To find a rate of reaction from the results I collect, I will take an average out of all three sets of results. To do this I will look at each row of timings, and work out the average rate of reaction for each recording. This provides me with directly comparable rates of reaction, because as the time gets longer, the rate is much slower. I will repeat the whole experiment three times, in order to reduce the effects of any mislead results, which may occur, by using averages to make a conclusion. So that I am clear on what substances are what, I will label all measuring cylinders I use.
Variables
Other variables, which may affect the outcome of this experiment, if not kept constant, are as follows:
Temperature of hydrochloric acid- I believe that the hotter the temperature, the faster the rate of reaction. This is because when a substance is heated up, each particle has more energy, resulting in more collisions between the free moving particles in a liquid, causing more particles to react, therefore increasing the overall rate of reaction. The second effect this has is that the particles collide with more force, increasing the chance of the particles reacting. I will keep this variable constant so that it does not affect the results by avoiding heating the hydrochloric acid in any way, and so it will stay at room temperature.
Concentration of sodium thiosulphate- The more concentrated the sodium thiosulphate solution, the faster the reaction will be, because there will be more particles in the same volume. The particles will be closer together, colliding more often, therefore increasing the speed of the overall reaction. I will keep the concentration of sodium thiosulphate constant, by using it from the same bottle each time, which has a fixed concentration.
Size of conical flask- Although this does not affect the rate of reaction, in that it doesn't affect the chemical reaction itself, but what this variable does affect is our measurement of how long the reaction has taken to complete. I will be judging the time taken for the reaction to finish as the time it takes for the reacted mixture of sodium thiosulphate to go completely cloudy so that I cannot see a cross drawn underneath the flask. If the conical flask is wider then the cloudiness will appear less concentrated, because it will be spread across the larger area of the bottom of the conical flask, and so it will take longer for the cross to disappear, and the results will increase. I will keep this variable constant by using the same conical flask for each experiment.
Catalyst- By using a catalyst, this experiment could be speeded up because catalysts lower the energy needed by the chemicals in order for a reaction to occur. However I will not be using a catalyst in this experiment, and I will make sure that there are no other materials present, apart from those chemicals I want to react. This way, I can be sure that no catalysts are affecting the results in any way.
Cross- The item of stationary, of which I use to draw the cross underneath the conical flask, could affect how easily the cross is seen. This in turn could affect the results because if the cross is seen more easily, drawn with a thick marker, our measurement of how long the reaction took to complete would be longer and so the rate of reaction would seem slower. Equally, if the cross was hard to see, then it would disappear more quickly and the rate of reaction would seem quicker. I will keep this constant by using the same black pen to draw the cross each time, if I am unable to use the same cross each time, which is preferable, then I will do so.
Volume Of Sodium Thiosulphate And Hydrochloric Acid - The larger the volume of hydrochloric acid and/or sodium thiosulphate, the greater total number of particles in the solution. This means that to completely react, and go cloudy, it will take longer. I will keep this variable constant by using 50cm³ of sodium thiosulphate solution with 5cm³ of hydrochloric acid each time. I have found from previous experiments that this volume (55cm³) is adequate because if too little is used then it is hard to see changes in it.
Movement Of Reacting Mixture - If I shake the conical flask while it is reacting, then the reaction may speed up and therefore the rate of reaction would be quicker. Likewise, if I stir it then it will increase the rate of reaction. This is because there will be a greater chance of the particles colliding with other particles which they can react with because the two types of particles will be more mixed together and also, will have more kinetic energy because they will be moving while being stirred. I will prevent this from affecting the results by not stirring or shaking the mixture in any way.
To make the experiment safe, I will be wearing goggles throughout, in case something unexpected occurs. I will be washing my hands regularly and thoroughly washing the equipment before each reaction to make sure that it is not only a fair test, but that it does not cause something dangerous to happen. I’m aware not to lean over the beaker, as I understand that the solution made is bad for my health, as well as and especially for those who are asthmatics.
Results
The results which I have collected, without any changes to the planned method, are shown below. I have also plotted them onto a graph so that the trend can be seen more easily.
