More collisions increases the Rate of reaction
The theory also states that:
Faster collisions are ONLY caused by increasing the temperature
Reactions ONLY happen if the particles collide with enough energy. At a higher temperature there will be more particles colliding with enough energy to make the reaction happen. This initial energy is known as the ACTIVATION energy, and its needed to break the initial bonds.
Method:
As mentioned above, I will be using the precipitation method and will therefore need to mark a cross on a spotting tile and time how long it takes for the cross to disappear. I know the solution will cloud because the reaction produces sulphur which I know from past experience turns a solution cloudy.
To carry out the experiment, I will fill a beaker with the correct volume of HCl (as determined in the table shown below) and then pour in 50cm3 of Na2S2O3 . At this point the timer starts, and I will stir the beaker constantly to act as an ‘artificial catalyst’ – to make sure the reaction is taking place at all places in the beaker. By my judgement when the cross marked on the spotting tile has completely been obscured from view I will stop the timer and record the time. I will produce three repeats for each concentration which allows me to exclude any obvious anomalies and record an average. I have chosen to study four different concentrations of solution, in order to allow me to compare results and plot an accurate graph for my analysis.
Apparatus:
I will be setting up the experiment in the following way, using the following equipment:
Fair test considerations:
There are a number of things I must take into account to make my test a fair one. Firstly I must make sure it is only one person making a judgement about when the ‘X’ mark disappears as different people will without question have different matters of opinion. There must also be clean apparatus every time, so every time a beaker of measuring cylinder is used, it is washed very thoroughly to avoid contamination. I also used the same person on the stopwatch every time.
I will also be taking three repeats for each concentration in order to exclude anomalies and produce an average for each dilution.
The final point is I have to make the concentrations of the solutions accurately and constantly. There is no point in being rough when pouring out solutions as this will only lead to inaccurate results which will appear anomalous when plotted on graphs. The concentration dilution figures are shown below in the table:
Safety:
I will be taking into account some safety precautions to ensure safety in the lab:
- Wear safety goggles to protect the eyes.
- Avoid chemical contact to hands/skin.
- Pour out chemicals under supervision.
- Remember to wash hands thoroughly after experiment.
Preliminary tests:
For my preliminary tests it is necessary to find out which chemical I will be keeping as a constant and which one I will be using as a variable. I will initially test Hydrochloric acid (HCl) as a variable by varying the dilutions and I will keep Sodium Thiosulphate (Na2S203) as a constant. I will be using the same method and apparatus as shown in the relative sections of my plan.
The results for the preliminary work are shown below:
Varying the concentration of HCL:
What do the results tell us?:
The above results therefore lead me to believe that Hydrochloric acid has NO effect on the rate of reaction and will not be a studied variable; it will be used as a constant. It is clear that whatever concentration of HCL is included in the solution does not make a difference, as the averages for the four repeats are roughly the same. This indicates that there must be some relationship between Na2S2O3 and the time taken for the cross to disappear on the spotting tile. I do not need to draw graphs for these results as all the graph would show would be a straight horizontal line across the sheet.
Preliminary background work:
- Glassware – I chose to use beakers in my experiment in preference to conical flasks for a few simple reasons. Conical flasks have large bases and narrow necks, while beakers have a consistent shape and has the same depth throughout. Conical flasks are more awkward to stir and due to their inconsistent shape my cause different rates of reaction in some part of the flask due to different surface areas.
- Stirring – this was necessary as it made sure the chemicals were mixed and the reaction was taking place under conditions of maximum efficiency.
Prediction:
I predict that varying the concentration of Sodium Thiosulphate will have a positive effect on the speed of the reaction. Hydrochloric acid has no effect on the rate of reaction as shown in my preliminary tests. I believe that when I vary the concentration of Sodium Thiosulphate the results will be inversely proportional to time due to the fact that it makes sense if results take less time when they react faster (inverse proportion). This means that as the concentration of Sodium Thiosulphate increases, the time for the black ‘X’ to obscure will be even less and less. This theory is backed up by my knowledge of the collision theory, explained below in my preliminary background work. I also believe my results to show that the inverse proportion of time to concentration theory will be proved – for example if I double the concentration of HCl from 1 mol to 2 mol the time taken for the ‘X’ to obscure will be halved. This is better shown in the graphs below:
I predict that the graphs I plot will show the following trend:
Creating the different concentrations of Na2S2O3 , a table showing the ratios:
I used the 0.15 molar solution (the stock strength solution) as a base and made dilutions from there. The above table explains how I diluted the Na2S2O3 into the necessary solutions.
Results for the varying concentrations of Na2S2O3:
By collecting the results using the same method as shown above I tabulated my results for the varying concentration of Sodium Thiosulphate.
