I can clearly see from this graph that as the concentration increases, the time for the reaction to take place decreases, therefore, rate is proportional to concentration.
This experiment shows exactly what I should find in the experiment I will do for my coursework. As the concentration increases, the rate of reaction also increases.
There are other factors I will have to keep constant in this reaction. The first is temperature. Since this is an exothermic reaction, I will have to find a way to keep the solutions and the atmosphere around at a certain temperature. If there is a high temperature, the energy between the particles will increase, therefore there will be more collisions per second, which will affect my results. Heat causes molecules to speed up and therefore changes the rate.
The second factor is the volume of thiosulphate. I will probably be using around 20cm³ of sodium thiosulphate as if I use different amounts with each of the concentrations, my results will be unfair. More thiosulphate will mean more particles for the hydrochloric acid molecules to collide with. I will need to keep the total volumes the same for all six solutions as well as this will also affect my results if varied. I will probably be using 40cm³ of solution in total, but with varying concentrations of hydrochloric acid. Atmospheric pressure may also need to be kept constant as on a colder day, the reaction will happen slower than if on a hotter day. I will need to make sure I complete the experiment in the shortest amount of time possible and on the same day. The apparatus is also another important factor to keep control of. I will need to make sure that I use the same apparatus for each concentration because if it is varied, a solution that is of a higher concentration may enter a solution that is of a lower concentration. I will need to clean the conical flasks , beakers and thermometers because the bi-products of this experiment have been known to leave some sort of residue on the glass, therefore obscuring my vision and my results. Another good idea would be to label each flask as not to get them mixed up when doing my experiments. This could cause everything to go wrong as if I get the conical flasks muddled up then I would have to start from scratch due to atmospheric pressure differences. To ensure accuracy I will repeat my experiment twice, unless my results don’t match, in which case I will attempt the experiment a third time.
Safety
Some of the chemicals used in the experiment are quite dangerous. I will need to take many precautions. I will wear a lab coat and goggles so if any chemicals splash, they will not harm me at all. I will make sure that when I empty my solutions, I will pour them into the fume cupboard sink so I do not damage the sinks or any who may touch the sink. I will ensure that I do not run around whether or not I am holding the apparatus. I will keep my experiment in the centre of the table with all my books packed away apart from a sheet of paper I will write my results on. If I manage to spill acid on myself, I will walk calmly to the sink whilst screaming in pain and wash it off. I will also make sure I do not inhale the solution as this may irritate my body and finally I will make sure I report and spills to my teacher immediately.
Apparatus
Hydrochloric Acid- The amount will change during my experiment
Sodium Thiosulphate- This will be kept the same during my experiment at around 20cm³
Beakers- I will use these to pour the raw chemicals into. They will be 250cm³ as I will not need any bigger and smaller would be unsafe.
Conical Flask- This is the piece of equipment where the apparatus will take place. This will be around 250cm³ also.
Thermometer- This will be used to keep the temperature constant. It will probably go up to 100 degrees centigrade as the heat produced will not be more than that.
Distilled Water- This will be used to clean the apparatus and to mix the hydrochloric acid with to obtain my separate concentrations.
Burette- This will be used to measure out the exact amount of hydrochloric acid and sodium thiosulphate and water. 100cm³ should suffice.
Retort stand and boss head clamp- This will be used to hold the biurette.
Stop clock- This will be used to measure the length of the reaction. Millisecond accuracy should suffice.
White tile and black cross- This will be used for accuracy and to judge when to stop the timer.
Diagram
Preliminary Investigation
The preliminary investigation was very important because it told me what elements within my experiment that I may have needed to change or methods I use that would make my experiment inaccurate. I took the preliminary investigation by assembling all the apparatus above, and followed my experiment through with only two different concentrations: the highest- 2 molars- and the lowest- 0.25 molars. I poured 20ml of sodium thiosulphate from a burette into a beaker and 20 ml of hydrochloric acid into a separate one. I measured the temperature of both substances and recorded them. I mixed the substances in a conical flask which was over a black cross and took measured the time taken.
