I will attempt to ensure that the experiments I do will be fair tests. I will do this by naming some things as variables and some things as constants.
Variables
- I will change the concentration of the sodium thiosulphate, by adding water to it, to observe the results at higher and lower concentration.
- The volume of the acid may be changed in the preliminary experiments, to find a sensible time for the cross to disappear in the final experiment.
Constants
- The concentration of the hydrochloric acid will remain the same.
- The volume of the hydrochloric acid will remain the same.
- All the apparatus used – the conical flask, the piece of paper with a cross on it, the measuring cylinders and the stop clock will be the same for each experiment.
- I will use the same eye, as some people have different eyesight in each eye, and I know for a fact that I do. I will be using my right eye because it is my better eye.
- I will also perform the experiments in the same part of the room, as sunlight or shadow might have an effect on the results.
- I will also keep the temperature of the solutions constant - at room temperature. I will check this with a thermometer, and if the temperature is too cool, I will place the solutions in a warm water bath. If the temperature is too high, I will cool the solutions by placing them in an ice bath, but this is unlikely to happen.
I will keep constant the temperature because a higher temperature increases the number of collisions in a reaction. If the temperature is increased, the reactant particles move faster. When they move faster there are more collisions, which leads to more successful collisions, which leads to a faster rate of reaction. If I increased the temperature for this experiment, it would make it an unfair test because I want to observe the effect of concentration, not the effect of temperature.
The surface area of a reactant can also speed up the rate of reaction. This is because if a reactant solid is in small particles rather than large lumps of solid, its surface area is increased. This means that the particles around it in the solution will have more area to work on so there are more collisions, so there are more successful collisions, leading to a faster rate of reaction.
I will not have to keep this constant as I am using two solutions in my experiments.
A catalyst can also speed up a reaction. A catalyst works by attracting the particles to it, so they all collide and so there are more successful collisions, leading to a faster rate of reaction.
I will not have to keep this constant, as I am not using a catalyst in my experiment.
I performed five trial experiments. I did this to find good concentrations and volumes for my final experiment. I did the experiments with the highest concentration and the lowest concentration first to make sure that they didn’t take too long or too short a time. I then did another experiment with concentrations in between the highest and the lowest, and repeated it twice, increasing the volume of acid by 5 cm3 each time to see if this would make any difference.
I poured out the correct amount of sodium thiosulphate, hydrochloric acid and water into separate measuring cylinders, and then I poured the sodium thiosulphate into the conical flask. I then added the hydrochloric acid to the conical flask and started the stop clock. I observed how long it took for the solutions to go cloudy, from a clear colour, and stopped the stop clock when I could no longer see the cross. In between experiments, I washed the conical flask with distilled water.
Here is a table of results for my trial experiments:
I have found from these trial experiments that the reaction was slightly faster when more hydrochloric acid was added. As I increased the volume of the hydrochloric acid, the number of hydrogen ions increased, increasing the speed of the experiment, which is what I expected.
Therefore I will use 20 cm3 of hydrochloric acid in my final experiment because this will give a slightly faster and more sensible time for the reaction to occur.
OBTAINING EVIDENCE
- Conical flask.
- Laminated paper with a cross drawn on it.
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3 measuring cylinders – with volumes 50 cm3 for water, 50 cm3 for sodium thiosulphate, 25cm3 for hydrochloric acid.
- Stop clock.
- Distilled water bottle.
- Thermometer.
- Beaker for warm water/ice bath (if necessary).
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Sodium thiosulphate solution, 0.12 Mol 1-.
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Hydrochloric acid solution, 1Mol 1-.
- Firstly, I will ensure that all preliminary safety procedures have been carried out, for example, putting on my lab coat and goggles, clearing my work area and making sure there are no reactive substances close by.
- Then I will gather together all the apparatus needed for the experiment.
- I will measure out the amounts of the solutions needed into three separate, labeled measuring cylinders.
- I will then add the sodium thiosulphate to the conical flask.
- To the conical flask, I will also add the amount of water required (if any).
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I will take the temperature of the sodium thiosulphate (and water). The constant temperature for my experiment is 22°C.
