Two Different Iron Oxides
Both versions of iron oxide are ionic compounds, which means they combine with negative ions in the correct ratio so that the compound is electrically neutral. Let's look at the equation for the formation of ferric oxide.
2Fe+3 + 3O-2 → Fe2 O3
The formation of ferrous oxide is represented by the equation:
Fe+2 + O-2 → FeO
The difference in the ratios of iron to oxygen in these two compounds make for differences in properties of the compound. Let's see what these differences are.
Comparison of Iron Oxides
Ferric oxide is the compound generated when iron reacts with oxygen in the air. It is reddish-brown in color and is also known as the mineral hematite and rust. It is used to make color dyes, and the iron is extracted in chemical processes for numerous uses. Ferrous oxide is black in color and is known as the mineral wustite.
Since ferric oxide has two iron atoms and three oxygen atoms, it has a larger molar mass (159.69 g/mole) compared to ferrous oxide (71.84 g/mole). Ferric oxide has a slightly higher melting point than ferrous oxide at just over 1500° C, compared to just over 1300° C for ferrous oxide.
The early Earth didn't have the oxygen rich atmosphere it does now. Iron(II) is water-soluble and was readily available for microorganisms to use to generate oxygen. As oxygen levels increased, the iron reacted with the oxygen to form solid ferric oxide. This caused the microorganisms to evolve to be able to acquire the iron they need. Ferric oxide is not soluble in water.
Lesson Summary
Ferric and ferrous oxides are iron oxides. Ferric means the iron(III) ion Fe+3, and ferrous means iron(II), which is Fe+2. Both of these ions react with oxygen, but ferric oxide is more common. Ferric oxide is Fe2 O3, and ferrous oxide is FeO. Ferrous oxide is reddish in color and is known as the mineral hematite. Ferric oxide is black and is the mineral wustite.
Early Earth had little to no oxygen in the atmosphere, and iron(II) was dissolved in ocean water. This allowed microorganisms to generate oxygen gas. When oxygen levels reached significant levels, iron reacted with oxygen, forming ferric oxide.
