Pure iron and its reactions
Pure iron is soft, malleable and ductile, and magnetic. It melts at about 1538 °C and has a density of 7.87 g/cm³. Being soft, pure iron has little industrial use, and the properties of the iron we use depend on its purity. Its configuration is [Ar] 3d⁶ 4s². It shows +2 (the two 4s electrons lost) and +3 (3d⁵, half-filled, the more stable state), while states above +3 are unimportant. A strong oxidising agent such as chlorine takes iron to +3; weaker ones, sulphur or the hydrogen ions of a dilute acid, take it only to +2. The reactions below are paper equations (red-hot iron is never a home experiment). With air: 3Fe + 2O₂ → Fe₃O₄. With steam: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. With chlorine: 2Fe + 3Cl₂ → 2FeCl₃. With sulphur: Fe + S → FeS. With dilute acids iron(II) salts form: Fe + 2HCl → FeCl₂ + H₂ and Fe + H₂SO₄ → FeSO₄ + H₂. For balancing practice, hot concentrated sulphuric acid gives 3Fe + 8H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4SO₂ + 8H₂O (the real products depend on conditions). In concentrated nitric acid iron becomes passive: a thin protective oxide layer forms, which rubbing or dilute hydrochloric acid removes. Concentrated acids are handled only by trained people in a laboratory.
| Iron reacts with | Products | Iron’s state in the iron compound |
|---|---|---|
| air (red-hot) | Fe₃O₄ | +2 and +3 (mixed) |
| steam (red-hot) | Fe₃O₄ + H₂ | +2 and +3 (mixed) |
| chlorine | FeCl₃ | +3 |
| sulphur | FeS | +2 |
| dilute HCl | FeCl₂ + H₂ | +2 |
| dilute H₂SO₄ | FeSO₄ + H₂ | +2 |
| Atom | Left side | Right side |
|---|---|---|
| Fe | 3 | 1 + 2 = 3 |
| S | 8 | 1 + 3 + 4 = 8 |
| O | 8 × 4 = 32 | 4 + 12 + 8 + 8 = 32 |
| H | 8 × 2 = 16 | 8 × 2 = 16 |
A slow-rust test, with a grown-up. Put one iron nail or steel paper clip in each of three clear plastic cups: (1) cold tap water, (2) cold tap water with a spoon of salt, (3) cold tap water covered with a thin layer of cooking oil. Predict what you will see in each, leave the cups for several days and see what happens, without tasting or touching the water with bare cuts. Test the nails with a fridge magnet before and after. Pour the water away with plenty of water and wash your hands. The reddish-brown solid is rust, which is hydrated iron(III) oxide (extra).