Apparatus.
1. Bunsen burner
2. Tripod
3. Gauze
4. Crucible and Lid
5. Tongs
6. Heatproof mat
7. Safety glasses
8. Magnesium strips
Safety.
- As we were dealing with chemicals and heat during the course of this test it was vital that it was carried out with great caution. To achieve this we:
- Made sure that the work surface was clean and tidy with no unnecessary objects lying around on it.
- Were cautious about burning magnesium as it burns a bright white light, which is possible to blind any one looking straight at it.
- Were careful to wear out protective goggles on at all times.
- Used tongs to move the equipment about as we were dealing with hot things.
- Stood up during the experiment.
- Never left the apparatus unattended and kept a close eye on it at all times noting the changes taking place.
- Put the Bunsen burner on the safety yellow flame when not in use to prevent people from not seeing it.
- Made sure we didn’t inhale any ‘smoke’ when the magnesium was being burned.
Method.
The crucible was weighed with both the magnesium strip and the lid on it. The weights were recorded. From this when the magnesium had been heated the added weight could be found, this being the oxygen.
The magnesium would need to be reacted with oxygen, to do this most efficiently, the magnesium should be heated in oxygen, but not excess oxygen because the magnesium would burn in excess oxygen. So the magnesium was heated with a crucible and its lid. At frequent intervals, remove the burner and slightly lift the crucible lid using tongs, quickly replacing the lid so as to lose as little magnesium oxide 'smoke' as possible. Repeat the process patiently until the magnesium ceases to flare up, then remove the lid and heat strongly to make sure that combustion is complete and all the metal has been converted into oxide. You should not be able to see any shiny metallic surfaces. Allow the crucible to cool then replace the lid and reweigh the whole. Record the mass.
Fair Test.
To make this a fair test we weighed the empty crucible and lid before adding magnesium. This means that the different weights of the crucibles could be accounted for. This allowing any minor errors in the experiments result to be averaged out. To make this a lot more accurate we carried the test out 5 times.
Before the experiment we carried out the experiment we cleaned the crucible and lid thoroughly making sure that there was no excess foreign materials of any sort to tamper with the chemical reaction.
Variables.
The amount of magnesium used in the experiment.
How long it had been left burning for.
How much air you let in it when lifting up the lid.
Results.
Conclusion.
The expected ratio of 1 to 1 can be seen in several sets of the results, but the rest are either side of being perfect.
I think that if this experiment had been carried out in a proper laboratory with no interferences you would find that the ratio for this combustion would be very close to 1:1. Which would justify my prediction to be correct.
Evaluation
The experiment went very well taking into consideration that the experiment was very hard to perform without error. The environment was not perfect, the % of air entering the crucible could not be certain, these were part of the many errors which disallowed a perfect set of results. With many people in the laboratory all performing the same experiment for results for the same experiment, it is almost certain that many of the groups did have different methods and thus have different errors. The groups all had different intervals for “lifting the lid” and different heating times. This all resulted in slightly different results.
Potential reasons for error
Hot Magnesium will react with Nitrogen in the air if there is insufficient O2 forming Magnesium Nitrate (Mg3N2).
When magnesium metal is burned in pure oxygen, the only product is magnesium oxide. If a sample of Mg is weighted before and after combustion, then the increase in mass is equal to the mass of oxygen that is combined with Mg. From a knowledge of the mass of Mg and the mass of combined O, the empirical formula of magnesium oxide can be calculated.
There is only one problem with this straightforward approach. It is more usual to burn things in air than in pure oxygen. Air is about 80% nitrogen. So burning Mg in air results not only in the formation of magnesium oxide, but also a small amount of a "by-product", magnesium nitride.
Not all the Mg was burned. There is not much to do about this, just make sure that you have followed the method correctly and look out for all the signs of the magnesium being present.
Improvements
If I were to do this experiment again I would carry it out in a fume cupboard to measure the gases allowed to come in contact with the heated magnesium. The laboratory would not have as many people in it and the environment would be controlled, allowing no draughts of air and risk unwanted particles getting into the experiment. I would also clean any existing oxide off the magnesium ribbon and any impurities. This was not done.