The preliminary investigation showed results that I did not expect. The first reading I took for the solution with 2 molars of hydrochloric acid showed a time of 12.56 seconds and for the second try, 11.21 seconds. This was expected because the acid and sodium thiosulphate should react quickly under these conditions of concentration. This is because the collision theory states that as more particles of hydrochloric acid are available, the reaction speed increases. The third result I obtained also backed up this because the speed was 12.58 seconds. There were a few problems with the diluted hydrochloric acid of 0.25 molars. This is because the results I took were only slightly faster than the high concentration of hydrochloric acid. The results I took varied from 17.34 to 19.36 seconds. This is only a difference of about 7 seconds, which is not what I had expected. I would have thought that there would have been at least a 60 second difference because there was much less concentrated acid placed into the second flasks. The collision theory states that as the particles of acid are reduced, there will be fewer reactions per second and therefore a longer time. The results I obtained may have come up because through chance, more particles were colliding and with the correct orientation. This may lead to a closer result. In table form, the preliminary results looked like the following:
The temperatures I took for the experiments showed that there was a change by 1 degree. This meant that the temperature went down, instead of going up, as I had expected. This shows that the solution was cooling as the reaction progressed. This is unexpected because usually when a reaction such as this happens; the energy produced causes heat and not cools the surroundings. I say this because the collision theory states that when two particles hit each other, they release energy in the form of heat. This may not have been the case in my preliminary investigation because perhaps the atmospheric temperature changed slightly whilst I was doing the experiments, which may have lead to the fluctuations in my data. Another factor could have been the temperature of the distilled water of hydrochloric acid that I added to the sodium thiosulphate.
Method
- Collect equipment and lay out on desk.
- Fill a conical flask with 20ml of sodium thiosulphate using the burettes.
- Using a piece of plain paper and a permanent marker, draw out a cross or use a cross provided.
- Take a beaker and fill it with the first concentration of hydrochloric acid.
- Take a conical flask that contains the sodium thiosulphate and place it over the cross. Add the hydrochloric acid and start the timer simultaneously.
- Watch the conical flask from about nose length away and check to see when the solution turns an opaque yellow, so much so that the cross is no longer visible. Stop the timer when the cross is no longer visible and record the time on the table. Measure the temperature.
- Empty the contents into the fume cupboard sink and wash out the flask thoroughly to prevent contamination of further experiments. Dry out the flask also to prevent further contamination.
- Repeat with this same concentration for accuracy.
- Continue following this method with all the concentrations, making sure that the temperature of all the solutions is kept constant.
Results and Analysis
What I saw happening was that as I decreased the concentration, the solution took longer to turn a yellow cloudy colour. This graph shows clearly what I expected to happen during my experiment. The general shape of the graph shows exactly what I had expected to happen and is almost a mirror image of my graphs in the prediction. The only difference is that the reaction happened in a quicker time than I had expected, possibly due to miscalculations by myself in the prediction. This is however the general shape that I had expected because it shows that as the time increases, the concentration decreases. I can see that at my first concentration, which was the strongest, the time taken for the solution to turn opaque yellow was an average of 16.61 seconds, which was the smallest. The collision theory states that at a higher concentration, the less time it takes for two substances to react. This is because there are many particles that undergo successful collisions and very quickly. The particles meet at the perfect orientation, and because there are so many particles due to the high concentration of the substance, there are many successful collisions per second. My second concentration of 1.75 molars showed a slightly higher average time of 17.22 seconds. This would mean that fewer particles are colliding with the correct orientation and with the right amount of energy. 1.5 molars show that the sodium thiosulphate and hydrochloric acid took 18.28 seconds on average to react. This tells me that there has so far been an increase in time of 1.67 seconds. This would be expected from this experiment because less and less successful collisions are happening when the concentration of the acid is diluted. This relates back to my theory of the collision theory. My fourth result showed that it took 19.33 seconds in average, for the solution to turn opaque yellow. This was in a concentration of 1.25 molars. I had expected this because the more diluted the solution is the fewer collisions per second and therefore a longer time for the two substances to react. My middle result of 1 molar showed that the time taken to react had increased again. It now took an average of 20.41 seconds for the substances to react until I could no longer see the black cross. This means an increase in time of 3.8 seconds. Again, I am reminded of how the collision theory states that fewer particles collide with the correct amount of energy and the correct orientation when the acid is diluted. My sixth result, which was 0.75 molars, showed that even more time was being taken for the substances to react. This again leads to the collision theory. This is the same as my seventh result in a concentration of 0.5 molars as the concentration is significantly decreasing. My final result of 0.25 molars showed an average time of 27.64 seconds. This tells me that the time has increased yet again and that it has taken 11.03 seconds on average longer than the first experiment with 2 molars of hydrochloric acid. This relates back to the collision theory because molecules will take longer to hit each other with the correct orientation, due to lack of energy under diluted substances.