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If the temperature is more than 22°C, I will place the conical flask into an ice-bath. I will do this by adding ice to an appropriately sized beaker, and placing the conical flask into this. I will keep taking the temperature of the sodium thiosulphate (and water) until it reaches the desired temperature (in this case, 22°C). Then I will remove it from the ice-bath and continue as normal.
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If the temperature is less than 22°C, I will place the conical flask into a warm-water bath. I will do this by adding warm water to an appropriately sized beaker. If necessary, I will heat the water over a Bunsen burner, standing the beaker on a tripod and gauze. Then I will add the conical flask to this. I will keep taking the temperature of the sodium thiosulphate (and water) until it reaches the desired temperature (in this case, 22°C). Then I will remove it from the warm-water bath and continue as normal.
- Then I will add the hydrochloric acid to the solution in the conical flask.
- As soon as the hydrochloric acid has been added, I will start the counter on the stop clock and swirl the conical flask slightly to ensure the solutions are all mixed.
- I will look into the conical flask from directly above, using one eye only, and stop the stop clock immediately the cross disappears from view.
- I will note the time taken, and then empty the contents of the conical flask into a sink. Then I will wash the flask well in distilled water.
- I will repeat steps 3 – 10 as many times as necessary.
I have carried out my experiment according to my plan, and I have performed it according to all the rules of safety and fair test I mentioned earlier.
ANALYSIS
On my first graph, it shows that as the concentration of the sodium thiosulphate is increased, the time for the reaction to occur decreases. This is shown by the fact that when the concentration is at its lowest, 0.024 Mol 1-, the time taken for the experiment is its longest, at 342 seconds. When this is compared to the highest concentration, 0.120 Mol 1-, which reacts in a time of just 40 seconds, we can see that the higher the concentration of the sodium thiosulphate, the shorter the time it takes to react.
On my second graph it shows that as the concentration of the sodium thiosulphate is increased, the rate of reaction also increases. This is shown by the fact that when the concentration is at its highest, 0.120 Mol 1-, the rate of reaction is also at its highest, at 0.025. When this is compared to the lowest concentration, of 0.024 Mol 1-, which has the lowest rate of reaction, of 0.003, we can see that the higher the concentration of the sodium thiosulphate, the higher the rate of reaction. This shows a similar result to my previous graph – the higher the concentration of the sodium thiosulphate the faster the reaction takes place – i.e. the faster the rate of reaction. As the rate of reaction has increased, this is the same thing as the time decreasing – it takes a shorter amount of time for the reaction to occur the higher the concentration of the sodium thiosulphate.
As I stated in my plan, the rate of reaction and concentration are directly proportional. If the concentration was doubled, the rate of reaction would also be doubled. This is because there would be twice as many particles to react, so it would take half as much time to react, leading to a rate of reaction which is twice as fast – i.e. doubled. I can now test this using my graph. The rate of reaction at 0.004 Mol 1- is 0.008. Therefore the rate of reaction at 0.008 Mol 1- should be 0.016. If I look at my line of best fit I can see that this is true. This means that my results were accurate.
These results can be explained by the particle theory, which states that the particles must collide for them to react. If there is a greater concentration of sodium thiosulphate it means that there will be more sodium ions, which means that the time it takes for them to react with the hydrogen ions in the hydrochloric acid will be shorter, and the rate of reaction will be increased. This is because there will be more collisions if there are more particles, therefore there will be more successful collisions, therefore the reaction will occur in a shorter time.
I predicted that ‘the higher the concentration of the sodium thiosulphate, the faster the reaction will be, so the cross will disappear in a shorter amount of time. The rate of reaction will increase the higher the concentration of the sodium thiosulphate.’
If I compare this to what I have found from my experiment results and my graphs, I can see that I was correct and that my results support my prediction that the rate of reaction is at its highest when the concentration of the sodium thiosulphate is at its highest.
From this I can conclude that the higher the concentration of the sodium thiosulphate, the higher the rate of reaction was, and the reaction occurred in a shorter amount of time, and that this could be explained by applying the particle theory.