I then used the average column for each result and plotted the (inversely proportional) rate over time figures as shown below:
Observations:
While the reaction was taking place, the solution produced an egg like odour. As shown in the chemical equation for the reaction on sheet 1, this is due to the sulphur oxide that is being produced. I could obviously see the solution turning cloudier after a period of time – as this is what I measured, which was due to the formation of insoluble sulphur in the beaker. We used a stopwatch accurate to 1/100th of a second, i.e. it had 2 decimal places. I obviously knew that the way to increase precision with this type of experiment was to collect as many repeats as possible. I noticed before we began the experiment the large range size (2 molar to 0.5 molar solutions).
What the results tell me:
As you can see from the results, the speed of the reaction was inversely proportional to time as concentration increased in the sodium thiosulphate. This fact agrees with my prediction. In this case the inverse proportion indicates that the higher the concentration of sodium thiosulphate the lower the time it took for the cross to become obscured. From the results I plotted the two following graphs:
Graph 1: Concentration of Na2S2O3 against the time taken for the mark to obscure
Graph 2: Concentration of Na2S2O3 in mols against rateˉ¹
As you can see the graphs I drew matched the ones I had predicted, proving my hypothesis correct. There is clearly a quantitative relationship between the concentration of Na2S2O3 and the time it takes for the mark to be obscured – again matching my prediction. This is proved by looking at the graph – for example as the concentration doubles from 0.075 mols to 0.15 mols the time goes from 64.4 seconds to 31.2 seconds which is extremely close to half. To put this more simply, I conclude that from my experiment that the greater concentration of sodium thiosulphate the faster the reaction in inverse proportion to time.
Summary of findings and Evaluation:
During the experiment I have discovered a number of things. The first is that it is the concentration of Sodium Thiosulphate that affects the rate of reaction of this experiment, and NOT Hydrochloric acid. I have also discovered that, as my hypothesis supports, that the rate of reaction was inversely proportional to time. I drew two graphs based on my findings, the first of which shows a curved line on a steady decline which begins to level out as the concentration of Na2S2O3 decreases. This indicates to me that my theories were correct. The second graph indicates that when the results are plotted as an inverse proportion to rate that there is a definite straight-line pattern.
I did not have any anomalies to identify but had to repeat one set of data (0.0325 molar) as the results were all different. When I was recording my results for this set the three repeat that I obtained were 98.63, 163.8 and 120.49 seconds, a clearly For all of these reasons I can conclude that the experiment was an almost complete success.
The proportional results prove to me that the collision theory affects the atoms in such a way that when the solution concentration is doubled, the atoms collide twice as much, giving an inversely proportionate set of results.
In conclusion I must state that I felt my experiment was an almost complete success. There were no anomalous results to be identified which allowed me to collect a much more accurate set of data. There was a problem when using the 0.0325 molar solution however, as when I first did this test I had three very varied results – 98.63, 163.8 and 120.49 seconds. I then redid this set of results in order to make my experiment more accurate. The possible reasons for the strange results were most likely down to human error – for example not cleaning the apparatus properly or a mix-up in chemicals.
The apparatus I used in the experiment proved itself to be fairly accurate as my results are shown to be consistent and reliable by their small group sizes. The greatest cause for human error within my results would probably be down to the human judgement factor, as it is fairly impossible to accurately tell when the mark had been obscured. The possible solution for this problem is to use a light-gate and a computer to record the results, but this is much more time consuming that using the human eye. I made sure that the apparatus I used was clean every time and that I was the only person making the judgements on when the mark became obscured in order to make the test fairer. To improve the accuracy of the concentration dilutions I could have used a burette or pipette as these allow a chemist to make more accurate solutions. It was also necessary to keep the stirring of the solution at a constant rate. No matter how steady your hand may be it is impossible to keep the same stirring speed throughout the experiment and prompts perhaps an ICT solution to replace a human stirrer.
My results I gathered were both accurate and relevant. I was able to identify that hydrochloric acid was a useless variable to study after my preliminary tests and that I needed to use sodium thiosulphate. I collected and tabulated a set of results concisely, fairly and accurately, and this is proved on the graphs that I’ve drawn and the theory I’ve researched. This thus means that my conclusion is valid.
I also needed to consider other variables to study in greater detail, and how they might increase other variables leading to increased reactions – for example temperature; is it true to say that an increase in rate might be down to temperature, and then a further increase of temperature causes an even increased reaction time?
There is also the matter of the apparatus we used. In an ideal world we would use new, unused, perfectly clean and flawless glassware and measuring equipment. This however, is not feasibly possible and we were not using totally dry glassware for each test, which may lead to a false reading in our results – again something incredibly time-consuming to work around.
By James Muir
