Discovering the formula of MgO
Report 6: Reaction between Magnesium and Oxygen
The aim of our experiment was to observe the effect of burning pure Magnesium (Mg) in oxygen (O2) and to confirm the chemical formula of magnesium oxide by comparing the masses of pure solid magnesium (prior to any reaction) and magnesium oxide solid after a reaction between magnesium and oxygen from the air when heated from a Bunsen burner in a crucible. Using molar masses of both magnesium and oxygen, an expected percent composition, by mass, was found and compared to our experimental results.
Theory: Magnesium oxide is easily made by burning magnesium ribbon which oxidizes in a bright white light, resulting in a powder.
2 Mg(s) + O2 (g) 2 MgO (s) ΔH = −1204 kJ
% composition of oxygen in Mg2O= 16.00/ ((24.01*2) +16) =25.0%
% composition of oxygen in MgO=16.00/ (24.01+16) =40.0%
% composition of oxygen in MgO2=16.00/(24.01(16.00*2))=57.1%
Prediction: Weight of material would increase when magnesium burns.
(i)Independent variable: Mass of Magnesium ribbon
(ii)Dependent variable: Mass of Magnesium Oxide
The independent variable will be the mass of magnesium ribbon, this is because the mass of the product i.e. Magnesium Oxide, depends on how much Magnesium is added. The dependent variable will be the mass of Magnesium Oxide, this is a measured variable as the mass of oxygen can be calculated and will enable us to determine the formula for Magnesium Oxide.
- Container used
- Surface area of Magnesium
- Concentration of oxygen in the container
- Temperature of flame
- A crucible with a lid is used and is filled with a layer of filter paper, in order to allow combustion in a closed environment, preventing the loss of Magnesium oxide powder.
- Magnesium ‘ribbon’ will be used in all cases.
- The experiment is performed in the same laboratory on the same day i.e., the atmospheric conditions and oxygen concentration will be the same.
- The same Bunsen burner was used every time, with the air hole completely open.
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Apparatus and chemicals:
- Crucible with lid
- Bunsen burner
- Pipe clay triangle
- Magnesium ribbon
- Inhalation of magnesium oxide fumes can cause therefore ensure that the room is well ventilated.
- The bright flame of MgO is very hard to extinguish and it emits a harmful intensity of UV light therefore do not look at the burning Mg for prolonged periods of time and wear eye protection.
- Hot apparatus could be a hazard, it can cause skin damage therefore use tongs to handle the crucible lid and do not attempt to weigh crucible until it is cool.
Take 2-3 cm of Magnesium ribbon and observe how it looks. Then using tongs hold it over a Bunsen burner, when it burns hold the magnesium over a little beaker and observe the product formed.
Now the experiment should be done under more controlled conditions. We start by tearing off a piece of Mg approx. 5cm and weigh it on a balance, if magnesium looks tarnished or black then it should cleaned using the emery paper. Next we take a suitable crucible-wash dry and then weigh it. Subsequently we fill the crucible with a single layer of filter paper and coil the magnesium ribbon, so that it fits into the crucible. The magnesium should only be touching the paper and not the crucible. e. Place the crucible containing the magnesium in the pipe clay triangle on a tripod and put the lid on. Heat the crucible for another 4-5 minutes .
Once the crucible is hot, gently lift the lid with the tongs a little to allow some oxygen to get in. You may see the magnesium begin to flare up. If the lid is off for too long then the magnesium oxide product will begin to escape. Don't let this happen.
Keep heating and lifting the lid until you see no further reaction. At this point, remove the lid and heat for another 8-10 minutes, until all the magnesium has turned into a white powder. Turn off the Bunsen burner and allow the apparatus to cool. Re-weigh the crucible containing the product. Filter paper will burn and ashes are supposed to weigh nothing. The experiment was repeated four times for accurate results.
Data Collection and processing
Original: The piece of Mg ribbon we took was silver-black.
Experiment done by holding the Mg ribbon above the Bunsen flame: The Mg ribbon no longer stayed into one piece and it turned white black.
Experiment with more controlled conditions: Once the magnesium was in the crucible and was being heated by the Bunsen burner, it glowed for a brief time. It then caught fire before the lid was placed on top. When it came into contact with the oxygen, the magnesium started glowing extremely bright, and intensely white. The glow became orange after some time. The magnesium ribbon then turned into a white powder.
Mass of Product= (Mass of crucible with product)-(Mass of crucible)
Uncertainty in mass of product= (Uncertainty in mass of crucible with product) + (Uncertainty in mass of crucible)
Mass of Oxygen== (Mass of product)-(Mass of magnesium)
Uncertainty in mass of oxygen= (Uncertainty in mass of product) + (Uncertainty in mass of magnesium)
Percentage composition of oxygen= (Mass of oxygen/Mass of product)*100
Conclusion and Evaluation:
Formula of magnesium oxide suggested by our Experimental results:
From our results we can see we obtained values closer to 40% each time we repeated the experiment, which suggests this formula of magnesium oxide (MgO).
The percentage errors for each reading are included in the processed table above. We take our most accurate result which gives the percentage composition of oxygen (38, 1%) closest to our theoretical value (40%) and thus has the minimum amount of percentage error.
% Error= (| (38.1 - 40)/40 |)*100= 4, 90 %
Proving the formula of magnesium oxide: Now in order to prove our results we need to find the formula of magnesium oxide in theory, for that we use the results which gave the most accurate percentage composition by mass of oxygen and minimum amount of percentage error.
Mass magnesium = 0,101g
Mass magnesium oxide = 0,163 g
So mass oxygen = 0,062g
Number moles of Mg = 0,101/24,000 = 0,004
Number moles of O = 0,062 /16,000 = 0,004
Dividing by the smallest ratio: we get ratio approx. 1 Mg : 1O
This would suggest a formula of MgO, which is the correct formula.
Hence we prove that the equation is: 2Mg + O2→ 2MgO.
Formula of magnesium oxide according to the theory of the charges of different ions: Furthermore, theoretical value is 40% since Mg which is in group 2 has a charge of +2 whereas oxygen which is in group 6 has a charge of -2, the charges are exchanged and cancelled out so we get the formula of Magnesium oxide should which is MgO.
Standard graph showing
Errors: It is worth evaluating our experiment as there could be several reasons for not getting exact results:
- the magnesium oxide product may escape as they lift the lid- there would be a less amount of magnesium oxide than expected at the end of the experiment.
- not all the magnesium may have reacted (the product may still look a bit grey rather than white)
- Possible sources of error in this experiment include the inaccuracy of measurements, as correct measurements are vital for the experiment for example not making the balance reading zero before weighing.
- having the magnesium coiled too tightly so that not all of it reacts
- Not cleaning Mg ribbon and crucible before starting experiment. This would mean that not all the magnesium reacted to form magnesium oxide, and so there would be a smaller mass of magnesium oxide than expected.
- The reaction not properly finishing should also be counted as a possible source of error. If you did not burn the magnesium in the crucible for the right amount of time, some of the magnesium would not have time to react and form magnesium oxide; thus, there would be less magnesium oxide at the end of the experiment than expected.
Further improvement: The crucible and lid can be heated at the beginning of the experiment before being weighed so that any moisture in the crucible is burned away. Moisture is heavy, and thus it can change the results of the experiment, as we only want the weight of magnesium and the magnesium oxide.