Iron (Fe): Properties and Reactions

Chemical Properties Overview

Iron (Fe) is a d-block transition metal, atomic number 26. It exhibits variable valency, primarily +2 (ferrous) and +3 (ferric), with +3 being more stable under ordinary conditions.

Reactivity Series Position

Iron is placed above hydrogen in the reactivity series (more reactive than H), but below more reactive metals like Na, K, Ca, Mg, and Al. This indicates its ability to displace hydrogen from acids and steam.

Electronegativity

The electronegativity of Iron (Pauling scale) is approximately 1.83. This value is characteristic of metals and indicates its tendency to lose electrons in chemical reactions.

General Reactivity

Iron is a moderately reactive metal. It readily undergoes oxidation, especially in the presence of moisture. It reacts with acids, steam, and halogens, forming ionic compounds.

Action of Air and Oxygen

1. Reaction with Dry Air/Oxygen

At room temperature, iron does not react with dry air or oxygen.

2. Reaction with Moist Air (Rusting)

In the presence of both oxygen and moisture, iron undergoes corrosion, commonly known as rusting, to form hydrated ferric oxide. This is an electrochemical process. Overall simplified reaction: 4Fe(s) + 3O2(g) + 2xH2O(l) → 2Fe2O3⋅xH2O(s) (Reddish-brown hydrated ferric oxide, commonly known as rust)

3. Reaction with Oxygen on Heating

When heated in air or oxygen, iron burns to form magnetic iron oxide (iron(II,III) oxide). 3Fe(s) + 2O2(g) \xrightarrow\{\text\{heat\}\} Fe3O4(s) (Magnetic iron oxide)

Action of Water and Steam

1. Reaction with Cold Water

Iron does not react with cold or boiling water.

2. Reaction with Steam

When red hot iron is passed over steam, it forms magnetic iron oxide (Fe3O4) and hydrogen gas. 3Fe(s) + 4H2O(g) \xrightarrow\{\text\{red heat\}\} Fe3O4(s) + 4H2(g)

Action of Acids and Bases

1. Action of Acids

a. Dilute Hydrochloric Acid (HCl)

Iron reacts with dilute HCl to form ferrous chloride and hydrogen gas. Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g) (Ferrous chloride)

b. Dilute Sulfuric Acid (H2SO4)

Iron reacts with dilute H2SO4 to form ferrous sulfate and hydrogen gas. Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g) (Ferrous sulfate)

c. Dilute Nitric Acid (HNO3)

Iron reacts with dilute nitric acid to form ferrous nitrate, nitric oxide, and water. 3Fe(s) + 8HNO3(dilute)(aq) → 3Fe(NO3)2(aq) + 2NO(g) + 4H2O(l) (Ferrous nitrate)

d. Concentrated Sulfuric Acid (H2SO4)

With hot concentrated sulfuric acid, iron reacts to form ferric sulfate, sulfur dioxide, and water. 2Fe(s) + 6H2SO4(conc.)(hot)(aq) → Fe2(SO4)3(aq) + 3SO2(g) + 6H2O(l) (Ferric sulfate)

e. Concentrated Nitric Acid (HNO3)

Concentrated nitric acid renders iron passive due to the formation of a thin, impervious, protective layer of iron oxide on its surface (e.g., Fe2O3 or Fe3O4). This prevents further reaction.

2. Action of Bases

Iron is generally unreactive with aqueous solutions of alkalis (bases).

Key Laboratory Test/Identification Reactions

Identification of Ferrous (Fe²⁺) Ions

1. With Sodium Hydroxide (NaOH) Solution: A dirty green precipitate of ferrous hydroxide, insoluble in excess NaOH. Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)2(s)

2. With Ammonium Hydroxide (NH4OH) Solution: A dirty green precipitate of ferrous hydroxide, insoluble in excess NH4OH. Fe²⁺(aq) + 2NH4OH(aq) → Fe(OH)2(s) + 2NH4⁺(aq)

3. With Potassium Ferricyanide (K3[Fe(CN)6]) Solution: Formation of a dark blue precipitate known as Turnbull’s Blue. 3Fe²⁺(aq) + 2[Fe(CN)6]³⁻(aq) → Fe3[Fe(CN)6]2(s)

Identification of Ferric (Fe³⁺) Ions

1. With Sodium Hydroxide (NaOH) Solution: A reddish-brown gelatinous precipitate of ferric hydroxide, insoluble in excess NaOH. Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)3(s)

2. With Ammonium Hydroxide (NH4OH) Solution: A reddish-brown gelatinous precipitate of ferric hydroxide, insoluble in excess NH4OH. Fe³⁺(aq) + 3NH4OH(aq) → Fe(OH)3(s) + 3NH4⁺(aq)

3. With Potassium Ferrocyanide (K4[Fe(CN)6]) Solution: Formation of a dark blue precipitate known as Prussian Blue. 4Fe³⁺(aq) + 3[Fe(CN)6]⁴⁻(aq) → Fe4[Fe(CN)6]3(s)

4. With Potassium Thiocyanate (KCNS) or Ammonium Thiocyanate (NH4CNS) Solution: A blood-red coloration due to the formation of ferric thiocyanate complex. Fe³⁺(aq) + 3CNS⁻(aq) → Fe(CNS)3(aq) (Blood-red coloration)