Iron: reactions of elements
Reaction of iron with air
Iron metal reacts in moist air by oxidation to give a hydrated iron oxide. This does not protect the iron surface to further reaction since it flakes off, exposing more iron metal to oxidation. This process is called rusting and is familiar to any car owner. Finely divided iron powder is pyrophoric, making it a fire risk.
On heating with oxygen, O2, the result is formation of the iron oxides Fe2O3 and Fe3O4.
4Fe(s) + 3O2(g) → 2Fe2O3(s)
3Fe(s) + 2O2(g) → Fe3O4(s)
Reaction of iron with water
Air-free water has little effect upon iron metal. However, iron metal reacts in moist air by oxidation to give a hydrated iron oxide. This does not protect the iron surface to further reaction since it flakes off, exposing more iron metal to oxidation. This process is called rusting and is familiar to any car owner.
Reaction of iron with the halogens
Iron reacts with excess of the halogens F2, Cl2, and Br2, to form ferric, that is, Fe(III), halides.
2Fe(s) + 3F2(g) → 2FeF3(s) (white)
2Fe(s) + 3Cl2(g) → 2FeCl3(s) (dark brown)
2Fe(s) + 3Br2(l) → 2FeBr3(s) (reddish brown)
This reaction is not very successful for iodine because of thermodynamic problems. The iron(III) is too oxidizing and the iodide is too reducing. The direct reaction between iron metal and iodine can be used to prepare iron (II) iodide, FeI2.
Fe(s) + I2(s) → FeI2(s) (grey)
Reaction of iron with acids
Iron metal dissolves readily in dilute sulphuric acid in the absence of oxygen to form solutions containing the aquated Fe(II) ion together with hydrogen gas, H2. In practice, the Fe(II) is present as the complex ion [Fe(OH2)6]2+.
Fe(s) + H2SO4(aq) → Fe2+(aq) + SO42-(aq) + H2(g)
If oxygen is present, some of the Fe(II) oxidizes to Fe(III).
The strongly oxidizing concentrated nitric acid, HNO3, reacts on th surface of iron and passivates the surface.