I can be fairly certain about my conclusion. I can only be fairly certain because in some of my results there are a few inaccuracies. For example, in experiment 5 there is a range of 31.77 seconds between the highest and lowest times it took for 10 cm3 of sodium thiosulphate and 40 cm3 of water to react with 20 cm3 of hydrochloric acid. The most accurate results are for experiment 1, with only + or – 1 cm3. However, when the averages are used to plot the graphs, there are no anomalous points, so this shows that the inaccuracies in my results have made no drastic differences. This means that my results are accurate enough to support my prediction.
EVALUATION
On the whole, I think that my experiment was fairly successful. My results are quite accurate and fit in with my prediction. However, there are always improvements to be made in any experiment.
There are no very anomalous points on my graph for rate of reaction, which means that my results are accurate enough to support my prediction. There are some points which do not fit exactly the line of best fit, and this shows some of the inaccuracies in my experiment.
I also think that my results are reliable because the repeat readings are fairly similar. However, in experiment 1 there is a range of 1.53 seconds between the highest and lowest value, which, being 4%, is quite high.
To gain additional evidence to make my investigation more clear, I could extend my experiment to include even more repeats and additional values of the concentration of sodium thiosulphate.
There are several inaccuracies in my experiment which are caused by human error. These occur because of inadvertent errors made by the person carrying out the experiment. The following are human errors:
People do not always have the same eyesight in each eye. I know for a fact that I do not, and that my left eye is the weakest. Therefore in my experiment I always used my right eye when looking into the conical flask.
Another factor that might affect the results would be when the stopclock is started. It is meant to be started when the hydrochloric acid is added to the sodium thiosulphate and the water in the conical flask. However, it would be impossible to start the clock at the exact instant the acid is mixed with the sodium thiosulphate and water each time the experiment is done, especially if one is working alone. There are also inaccuracies when the stop clock is stopped.
To find out exactly the inaccuracies of the apparatus in my experiment, I can calculate the percentage errors to give me an idea of how great these inaccuracies are.
I have calculated the percentage errors for my experiment using the equation:
% error = instrument error on reading(s) x 100
quantity measured
These are shown in the following table:
* I have chosen 10 cm3 because it will show the greatest error when using the 50 cm3 measuring cylinder.
** 40.49 is my lowest average time. I have used this time because it will show the greatest error when using the stop clock.
So the total percentage error for my experiment is 13%. This means that there will be a certain amount of inaccuracy in my experiment. However, although it is impossible to perform any experiment without some errors, I think 13% is too high a percentage.
Also, this percentage can only allow for apparatus error, not human error. Therefore the total error could be greater than 13%, but probably not by a huge amount.
To reduce the apparatus error I could increase the volumes of the solutions. This would decrease the percentage error, but it could be impractical to use very large amounts.
I think my method was suitable and was performed as accurately as possible. However, some changes in apparatus or method might make it more accurate if I were to do my experiment again.
For example, a burette instead of a measuring cylinder for the sodium thiosulphate, water and hydrochloric acid would have been more accurate. A burette has an error of only ± 0.05 cm3 compared to one of ± 0.5 cm3 for a 25 cm3 measuring cylinder, and ± 1 cm3 for a 50 cm3 measuring cylinder.
Another change that could be made to the method would be to swirl the conical flask in the same direction, i.e. clockwise, every time it is swirled. This may seem like a small detail, but it may make some difference.
One solution to the problem of knowing when to stop the stop clock might be to have some sort of light sensor attached to it that would stop it when no light is let through, i.e. when the cross would no longer be visible because the solution would be cloudy. This would be a better way of finding the time, because eyesight is unreliable.
I could also improve the temperature error by using a controlled water bath with a thermostat on the front. This would eliminate the possibility of the temperature changing during the experiment and affecting the results.
I could have extended the range of concentration to gain more evidence and more results to make the investigation more clear.
So the evidence I obtained, having so many errors, is not entirely accurate. However, there are only a few slight anomalies on the graphs and, despite these, it is still sufficient to support the conclusion because my results were basically, but not exactly, correct.
So, in conclusion, I have proved my hypothesis correct by using the results of my experiment and explaining them with the particle theory, and I think that although there were some inaccuracies in my experiment, it has nevertheless been a success.
