Iron: the metal and its reactions
Iron has symbol Fe and atomic number 26. It is a grey metal, strong, and a magnet pulls it. It is the second most common metal in the Earth's crust, after aluminium. Pure iron is rather soft; most iron is made into steel.
The outer electrons of an iron atom are 3d⁶4s². The atom can lose 2 electrons to make Fe²⁺, or 3 electrons to make Fe³⁺. The stronger the substance that attacks iron (the stronger the oxidising agent), the more electrons iron gives up.
| Iron meets | Result | Iron ion |
|---|---|---|
| Dilute hydrochloric or sulfuric acid | Fe + 2H⁺ → Fe²⁺ + H₂ | Fe²⁺ (pale green) |
| Chlorine gas (strong oxidiser) | 2Fe + 3Cl₂ → 2FeCl₃ | Fe³⁺ |
| Steam (red hot) | 3Fe + 4H₂O → Fe₃O₄ + 4H₂ | Fe²⁺ and Fe³⁺ |
| Damp air (oxygen and water) | Rust, hydrated Fe₂O₃ | Fe³⁺ |
Note: iron with dilute acid gives Fe²⁺, not Fe³⁺. The hydrogen ion is only a mild oxidiser.
Iron oxides: FeO, Fe₂O₃ and Fe₃O₄
Iron makes three oxides. They are all solids that do not dissolve in water.
| Oxide | Name | Colour | Iron ions |
|---|---|---|---|
| FeO | iron(II) oxide | black powder | Fe²⁺ only |
| Fe₂O₃ | iron(III) oxide (hematite; rust is its hydrated form) | red-brown | Fe³⁺ only |
| Fe₃O₄ | magnetite (a mix of FeO and Fe₂O₃) | black, magnetic | 1 Fe²⁺ and 2 Fe³⁺ |
Check the charges: in Fe₃O₄ there are 3 iron ions and 4 oxide ions (O²⁻). Total negative = 8. Positive = 2 + 3 + 3 = 8. The compound is neutral, so one Fe²⁺ and two Fe³⁺ is the right mix.
All three oxides are basic oxides: they react with acids to give a salt and water. FeO + 2HCl → FeCl₂ + H₂O. Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O.
Iron hydroxides: Fe(OH)₂ and Fe(OH)₃
Add sodium hydroxide solution to an iron salt and a solid hydroxide drops out (a precipitate). Iron hydroxides do not dissolve in water.
- Iron(II) salt + alkali: Fe(OH)₂, a white-green solid. Fe²⁺ + 2OH⁻ → Fe(OH)₂.
- Iron(III) salt + alkali: Fe(OH)₃, a red-brown jelly-like solid. Fe³⁺ + 3OH⁻ → Fe(OH)₃.
White-green turns red-brown in air
Fe(OH)₂ is not stable in air. Oxygen and water change it to Fe(OH)₃: 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃. You see the white-green solid go grey-green, then red-brown. This is a nice way to see Fe²⁺ turn into Fe³⁺.
Both hydroxides dissolve in acids to give the matching salt: Fe(OH)₃ + 3HCl → FeCl₃ + 3H₂O. Heating Fe(OH)₃ gives Fe₂O₃ and water.
Fe²⁺ and Fe³⁺ conversion
Fe²⁺ and Fe³⁺ differ by one electron: Fe²⁺ ⇌ Fe³⁺ + e⁻.
Fe²⁺ to Fe³⁺: oxidation
An oxidising agent takes one electron from each Fe²⁺. Examples: chlorine water, hydrogen peroxide, acidified potassium permanganate, oxygen of the air. Colour changes from pale green to yellow-brown. Example: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻.
Fe³⁺ to Fe²⁺: reduction
A reducing agent gives one electron to each Fe³⁺. Examples: iron metal, zinc, tin(II) chloride, sulfur dioxide. Colour fades from yellow-brown to pale green. Example: 2Fe³⁺ + Fe → 3Fe²⁺.
Why do we keep iron(II) solutions with a nail in them? The iron changes any Fe³⁺ that forms back to Fe²⁺.
Fe³⁺ is more stable in air, so iron(II) compounds slowly turn into iron(III) ones in open air. That is why an old bottle of ferrous sulfate crystals gets a brown coat.
Try it: predict, then check
Use the free-play picker in the 3D. Before you tap a compound, say its colour and its iron charge. At home: leave a bright iron nail in a glass of water with a pinch of salt for two days and watch the red-brown rust (Fe³⁺ compound) form. Wash your hands afterwards.
Key formulas and definitions
- Iron: Z = 26, Fe → Fe²⁺ + 2e⁻ and Fe → Fe³⁺ + 3e⁻
- Fe + 2H⁺ → Fe²⁺ + H₂ (dilute acid)
- 2Fe + 3Cl₂ → 2FeCl₃
- 3Fe + 4H₂O(steam) → Fe₃O₄ + 4H₂
- Oxides: FeO (Fe²⁺), Fe₂O₃ (Fe³⁺), Fe₃O₄ = FeO · Fe₂O₃
- 4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃
- Fe²⁺ ⇌ Fe³⁺ + e⁻: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻; 2Fe³⁺ + Fe → 3Fe²⁺
Worked examples
1. Name the iron compound in each: (a) FeO (b) Fe₂O₃. What is the charge on iron?
(a) FeO is iron(II) oxide: one O²⁻ needs one Fe²⁺. (b) Fe₂O₃ is iron(III) oxide: three O²⁻ (total 6−) need two iron ions of 3+ each (6+).
2. Show that magnetite, Fe₃O₄, has one Fe²⁺ and two Fe³⁺.
Four O²⁻ give 8−. Three iron ions must give 8+. If x ions are Fe²⁺ and (3 − x) are Fe³⁺: 2x + 3(3 − x) = 8, so 9 − x = 8, x = 1. So 1 Fe²⁺ and 2 Fe³⁺.
3. A pale green solution turns yellow-brown when hydrogen peroxide is added. What happened to the iron?
The pale green solution holds Fe²⁺. Hydrogen peroxide is an oxidising agent, so it took one electron from each Fe²⁺ and made Fe³⁺ (yellow-brown). The iron was oxidised.
Common mistakes
- Saying iron with dilute acid gives iron(III). It gives iron(II) and hydrogen: Fe + 2H⁺ → Fe²⁺ + H₂.
- Mixing up the colours: Fe²⁺ is pale green, Fe³⁺ is yellow-brown or red-brown. Low charge, light colour.
- Writing the formula of rust as FeO. Rust is hydrated iron(III) oxide, Fe₂O₃·xH₂O.
- Thinking Fe₃O₄ has only one kind of iron ion. It has both: one Fe²⁺ and two Fe³⁺.