* Surface area (Type ie concentration is controlled) - increasing the surface area increases the number of surface particles exposed. This means that there is a higher chance of particles colliding with one another and reacting. Although sodium thiosulphate does come in powder form, surface area would be quite difficult to measure. Continuous data would be near impossible to obtain unless each granule was carefully measured and increased by a set amount. Using big, medium and fine granules could produce non-continuous data, although this couldn’t be used to make a line graph.
* Pressure (Type ie concentration is controlled) - by increasing the pressure put upon a gas you increase the chance of particle collision by pushing the particles closer together, thus speeding up the reaction time. As my reactants aren’t gases, this is not applicable to my experiment.
* Catalyst (Type ie concentration is controlled) - a catalyst reduces the activation energy for a certain reaction. It does this by providing a surface for the reactants to collide upon, and helps to break bonds. Although I have researched over the Internet, I cannot find a suitable catalyst for my reaction. Even if I was to trial different substances I cannot get continuous data.
* Concentration (Type ie concentration is controlled) - increasing concentration increases the amount of active reactant particles in a given volume. This means that there is an increased chance of collision so the frequency of collisions increases, and the reaction rate will speed up. Performing my experiment with increasing concentrations of hydrochloric acid would be relatively straightforward to do and would produce a line graph of continuous data.
You could also include a variable table:
I have decided to investigate concentration, as it is the simplest experiment to perform with the equipment I have available. If I had access to water- baths I could investigate temperature as well, but I don’t, and I would be unable to keep a set constant temperature. This would mean my results would be very scattered. Because of this, I have chosen to use concentration as my independent variable.
I predict that as you increase the concentration of HCl, the reaction time will decrease. This is because of particle theory explained above under concentration: as the concentration is increased, the number of reactant particles in a given volume increase, increasing the chance of collision and speeding up the reaction. I also predict that the relationship between the rate of reaction and concentration will be directly proportional, and will show up on a graph as a straight line of x=y. This I will show as a graph after I have obtained my results.
To get valid data I will have to keep all other factors the same and change only concentration. This means that temperature, surface area, pressure, amount of light, cross and apparatus will all be kept the same. The temperature will be kept fairly constant by performing my experiment on the same day, away from direct sunlight, in a cool classroom. Surface area and pressure are irrelevant for this reaction as the reactants are both aqueous, which means the surface area cannot be changed and pressure will not have an effect. The amount of light will affect how easy the cross is to see, and so this will be kept constant by keeping it in the shade out of direct sunlight. The cross will be the same each time so it doesn’t become easier or harder to see by being slightly less striking or defined. My apparatus will be the same for each experiment and is shown in the diagram of the experiment.
Before I do this experiment I will need to do some preliminary work to find the limiting factors. I will need to decide what concentrations to use finding out what concentration is so fast it is instantaneous, and how low the concentration can be so that it still produces enough Sulphur to extinguish the cross. If I take the concentration down too low there will become a point where enough Sulphur is never produced, and this I will need to find.
This is a diagram of my experiment; finding the limiting factors of concentration:
The results of my preliminary experiment are shown below. I have rounded each result to the nearest second, as my timings will not be completely accurate because of my reaction times and slight human error. Milliseconds are inappropriate as they are too fine a measure. 10ml of varying molarity acid has been added to 50ml of Sodium Thiosulphate in a glass beaker with a cross marked on paper beneath it, the stopwatch started, and the time taken for the cross to no longer visible is recorded. The results of my preliminary experiment are shown below:
Overleaf is a sketch-graph of these results to illustrate the curve that I am expecting from my main experiment. From the results I now know that there is no point of using a concentration any lower than 0.02M HCl as the reaction takes too long, and may never produce enough Sulphur precipitate to extinguish the cross. If I take concentrations greater than 0.10M, the difference in time taken between the results becomes progressively smaller and my experiment results would be more liable to error and the trend of data would be more difficult to decipher. For these reasons I will be taking the data range from 0.03M – 0.10M at 0.01M intervals.
Each molar concentration used will need to be precise in order for me to find a pattern, so I shall be measuring each concentration out separately from a base of 0.10M acid. 10ml of varying strength acid shall be added to 50ml Sodium Thiosulphate so that the total volume is constant; unlike if varying amounts of 0.10M acid were added which would mean that the total volume would change each time. 10ml of each concentration will be made up, and the ratio of acid to water are shown below:
Method
- Gather equipment, Sodium Thiosulphate and 0.10M HCl. For each repeat of the experiment you will need approx. 60ml acid and 400ml Sodium Thiosulphate. Put some water in a beaker and put a pipette in each of the beakers; water, hydrochloric acid and sodium thiosulphate. Mark each pipette so that cross-contamination does not occur. This is done because even a slight amount of acid placed into the sodium thiosulphate would off the reaction.
- On a piece of plain white paper, draw a bold cross in permanent marker. Make sure that the cross is smaller than the diameter of the base of the beaker so as not to be visible round the edges. This is so that the cross is completely vanished once the solution becomes opaque with no guiding outlines to give a hint to the eye as to the whereabouts of the cross.
