Iron Compounds: A Revision Guide
Important Chemical Compounds of Iron (Fe)
Introduction to the Major Compounds of Iron
Iron (Fe), a transition metal, forms a variety of compounds, primarily in +2 (ferrous) and +3 (ferric) oxidation states. These compounds are crucial in industrial processes, biological systems, and everyday applications. For high school chemistry, understanding their preparation, properties, and reactions is essential.
1. Iron(II) Sulfate Heptahydrate ($\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$)
- Chemical Formula: $\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$
- Common Name: Green Vitriol (due to its distinct green crystalline form)
Laboratory Preparation
Iron(II) sulfate is typically prepared by the reaction of iron metal (e.g., iron filings) with dilute sulfuric acid. The solution is then concentrated and cooled to allow crystallization.
Balanced Equation: $\text{Fe(s)} + \text{H}_2\text{SO}_4\text{(aq, dil)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{H}_2\text{(g)}$
The $\text{FeSO}_4$ solution is then crystallized to obtain $\text{FeSO}_4 \cdot 7\text{H}_2\text{O}$. To prevent oxidation of $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ during preparation, a small piece of iron metal is often kept in the solution.
Properties and Exam-Relevant Reactions
- Physical Properties: Crystalline green solid, soluble in water, efflorescent (loses water of crystallization on exposure to air).
- Oxidation: In aqueous solution, $\text{Fe}^{2+}$ ions are readily oxidized to $\text{Fe}^{3+}$ ions, especially in the presence of air and acid. $4\text{FeSO}_4\text{(aq)} + \text{O}_2\text{(g)} + 2\text{H}_2\text{SO}_4\text{(aq)} \rightarrow 2\text{Fe}_2(\text{SO}_4)_3\text{(aq)} + 2\text{H}_2\text{O(l)}$
- Thermal Decomposition: On heating, green vitriol first loses its water of crystallization and then decomposes into iron(III) oxide, sulfur dioxide, and sulfur trioxide. $\text{FeSO}_4 \cdot 7\text{H}_2\text{O(s)} \xrightarrow{\Delta} \text{FeSO}_4\text{(s)} + 7\text{H}_2\text{O(g)}$ $2\text{FeSO}_4\text{(s)} \xrightarrow{\Delta} \text{Fe}_2\text{O}_3\text{(s)} + \text{SO}_2\text{(g)} + \text{SO}_3\text{(g)}$
- Redox Reactions (as a reducing agent): $\text{FeSO}_4$ acts as a reducing agent, particularly in acidic medium, where $\text{Fe}^{2+}$ is oxidized to $\text{Fe}^{3+}$.
- With acidified Potassium Permanganate ($\text{KMnO}_4$): $10\text{FeSO}_4\text{(aq)} + 2\text{KMnO}_4\text{(aq)} + 8\text{H}_2\text{SO}_4\text{(aq)} \rightarrow 5\text{Fe}_2(\text{SO}_4)_3\text{(aq)} + 2\text{MnSO}_4\text{(aq)} + \text{K}_2\text{SO}_4\text{(aq)} + 8\text{H}_2\text{O(l)}$ (Purple $\text{KMnO}_4$ is decolorized as $\text{MnO}_4^-$ is reduced to $\text{Mn}^{2+}$)
- With acidified Potassium Dichromate ($\text{K}_2\text{Cr}_2\text{O}_7$): $6\text{FeSO}_4\text{(aq)} + \text{K}_2\text{Cr}_2\text{O}_7\text{(aq)} + 7\text{H}_2\text{SO}_4\text{(aq)} \rightarrow 3\text{Fe}_2(\text{SO}_4)_3\text{(aq)} + \text{Cr}_2(\text{SO}_4)_3\text{(aq)} + \text{K}_2\text{SO}_4\text{(aq)} + 7\text{H}_2\text{O(l)}$ (Orange dichromate is reduced to green $\text{Cr}^{3+}$)
2. Iron(III) Hydroxide ($\text{Fe(OH)}_3$)
- Chemical Formula: $\text{Fe(OH)}_3$
- Common Name: Ferric hydroxide
Preparation Process
Iron(III) hydroxide is typically prepared as a gelatinous precipitate by adding an alkali (like $\text{NaOH}$ or $\text{NH}_4\text{OH}$) to an aqueous solution of an iron(III) salt.
Balanced Equation: $\text{FeCl}_3\text{(aq)} + 3\text{NaOH(aq)} \rightarrow \text{Fe(OH)}_3\text{(s)} + 3\text{NaCl(aq)}$ (A reddish-brown precipitate is formed)
Properties and Exam-Relevant Reactions
- Physical Properties: Reddish-brown, gelatinous precipitate, insoluble in water.
