From the above results I can then go on to find out the number of moles of each substance and plot them on a graph. I know that the relative atomic mass (Ar) of magnesium is 24 and the Ar of oxygen is 16.
With the above results I can use them to analyse the results in two different ways. The first way is to look at the highlighted results and they show that a certain number of moles of magnesium will react with the same number of moles of oxygen. For example, 0.002 moles of magnesium atoms combine with 0.002 moles of oxygen atoms. This tells us that 1 atom of magnesium reacts with 1 atom of oxygen.
This can also be seen by plotting the moles of magnesium against the moles of oxygen, and then finding the gradient. The graph is shown on the next page.
From the graph I can calculate the gradient by finding the change in the Y axis over the change in the X axis.
Gradient = Change in Y
Change in X
If you look at the triangle I have drawn on the graph (in pink) the change in Y is 0.001 moles and the change in X is 0.001 moles.
Therefore the Gradient = 0.001 = 1 mole
0.001
From this you can see that 1 atom of magnesium reacts with 1 atom of oxygen. So, the empirical formula of the product, magnesium oxide, is MgO. In addition to this we can also look at the placement of magnesium and oxygen in the periodic table and the way in which they bond during the reaction.
When reacting a metal with a non metal, in this case a gas, they undergo ionic bonding. This is when an atom loses electrons and gives them to another atom which needs to gain electrons in order to have a complete outer shell, or in other words to gain the structure of a noble gas.
Magnesium is in the second group and has an electron arrangement of 2.8.2 and so needs to give away two electrons in order to have a full outer shell. Oxygen is in the sixth group of the periodic table and has an electron arrangement of 2.6 and so needs to gain 2 electrons to also have a full outer shell.
Diagram to show the reaction of magnesium and oxygen.
Now we know that when these two elements react they form magnesium oxide (MgO), which is made up of magnesium ions with a double positive charge (Mg2+) and oxide ions with a double negative charge (O2-). Therefore it must be true that 1 atom of magnesium reacts with 1 atom of oxygen, seeing as when they react their outer shells will be complete, because the magnesium atom needs to give away 2 electrons of which the oxygen atom needs to gain. In addition their charges will be cancelled out by the other atom (2+ + 2- = 0).
As a result, the formula for magnesium oxide is MgO.
Evaluation
After gaining the results from the experiment I had to do some calculations in order to find the number of moles within a certain number of grams of magnesium and oxygen. However, when calculating the result, the accuracy used is important. For example, if I take 0.13g of magnesium and want to find out the number of moles, I divide it by 24. The result is 0.005416666 moles, and so I rounded it to 3 decimals places to give 0.005 moles. This was also done for the amount of oxygen that reacted with this amount of magnesium, which was 0.08g. I divided it by 16 and the result was 0.005 moles, which is already correct to 3 decimal places. Therefore as a result I rounded all the figures to three decimal places in order to work with a set of easy numbers and keep a consistent accuracy amongst results.
However, I have to consider whether the results that were produced at the start of the experiment were reliable, rather than accurate. Judging from my results and the fact that the conclusion that I came up with in my analysis did support my prediction fully, and so the results were obviously reliable. However, if you look at the graph I drew, the circled crosses show the anomalies that occurred. These may have occurred due to one main problem when concocting the experiment.
At step 4 when the lid had to be lifted to allow air to enter, in order for the magnesium to react with the oxygen in the air, some of the magnesium oxide that had already reacted could have been lost as smoke. Therefore, changing the amount of magnesium that was started off with, and the amount of oxygen that should have reacted with, and so as a result the proportion of magnesium to oxygen would be slightly out of place. Also this could have been that the reaction had not fully taken place, and so not all the amount of magnesium was allowed to react with the oxygen in the air. This was the main reason why anomalies occurred.
To have avoided this happening, after heating, cooling and weighing the contents and the crucible lid. The magnesium oxide could have been heated again, and weighed until the mass was constant. This would have proved that the magnesium had fully reacted with the oxygen in the air. Despite the anomalies that occurred, it became apparent that they did not affect my results or conclusion in any way, as my prediction was supported.
To further this investigation I could:
- Try to react other metals from group 2 in the Periodic table with oxygen to see whether a pattern occurs in the result. For example, if I were to react Calcium, Ca, with oxygen then I would expect the empirical formula to be CaO. This is based on the idea that Mg reacted with one oxygen atom, and so Ca will react with one oxygen atom also, seeing as Mg and Ca both have similar properties.
- Try to react magnesium with other gases in group 6, such as sulphur, to see whether the empirical formula is MgS, on the basis of my knowledge of magnesium reacting with oxygen.