I am going to make this a fair test, so that each amount won’t turn out different than it should do, because for example, less magnesium wouldn’t take longer to react with the HCl. I am going to use the same amount of acid (10mls) and the same amount of magnesium (3cm strips) to achieve this. I will do this by measuring precise amounts of acids with a measuring cylinder, and a ruler to measure the strips. In fact, the only thing that I want to be different for each reaction is the concentration of the acid (with all six being between different concentrations between 0.1M and 4M, as I mentioned in the list above).
I am going to fill the 6 test tubes with the 10mls of hydrochloric acid, which will be measured with the measuring cylinder. I will then drop the magnesium strips into them. I will time the reactions for however long it will take the magnesium to disappear (i.e. when the reaction has finished).
I am also going to do more than one test, to make sure that I don’t do any of the tests wrong. If I were to only do one test, then any anomalous results will look more inaccurate, than they would do if I were to do two tests. This is because if one test had been done wrong and I didn’t know about it, and the other test had been done correctly, then the average result would reduce the deficit of the anomalous test by half. The only problem with this would be if I did all my tests wrong – because then my averages wouldn’t make a difference on the deficit of the results.
I will try to do two tests, in the two lessons allocated for the practical side of the experiment. In each lesson, I will run three reactions at once, twice. I will do this by running the 0.1M, 0.2M and 0.5M reactions first, and then once those have stopped reacting, I will set up the 1M, 2M and 4M reactions up. If my prediction is correct, then this would be the way to do it to be less time-consuming, as not a lot of time would be spent on the reactions involving the 0.1M, 0.2M and 0.5M acid.
The only problem I see with this method is the time factor. I'm expecting some of these reactions to run for a long time with the weaker (less concentrated) acids, meaning that I would have to keep watching the reactions for the long period. If I was to look or walk away from the reaction and find that the magnesium had finished reacting, then my results would be flawed – and I probably wouldn’t know by how much. This would be an unlikely scenario, though, as I would have no reason to be away from the reactions for long periods at a time (perhaps one or two minutes at the absolute most).
Results:
*Rounded down to nearest second
A graph of these results is on the next page
Although these results do show trends to a certain extent (e.g. times descend), I can’t analyse the results like this. This is because the different concentrations vary too much, meaning that the differences between each concentration and the preceding one are different depending upon which result you look at (e.g.
0.2M - 0.1M = 0.1M; but 4M – 2M = 2M difference). This is why I have set the ‘x’ axis on my graph 9the concentration), to reflect this, by using a scale, whereby every 10 little squares represents 0.5M.
Analysis:
These results show trends, but not in the way I expected them to. The results were pretty accurate, I felt, because apart from one minute at the maximum, the results for each test and its repeat were nearly the same.
The plot of results on both graphs representing the two tests and the average results all have the same trends, only these results weren’t proportional unlike I predicted them to be. There are basically two proportional lines, involving the points 0.1, 0.2 and 0.5 mole concentrations, and the 1, 2 and 4 mole concentrations. For some strange reason, somewhere between 0.5 and 1 mole concentration, the line’s sharp gradient declines, so it looks as if the reactions doesn’t have the same speed in contrast to the concentration of HCl it’s in.
Because of this decline being shown in the table, I wasn’t sure whether I had done the experiment correctly. I thought after the first test that the results would turn out to be completely different to show proportionality, but indeed they didn’t. The fact that the two tests were so alike suggested to me that the tests must have been correct, and my prediction as wrong, because it would be unlikely I would make the same mistake twice over a space of three days (the number of days between the lessons), not the mention the time I had used for planning the experiment carefully.
I think that there is a specific concentration, in which the HCl will overpower the amount of magnesium (although this concentration would vary supposing the 3cm I used wasn’t the amount used). This would be because, in simple terms, there would now be more molecules of hydrogen and chlorine than the number of atoms in the magnesium strip. This in turn would mean that not all the molecules of HCl would need to react (collide) with the magnesium, to the extent that all the magnesium would react, and there would therefore be a high chance that all the atoms would react with the acid molecules in a much quicker time. However, this theory may not be correct.
As far as my individual reaction times are concerned, there were two anomalous results, for the 0.5M and 1M reactions. Perhaps this ties in with my theory in the last paragraph, whereby there is a limit around about 0.5M and 1M acids, where there would be a similar number of Mg and Cl atoms in the solutions to react, therefore making the results for these concentrations (and concentrations between 0.5 and 1M) more unreliable.
So, I would conclude that the reaction rates get quicker as the concentration of the aqueous solution (in this case HCl) gets higher.
I could back this conclusion up by doing two things. Firstly, I could use my average time graph (graph 1) to work out what time any concentration would take to complete its reaction. This is especially because of the fact that all my results I tested followed a trend. What I can do is to look where any concentration level is in line with the line of best fit, and then I could take this point on the line of best fit and look to see where that point lines up with the time taken. An example is on the next page:
Here you can see that the point indicated as the intersecting point is where the concentration levels and time taken axis’ meet on the line of best fit. This is exactly how I could prove my conclusion correct, by taking any concentration, and then test the concentration in the same way as in my method to see if the time taken follows this pattern. If it does, then I could easily prove my conclusion to be correct.
Evaluation:
The times would have been very unlikely to be the same, particularly under the conditions of the fact that we had a strict time limit of two lessons, including getting the equipment out and clearing it up again. Overall, this was a very tight deadline to meet, and I eventually realised I needed to try and run my first and second tests simultaneously in the second lesson, as I was nearly out of time. This rush could easily have affected my final results in several ways, all because of the rush I was in to get all my results.
The magnesium floating in the HCl may have been a problem too. I don’t think it would have affected the results much, but because it was floating, not all of the magnesium was in the acid at the same time throughout the reaction, and therefore this may have meant the reaction took longer as it couldn’t react with all of the magnesium straight away.
I could have got over this by poking the Mg down, but that may have affected the results as well. If I were to use a substance that would react with the HCl in ANY way (even if it would be unnoticeable to the naked eye), then the reaction rates would have probably slowed down, because some of the HCl would have reacted with the other substance, leaving less of it to react with the Mg. The other (more obvious) point is that I would have found it hard to poke three pieces of Mg in different test tubes down at once.
The other way which I could have solved this, would be to make the density of the Mg larger, so that the Mg would sink naturally to the bottom of the test tube. This could have been done either by redoing the tests with more magnesium (a longer strip), or by using less HCl. I would have done this, but the tight time limit meant that I couldn’t redo all the tests that I have already done in time, before realising this.
Another way my results could have been affected is that because the 0.1 reading took a long time to complete, meaning that I may not have noticed that the reaction had finished straight away. I don’t think this happened, though, so I'm not too worried about this.
Finally, and this occurred in my second test which could indeed explain my ringed faulty result, I admit, time got the better of me and so I didn’t wash out my measuring cylinder out after measuring each concentration of acid. I may have measured smaller concentrations first and these concentrations could have mixed with the one I was using then to not give the mole that I said I was using for this test.
To get over this problem, I could either have rinsed out the cylinder before measuring each concentration, or even by sharing cylinders with other people, of which each cylinder would only be used for one concentration.
Overall, despite these problems, I don’t think I will have made a bad attempt at the experiment, because the results were similar both times and they did follow a trend on my two graphs.