- Amphoteric Nature (weakly): It is a basic hydroxide but shows very weak amphoteric character, dissolving slightly in strong alkalis to form a metaferrite. More prominently, it reacts with acids. $\text{Fe(OH)}_3\text{(s)} + 3\text{HCl(aq)} \rightarrow \text{FeCl}_3\text{(aq)} + 3\text{H}_2\text{O(l)}$
- Thermal Decomposition: On heating, iron(III) hydroxide decomposes to form iron(III) oxide and water. $2\text{Fe(OH)}_3\text{(s)} \xrightarrow{\Delta} \text{Fe}_2\text{O}_3\text{(s)} + 3\text{H}_2\text{O(g)}$ This reaction is crucial as it forms stable iron(III) oxide.
3. Important Iron Oxides and Rust
Iron forms several oxides, with the most common being Iron(II) Oxide ($\text{FeO}$), Iron(III) Oxide ($\text{Fe}_2\text{O}_3$), and Iron(II,III) Oxide ($\text{Fe}_3\text{O}_4$).
Iron(III) Oxide ($\text{Fe}_2\text{O}_3$)
- Chemical Formula: $\text{Fe}_2\text{O}_3$
- Common Names: Haematite (major ore), Ferric oxide, Red Ochre.
- Brief Guide: A reddish-brown solid. It is the most stable oxide of iron and the chief component of rust. It is diamagnetic.
- Preparation: Formed by the strong heating of $\text{Fe(OH)}_3$, $\text{FeCO}_3$, or $\text{FeSO}_4$. $2\text{Fe(OH)}_3\text{(s)} \xrightarrow{\Delta} \text{Fe}_2\text{O}_3\text{(s)} + 3\text{H}_2\text{O(g)}$
- Reactions: It is a basic oxide, reacting with acids to form iron(III) salts. $\text{Fe}_2\text{O}_3\text{(s)} + 6\text{HCl(aq)} \rightarrow 2\text{FeCl}_3\text{(aq)} + 3\text{H}_2\text{O(l)}$
Iron(II,III) Oxide ($\text{Fe}_3\text{O}_4$)
- Chemical Formula: $\text{Fe}_3\text{O}_4$ (Can be considered as a mixed oxide of $\text{FeO}$ and $\text{Fe}_2\text{O}_3$)
- Common Names: Magnetite (major ore), Black Iron Oxide, Ferrosoferric oxide.
- Brief Guide: A black, crystalline solid, which is ferromagnetic. It is formed when iron reacts with steam at high temperatures or during the slow oxidation of iron.
- Preparation: $3\text{Fe(s)} + 4\text{H}_2\text{O(g, steam)} \xrightarrow{\text{red hot}} \text{Fe}_3\text{O}_4\text{(s)} + 4\text{H}_2\text{(g)}$
Rust (Hydrated Iron(III) Oxide)
- Chemical Formula: $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$
- Brief Guide: Rust is the common name for the reddish-brown flaky substance formed when iron or its alloys (like steel) are exposed to oxygen and moisture for a prolonged period. It is an electrochemical process.
Rusting Process: Rusting requires both oxygen and water. The overall simplified process can be represented as: $4\text{Fe(s)} + 3\text{O}_2\text{(g)} + x\text{H}_2\text{O(l)} \rightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O(s)}$
Comparative Properties of Iron Ions ($\text{Fe}^{2+}$ vs $\text{Fe}^{3+}$)
| Property | Iron(II) ion ($\text{Fe}^{2+}$, Ferrous) | Iron(III) ion ($\text{Fe}^{3+}$, Ferric) |
|---|---|---|
| Electronic Config. | $[\text{Ar}] 3\text{d}^6$ | $[\text{Ar}] 3\text{d}^5$ |
| Colour (aqueous) | Pale green | Yellowish-brown (due to hydrolysis to $\text{Fe(OH)}_3$ in solution) |
| Stability | Less stable, easily oxidized to $\text{Fe}^{3+}$ | More stable |
| Magnetic Nature | Paramagnetic (4 unpaired electrons) | Paramagnetic (5 unpaired electrons) |
| Redox Character | Good reducing agent (gets oxidized to $\text{Fe}^{3+}$) | Weak oxidizing agent (gets reduced to $\text{Fe}^{2+}$) |
| Hydroxide | $\text{Fe(OH)}_2$, white/green precipitate, turns brown on oxidation | $\text{Fe(OH)}_3$, reddish-brown gelatinous precipitate |
| Sulfide | $\text{FeS}$, black precipitate | $\text{Fe}_2\text{S}_3$ or $\text{FeS}$ (hydrolyzes to $\text{Fe(OH)}_3$ and $\text{H}_2\text{S}$) |