Class 12 Chapter 2 Chromium Properties Reactions oxidation states of Chromium 

Introduction

Chromium (Cr), atomic number 24, belongs to the first row of the d‑block elements. Known for its brilliant colors and multiple oxidation states, chromium exhibits rich chemistry central to both industrial and biochemical processes.

Occurrence & Extraction

Chromium is primarily found as the ore chromite (FeCr2O4) in South Africa, Kazakhstan, and India. Extraction is carried out by roasting chromite with sodium carbonate followed by reduction with carbon:

4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2
Na2CrO4 + C → Cr2O3 + CO
Cr2O3 + 2 Al → 2 Cr + Al2O3

Physical Properties

  • Silvery‑steel grey metal, lustrous and hard
  • High melting point: 1907 °C; boiling point: 2672 °C
  • Density: 7.19 g cm⁻³
  • Paramagnetic due to unpaired d‑electrons

Chemical Properties

Chromium exhibits both metallic and non‑metallic behavior:

  • Reacts with oxygen: Forms protective Cr2O3 layer (passivation).
  • Reacts with acids: Dissolves in HCl/H2SO4 evolving H2; in oxidizing acids forms Cr(VI) species.
  • Reacts with alkalis: Forms chromite and chromate in hot and concentrated solutions.

Oxidation States

Chromium exhibits oxidation states from –2 to +6, the most common being +2, +3, and +6:

Oxidation State Species Color
+2 Cr2+ (chromous) Pale blue
+3 Cr3+ (chromic) Green
+6 CrO42– / Cr2O72– (chromate/dichromate) Yellow / Orange

Important Reactions

Chromium(III) Hydroxide Formation

Cr3+ + 3 OH → Cr(OH)3 (green precipitate)

Chromate–Dichromate Equilibrium

2 CrO42– + 2 H+ ⇌ Cr2O72– + H2O

Redox: Cr(VI) to Cr(III)

3 Cr2O72– + 14 H+ + 6 e → 6 Cr3+ + 7 H2O

Applications

  • Stainless steel production (adds corrosion resistance)
  • Electroplating (decorative & protective coatings)
  • Leather tanning (Cr(III) salts)
  • Pigments (chromate/dichromate compounds)
  • Wood preservation (sodium chromate)

Conclusion

Chromium’s versatile oxidation states and robust physical properties make it indispensable in alloys, coatings, and chemical industries. Understanding its reactions and redox chemistry is key for applications in materials science and environmental chemistry.

 

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