The dependant variable will be the rate of reaction; this should be a direct result of the independent variable.
Safety
While doing the experiments it is important to be cautious against possible dangers. At all times a lab coat and goggles should be worn to prevent unnecessary accidents.
The sodium thiosulphate is at a very low concentration and so isn’t very dangerous but the hydrochloric acid is an irritant. If either of these reactants were spilt on skin, the affected area should be washed and the teacher should be notified immediately. If any were to get in the eyes, although it shouldn’t due to goggles, they should be washed with running water for ten minutes; once again, a teacher should be notified. Any spillage should be cleaned straight away.
The products of the experiment are sodium chloride, sulphur dioxide and sulphur. The sodium chloride, which is a salt, has no risk of danger. Sulphur is flammable but because it is in a solution and there are no naked flames there is no potential risk. Sulphur dioxide is toxic in large quantities but not so much should be produced; nevertheless the room should be well ventilated.
Apparatus
- A laminated piece of card with a defined cross (X) – to define when the reaction has taken place to a suitable point.
- Two pipettes – one for sodium thiosulphate and one for hydrochloric acid so the chemicals do not get a chance to mix.
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100cm3 measuring cylinder for the sodium thiosulphate.
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Two 10cm3 measuring cylinders for the water and hydrochloric acid. I am using two measuring cylinders because the measurements are quite small and an excess or shortage of water or hydrochloric acid would make the test unfair.
- A conical flask – the reaction of the chemicals shall take place here.
- Stop clock – to time the speed of the reaction.
The following chemicals will be used:
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0.1 Mol/dm3 of sodium thiosulphate
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1.0 Mol/dm3 of hydrochloric acid
- (Tap) Water
Preliminary Results
I plan to use the following ratios of hydrochloric acid:water
Each experiment shall be done three times to increase accuracy. In graphs I shall use the average figure. Any results which seem anomalous will be repeated.
I chose these ratios because in preliminary experiments when the concentration of hydrochloric acid was lower than this it took far too long for the cross to become invisible and at times did not become invisible. Preliminary results may have differed from the final results due to more haphazard measuring (not using a pipette), not using different apparatus for the different substances (measuring water and hydrochloric acid in the same measuring cylinder for example), but otherwise the method was identical.
Prediction
From the results of my preliminary experiments I predict that as the concentration of the hydrochloric acid is higher the less time it takes for the chemicals to react and therefore the rate of reaction will be higher. This is because at lower concentrations of hydrochloric acid there are fewer HCl molecules per cm3 of solution. Due to this there are fewer successful collisions between the reactants and it takes longer for the two chemicals to react.
Method
Measure 60cm3 of sodium thiosulphate into a 100cm3 measuring cylinder. To do this first measure as close to 100cm3 as you can before measuring the final few drops with a pipette. As with all measurements, readings should be taken from eye level.
Add the sodium thiosulphate to the conical flask before measuring the specified amount of water from a tap (if necessary) into the 10cm3 measuring cylinder and then into the conical flask.
Now measure the amount of hydrochloric acid into the second 10cm3 measuring cylinder, adding the last few drops with a pipette as with the sodium thiosulphate.
Set the stop clock to ‘Count Up’ and begin to time the reaction just as the hydrochloric acid is being added into the conical flask.
Do not stir the mixture as the extra energy given to the molecules would increase the rate of reaction due to the collision theory.
From head height directly over the conical flask, the clock stop should be stopped just as the cross’ (X) formal shape is too unclear to be seen properly.
Results
Note: All results are in seconds. In results where experiments had to be repeated, the improved value is used to find the average. In all results the amount of sodium thiosulphate used was 60cm3 of 0.1 mole dm-3.
The result for 1/time is to 3 significant figures.
Analysis
These results show that as the concentration of the hydrochloric acid decreases so does the rate of reaction.
The graph titled ‘The time taken for a reaction between sodium thiosulphate and hydrochloric acid (HCl) to reach a certain point with differing concentrations of HCl’ show that there is a more or less steady slope going down from right to left. This shows that it did take longer for the reaction to take place when the hydrochloric acid was at lower concentrations because at lower concentrations there were less HCl molecules colliding successfully with the sodium thiosulphate molecules per second. At 1 mole dm-3 of HCl it took an average of 54 seconds for the reaction to take place, at half the concentration, 0.5 mole dm-3 of HCl it took an average of 78.3 seconds, roughly 0.69 times longer.
The graph entitled ‘Graph showing the rate of reaction between sodium thiosulphate and hydrochloric acid (HCl) with differing concentrations of HCl’ shows that the faster a reaction takes place, the shorter the time is needed for the reaction to finish; the speed of the reaction is inversely proportional to the time taken for the reaction to finish.
S∝1/Time, 1/Time∝Concentration ∴ speed of reaction∝ Concentration
The graph showing 1/Time (the rate of reaction) is more or less a diagonal line with a gradient of approximately 1.3
The scientific theory that more HCl molecules would react with the sodium thiosulphate to produce a higher rate of reaction is correct; as all the other variables were kept constant.
This supports my prediction (‘as the concentration of the hydrochloric acid is higher the less time it takes for the chemicals to react and therefore the rate of reaction will be higher’) as the graphs and results clearly show.
Evaluation
I believe the results attained in this experiment are accurate enough due to careful measuring and use of clean apparatus.
The one anomalous result could have been caused by an inaccurate measurement of the water, hydrochloric acid or sodium thiosulphate. It may also have been bad judgement on when the cross became un-visible. Ways to improve on this are given below.
The scale used for the graph was very good as it allowed clear points to be made and was as big as necessary, however if I was to redraw the graph I would put the time on the Y axis as it is the variable that changes as a result of the concentration of hydrochloric acid. The graphs are still suitable for noticing patterns and are correct though.
I think the range of concentration used was suitable enough to provide me with accurate results; as I mentioned before, when the hydrochloric acid was used in lower concentrations it seemed there would not be enough energy for the reaction to properly take place.
If I were to repeat this experiment there would be several ways in which I could improve it.
Firstly, as it is hard to judge when the cross becomes un-visible, I would use a light probe as shown in the diagram below. It would be connected to a computer to record the time it took for the light intensity to drop below a pre-determined level. This would make sure that the level of the reaction reached was the same each time.
To further increase the accuracy of the experiment I could use burettes to measure the quantities of hydrochloric acid, sodium thiosulphate and water.
All experiments could be done on the same day so there would be less variation in weather; for example it may be cloudy one day but extremely sunny another. This would ensure that factors you cannot control do not dictate the results of the experiment.
To be extra sure the temperature of the sodium thiosulphate and the hydrochloric acid could be measured to ensure the acids’ energy is equal throughout the experiments.