appear, stop the swirling when the “X” is becoming less visible. Observe very carefully, as soon as the “X” has vanished stop the timer immediately.
- Take down the time and record as shown in the table above. Wash out the conical flask 2-3 times using distilled water the last time, also make sure as much water as possible has been removed by shaking the flask.
- Repeat the experiment and use all the different concentrations.
The relative concentration of sodium thiosulphate can be worked out by using this series of calculations:
1) Moles = Concentration x Volume
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Concentration = Moles (from 1))
Volume
Although the concentrations do not have an obvious regular interval between them as in the second experiment, this experiment uses a greater number of concentrations, therefore the graph will be form more easily and show greater accuracy.
When varying the concentration of HCl (Sodium thiosulphate is in excess):
- Now that sodium has to be in excess, the given concentration of hydrochloric acid (2M) has to be diluted considerably (10x) to make sodium thiosulphate (0.4M) in excess:
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To make up these solutions, follow the volumes on the tables. However use a 5cm3 graduated pipette to measure out the single digit volumes and again washing out the pipette after each use, with distilled water. Therefore when testing the second concentrations, take out 5cm3 then 3cm3 of HCl (there is no need to wash out pipette as the same solution is being pipetted) the having washed out the pipette add 2cm3 of distilled water to the solution. The remaining 90cm3 can pipetted using a combination of 50cm3 and 20cm3. The procedure is repeated logically for making up each solution.
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The relative HCl solutions can be made up by pipetting the corresponding volumes of HCl and water (shown in the table) into a conical flask. Then add 10cm3 of sodium thiosulphate from a measuring cylinder as described in the first method, time and repeat as described before. The resultant concentrations are shown below, calculations using the known data can be used:
Concentration = Percentage vol. HCl x 2M
e.g: ( 8 x 100) x 2 = 0.16M
10
When this is applied to each of the relative volumes, a fair distribution of concentrations results as seen in the results tables. This is also shows that the sodium thiosulphate is always in excess, as the starting concentration for HCl is 0.2M and reduces down the table; in comparison the sodium thiosulphate is set at 0.4M.
Safety Precautions:
- Wear lab coat and safety goggles at all times
- Do not ingest, get on your hands or put into eyes reagents.
- Do not run in the lab
- When attaching pipette to pipette fillers, ensure pressure is only applied at the end near the filler so the pipette is less likely to cause any injury if the glass were to break.
Fair Test:
- The experiment should be done on the same day if possible to ensure variable factors such as temperature is constant
- Repeat each experiment twice to reduce human error
- The same person must take readings, in particular when the “X” disappears, everyone has different vision so the “X” will disappear at different times for different people
- Same equipment should be used to ensure greatest accuracy
Results:
When sodium HCl is in excess:
Refer to Graph overleaf
When sodium thiosulphate is in excess:
Refer to graph on the previous page
Analysis:
- Both graphs show first order reactions with respect to the relative concentrations of the varying concentration of the named reagent. This is in agreement to the graph in the background section. A linear line indicates a first order reaction, which the results show when applying a line of best fit.
- With the results I have obtained it is possible to work out the rate constant k by using the equation:
Rate = k[A]a[B]b
A and B represent the two reactants HCl and Na2S2O3 and the letters in lower case represent the order of reaction which is 1 for both the respective reactants. The rate is already known, as seen in the results. Therefore leaving k to be solved. At each concentration, k can be solved, which, if accurate will be pretty much be the same value. The equation can be rearranged to solve k:
k = Rate
[A]a[B]b
By substituting the known values k can be solved:
When sodium HCl is in excess:
Average of k = 0.083
When sodium thiosulphate is in excess:
.
Average of k = 0.397
Looking at k found for each concentration, they are all within a fairly close range and the averages reflect the approximate values for k. However for 0.06M when HCl is in excess, k is considerably low in comparison to the other values. This could be because it is an anomaly even though I attempted to minimise any human and/or technical errors.
- In general the results show that as the concentration of either HCl or sodium thiosulphate is increased, when the respective reagent is in excess, the faster the reaction occurred- the “X” disappeared in a shorter time
Evaluation:
- The Experiment was carried out in two parts, when HCl was in excess and when sodium thiosulphate was in excess. However the results were obtained for each on two separate days as time did not allow for it to be all done in one session. As a result this could have affected the results gained and a difference in results might have been created. For example temperature on the one day could have been very different, although in the same lab, the amount of sunlight differed, the second day was colder and dark as it was late in the afternoon. Therefore this may have had an effect on the overall rate of reaction with respect to the collision theory, if less energy is available in all the molecules, the likelihood of successful collisions is lower, reducing the rate.
- The fact that the investigation was carried out over two days only leaves greater margin for error when considering that the equipment used on the separate days differs. The accuracies of each pipette for example may differ slightly, which may affect the outcome of the experiment. In deeper consideration, when pipetting volumes, there is margin for error when manipulating the filler. I noticed that it was very difficult to maintain the meniscus of the reagent on the mark as the pipette is lifted out of the solution. This leaves margin for error.
- Another problem in the practical is the equality of swirling the mixture. There is no certainty that at times, the rate of reaction may have been altered by the swirling of the mixture, as the number of unreacted molecules that come into contact with the reacting molecules will differ. The swirling is essentially random and cannot be made to be precise, however if it was it may show a more accurate set of results, with a better line of best fit on the graphs.
- When the solution changes colour, due to the build up of sulphur, the intensity of the colour differed occasionally, even when testing a particular concentration. This may have been a result of different concentrations of sulphur forming due to a change in one of the factors. However this may have also effected the results, because the end point of the reaction is determined by the “eye” the reliability and consistency of the results can easily be questioned. I found that occasionally the end point colour would look a more intense yellow colour. I also found that the end point, for most of my reactions, occurred at a point where the colour although very cloudy, it was a milky white colour. Although this is good in the sense that I most likely stopped at the equivalent point for each reaction, if the reaction is allowed to continue, the colour does go very bright yellow, surely this is the end point? When the colour is milky white, it indicates that there is little sulphur or some other compound that is formed.
- More repeats for the second method could have been done to ensure an accurate set of results.
- A colorimeter could have been used to measure the change in colour, this would be far more accurate and improve results.
- The same apparatus could have been used.
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The equipment could be washed out and allowed to dry fully, the traces of distilled water may affect concentrations and cumulatively build up. For example, I noticed that sometimes after leaving a conical flask to stand for a while, there was about 3cm3 of water at the base of the flask.
The results are quite reliable, but when considering all the faults and inaccuracies there is a fairly considerable margin of error within these results. Referring to the graphs, there is not a precise correlation, although clearly a linear graph, there are still points that look more like anomalies than slightly inaccurate results. However it could be argued that this investigation is based on what the individual sees as the “end” of the reaction, which determines the amount of time it takes for the reaction to complete. Therefore if the individual is consistent the results may still show a true representation of the real outcome, even if not completely accurate. I think the results show good relationship, especially when HCl is in excess and the concentration of sodium thiosulphate is varied, all points are very close together.
On the whole the results prove the order of reaction with respect to the reagent, and even if results were made to be more accurate, the same outcome would arise. This makes the results accurate enough to justify the conclusions made.