This graph is a mirror image of my predictions at the beginning of this piece of coursework and shows that as concentration is lessened, a longer time is taken for the substances to react. This is because the collision theory states that diluted substances take longer to react due to particles having little energy (see theory for detailed explanation).
1/Time graphs show the rate of the reaction in a simple line graph. I can see from my experiment that as the concentration increases, the time taken for the experiment to complete decreases. From the general line, I can see that at 2.00 molars, the line begins to drop, steadily from 6 to 5.9. It continues to decrease for all of the results, but my last concentration of 0.25 molars dropped significantly faster than the other results. This may possibly be due to fluctuations in atmospheric pressure or mistakes in which my concentrations were set up. I may have possibly added too much water for this cylinder therefore making my results inaccurate. The general result shows that as rate increases, the concentration increases.
The temperature stayed the same for all the experiments I took, which I find surprising because I would have thought that more heat would be produced when there is more energy between the molecules. I would have expected there to be most heat with the highest concentration and least with the lowest because the particle theory indicates that in a higher concentration, more particles are colliding very quickly, and in a dilute concentration, less particles are colliding and quite slowly.
In conclusion, I can say that as concentration increases, the time taken for a reaction to happen complete decreases. This is outlined in the collision theory and as more particles of acid will increase the chances of there being successful collisions at the correct orientation and with the right amount of energy between the molecules. This proves my prediction.
Evaluation
I carried out the experiment well and this was a suitable method for finding out how time varies as concentration varies. However, there are a few things I could do to improve my experiments reliability.
My method was accurate because I used apparatus that allowed me to measure each chemical very accurately. I used a burette, which has a very small margin of error and therefore allows an accurate reading of the different substances to be taken. My method was also accurate because I tried to dry all of the equipment after I had used them to ensure that there would not be an excess of water, which would lead to inaccurate results.
I have one anomalous result in my experiment, which was the final concentration of 0.25 molars, which took slightly longer than I had expected. This may have been because a few drops were left in the conical flask that I was measuring the reaction in, and therefore diluted the hydrochloric acid even further. Another possible explanation for this is that I failed to press the stopwatch quick enough. I found that on the stop clock I used, the button was slightly sticky, which could lead to unfair results as I may not be able to stop it in time.
To improve my experiment, I could have done a few things. The first would be to find a more accurate way than my eyes of measuring how quickly the solution turns cloudy. To do this I would have to use some sort of light meter, which would enable me to record an accurate reading of how quickly it took for the solution to turn cloudy. The second would be some sort of electronic thermometer that could measure the temperature fluctuations to tenths of degrees. This would ensure that I could see if there was any change in temperature. Another improvement I could make, is to use a fresh conical flask for each concentration as this will ensure that each flask is 100% free of excess water that my obscure my results. This will also ensure that the residue on the conical flask after the experiment is finished will not affect my view of the solution. The final improvement I could have made is to include many more concentrations and possibly use hydrochloric acid that was more than 2 molars. This would give me much more variety to work with.
To extend the experiment even further, I would try to change the temperature of the solution so that I could see the effect of a higher or lower temperature of the solution. This would give me quite a broad set of results to analyze. In order to accomplish this, I would have to use water baths that contained water at different temperatures. I would probably use ten water baths with temperatures ranging from 0 degrees centigrade up to 90 degrees centigrade. I would only use one concentration or if I had a great deal of time, I could use these temperatures for every concentration. I would expect that if I did this experiment, the lowest temperature water bath would produce the slowest reaction time and the highest temperature water bath to produce the quickest reaction time. This is because there would be more energy in a hotter environment for the particles to move around in, which would stimulate many more successful collisions per second, therefore speeding up the reaction time. A cooler environment would slow the reaction time because there would be less energy for the particles to move around in. I would expect the graph of that experiment to look something like this:
This graph shows that as the temperature increases, the time taken for the reaction to happen will decrease. This is true because the collision theory states that more energy produces more collisions per second and heat is energy. In a lower temperature, there is less energy therefore the reactions are much slower.