First experiment
Second experiment
Third experiment
Averages
Conclusions
The results and the graph appear to show roughly what I predicted. As the concentration of hydrochloric acid is increased, then the rate of reaction increases also. What I believe to be happening is that as the concentration decreases, there are less hydrochloric acid molecules around in the reacting mixture, meaning that it will take longer for each sodium thiosulphate molecule to ‘find’ a hydrochloric acid molecule for it to react with… Overall, this decreases the rate of reaction because it takes longer for the mixture as a whole to react fully, because of the extra time it takes for the molecules to collide with each other. The results support this theory in that the relationship between concentration and rate of reaction is directly proportional that is to say that every time the concentration increases, the rate increases. This also supports the idea that it takes time for the molecules to ‘find’ each other in order to react, because if the concentration is less, then the molecules of hydrochloric acid will be more spaced out and there will be more space between themselves and the sodium thiosulphate molecules. If the molecules move around in the liquids at the same speed each time (because they have a constant temperature), then the further they have to travel to reach another molecule, the longer it will take for them to reach that molecule and then react. However, although my results give me a clear indication to a direct link to my theory, the third experiment was completely out of range and are completely out of target. This is due to the fact that the bottles we were taking the substances from were apparently combined together, forming a higher concentration and the experiment to be faulty. Therefore, the third set of results was not relevant to our experiment and I should have done it a fourth time having had the time, though.
My four line graphs give me a greater indication to how much of my results go to use and can actually give me an intelligent conclusion into my experiment. In experiment line graph one, I can see two anomalous results, one of which is extremely out of target! I can put this down to two things; not doing a trial experiment to be completely positive on what I was doing, and this resolves in my second possibility of not measuring accurately enough. I could have resolved this by either, if I had the time, of carrying out a trial experiment and/or being 100% sure on the accuracy of my measurements. However, from this graph I am able to make a very good judgement on the reaction. The steep curve symbolises that there was a dramatically faster in the rate of reaction and that the graph supports my theory.
Experiment line graph two indicates only one anomalous result, as you can obviously see (30 acid, 20 water), and this is most likely to be due to a measuring error. Other than that, this graph gives me a reasonably good analysis of the second experiment and the curve which nearly touches all points, tells me that as the concentration gets higher, as does the rate of reaction.
In my third experiment line graph also; it indicates another two anomalous results. the reason for this is that another member of the class combined the two substances, sodium thiosulphate and hydrochloric acid in which the bottles I was taking my samples from. Therefore, I should of, having had the time, repeated the experiment a fourth time as these results are of no use and leaves me with only two recordings to take a logical average from.
My average graph is the best graph out of them all, giving a greater indication into the rate of reaction and how my results should of turned out! However, I do have one flaw in this graph and that being an anomaly from the first experiment, the one of which was highly inaccurate. This does not, in theory, affect my results that much as it doesn’t affect the curve and you can clearly see how the concentration affects the rate of reaction. All in all, my results support my theory but were not entirely accurate and I should have done the whole experiment again just to be sure.
Evaluation
The results of this experiment appear to be quite accurate and so I can make a definite conclusion. From this, I can speculate that the experiment was conducted well, and that the results have only been affected by other variables by little or no amount. However, looking at the graph, I can see that there are some results, in the third experiment specifically, that do not follow the general trend, and show a higher or lower rate of reaction than would be expected at that point. Normally in circumstances like this, there is a change in another variable or an inaccuracy in the measuring of the input variable. This means that either the reacting mixture was shaken accidentally, moving the molecules around more, increasing their chances of reacting, or more likely, the concentrations of hydrochloric acid were incorrectly measured, and too much hydrochloric acid was used. This would reduce the distances between the molecules, and increase the rate of reaction. However, I was later informed that, unfortunately, someone else in the class had mixed the two substances to form different concentrations in both bottles, therefore, in the third experiment, the trend was altered.
I think that the only flaw in the experiment were in the measuring of the concentrations, and in the judging of when the reaction had completed of which corresponds to my actions.
It was very hard to get an exact volume of water/hydrochloric acid to get the right concentration, and so this may have thrown the last result in the first experiment off the trend, too. Also, there was not really any sure way of knowing when the reaction had finished. My method of looking at the mixture and judging by the eye when the reaction is complete is very inaccurate, and leads to confusion, especially, for example, if you look away for a moment after thinking the reaction is over, and then look again and realise it hasn’t.
If I was to repeat the experiment, I would use a much more accurate way of measuring the volumes of the reactants, and I would find a completely different way of measuring when the reaction has completed. I think if I were to use an electronic device rather than the judgement of the naked eye then the experiment would be of far much more use and would conclude with highly accurate results. A laser may be appropriate as you could then see the particles moving and colliding but, unfortunately, I didn't have a resource of that kind.
Also, I believe that with the use of a digital thermometer, I would keep track of the room temperature as that could also prove to alter the final results.
Another possibility is to insure that you know exactly what you are doing by practising with a trial experiment first, of which I didn’t do. I should have done this as I was perfectly equipped and able but I was under time restrictions and therefore, it was not possible for me to carry this out.
C:\My Documents\coursework.doc\Kelly Duggan\NDO
