Standard level
Many elements combine directly forming binary compounds. The chemical formula of every compound obeys the law of constant composition and has a specific ratio of each element in the compound.
Background
Magnesium metal burns in air forming magnesium oxide.
magnesium + oxygen → magnesium oxide
2Mg(s) + O2(g) → 2MgO(s)
This is a reaction that is often carried out at pre-diploma level to find the empirical formula of magnesium oxide. However, the results are never conclusive as the magnesium also reacts with nitrogen in the air forming magnesium nitride, Mg3N2.
The purpose of this experiment is to determine the ratio of magnesium oxide to magnesium nitride formed in the reaction.
magnesium + nitrogen → magnesium nitride
3Mg(s) + N2(g) → Mg3N2(s)
The theoretical yield of magnesium oxide can be calculated, assuming only magnesium oxide is made. The theoretical yield of magnesium nitride can also be calculated assuming only magnesium nitride is formed.
From these two values it is possible to determine the proportion of each compound in the final mixture.
Chemicals
- Magnesium ribbon
Apparatus
- Electronic balance
- Large Bunsen burner
- Tripod and pipe-clay triangle
- Heat-proof mat
- Crucible and lid
- Crucible tongs
- Measure exactly 20cm of magnesium ribbon and record its mass.
- Cut exactly 5cm of the ribbon and break into small pieces into a pre-weighed crucible with lid.
- Rest the crucible on the pipeclay triangle and heat strongly with a Bunsen flame.
- Using the crucible tongs, lift the lid from time to time until no further reaction is observed.
- Carefully transfer the crucible to the heat proof mat to cool down.
- Once cool, reweigh the crucible, residue and lid
Magnesium burns at over 2000ºC. Care is needed to ensure that there are no accidents.
The hot crucible must be placed on a heat-proof mat to cool down before weighing.