- Measure out 50ml of sodium thiosulphate using the 50ml measuring cylinder, and put into the 100ml glass beaker, on top of the piece of paper marked with a cross.
- Make up the first concentration using the 10ml measuring cylinder, which is 10ml of straight 0.10M acid. Get ready to push the “start” button of the stopwatch as the acid is poured into the glass beaker containing sodium thiosulphate. Press start, and swill the sodium thiosulphate and hydrochloric acid around in the beaker.
- Keep observing the reaction, checking whether the cross can still be seen. Once the solution becomes opaque and the cross is no longer visible, press “stop” on the stopwatch and write down the time to the nearest second.
- Repeat with each of the concentrations 0.09M, 0.08M, 0.07M, 0.06M, 0.05M, 0.04M and 0.03M.
- Repeat the whole experiment at least once, so as to get more data and make sure your results are reliable. Work out the averages of the repeats and enter this into a chart, the chart for my data is shown on the next page.
These results have been put into a graph of time vs concentration. This shows the curve of the data, and allows me to highlight any anomalous data. It clearly shows the curve that occurs, that as concentration increases, time decreases at a decreasing rate.
There is another way to show reaction rate that I have researched, and I have drawn a graph of this also. To find the reciprocal of the time taken for the cross to disappear against temperature, I have divided 1 by the time taken for the cross to disappear. This is because in this experiment:
Reaction rate = amount of sulphur produced
time taken for cross to disappear
In my experiment the amount of precipitate is the same each time for each concentration, as it takes the same amount of precipitate to extinguish the cross. So, this same amount of sulphur produced can be valued at ‘1’. So:
Reaction rate ∞ 1
time taken for cross to disappear
This I have drawn as a graph and you can clearly see a straight line, although my results are quite scattered. Results for each of the experiments are quite different, test 1 results being consistently slower than the time taken for test 2 results. I noticed that the sodium thiosulphate I used for test 2 results was slightly cloudy, which suggests that it had been contaminated slightly with Hydrochloric acid. If some acid had been misplaced into the sodium thiosulphate then the reaction would have already started, and this would mean my results for experiment 2 to be slightly quicker. This seems to relate to my data and because of test 2 results being too quick the line of best fit may be slightly out. If I was to do more repeats I could afford to discount the whole of test 2 results, but there is not enough time available for me to do this. The contamination of sodium thiosulphate with Hydrochloric acid was likely caused by mixing up the pipettes when measuring out different quantities for the individual tests.
The pattern from both graphs clearly shows that as concentration increases, the reaction rate increases also. This proves my predictions are correct: as the concentration is increased, the number of reactant particles in a given volume increase, increasing the chance of collision and speeding up the reaction. The rate has been proved to be directly proportional to concentration, as I predicted, but the line of best fit is offset from the origin.
The line of best fit in the concentration vs. time graph tends to infinity when the concentration is at zero. This is as you might expect because when the concentration of Hydrochloric acid is at zero then there is no reactant particles to react, as 0.00M HCl is purely water. However, at the other end of the line of best fit the reaction time tends to a constant level of approximately 50 seconds. This means that however much the concentration is increased over 0.10M, the reaction time will not decrease and so the reaction rate will not increase.
This is because of a limiting factor; that the Sodium thiosulphate is at a constant concentration. This means that there are a set number of sodium thiosulphate particles per given volume, and there comes a point where no matter how many acid particles are added, there are only so many sodium thiosulphate particles for a set amount of product. From the concentration vs. time graph we know that the reaction time reaches a constant of around 50 seconds, however if sodium thiosulphate concentration was increased the reaction should be faster when a higher concentration of hydrochloric acid is added. Conversely, if the sodium thiosulphate were less concentrated then the constant time would be slower.
Thus we can change the maximum rate of reaction by not only changing the concentration of hydrochloric acid but also sodium thiosulphate. There are other variables that have been limiting factors in my experiment that could be examined adjunct to this investigation, some of these are listed below.
In my experiment the tests took place at a constant temperature (room temperature), however I expect that varying the temperature will have an effect on the reaction rate. This is because increasing the temperature gives the particles more energy, which means the activation energy is reached easier, and the reaction will take less time to make a certain amount of precipitate. Conversely decreasing the temperature will lengthen the time taken for a reaction. This could be investigated further to find exactly how much effect temperature has on the reaction of sodium thiosulphate and hydrochloric acid.
Another factor that may affect the rate could be a catalyst. A suitable catalyst would increase the reaction rate by reducing the activation energy needed for the reaction to take place. Potential catalysts could be investigated adjunct to this work to increase the reaction rate, and to investigate by how much they speed things up.
My work would be a lot more accurate if more repeats had been done to replace the erroneous results of test 2, mentioned above. Even so, my work proves that concentration does have a substantial effect on the rate of reaction in the chemical reaction between Hydrochloric acid and Sodium Thiosulphate. As concentration increases, the time taken to produce a set amount of precipitate product reduces up to a certain point where limiting factors limit the decrease in time.
