Chromium (Cr): A Comprehensive Revision Guide

Introduction to Chromium (Cr)

Chromium (Cr) is a silvery-white, hard, and brittle transition metal. It is highly valued for its high polish, resistance to tarnishing, and high melting point. Its diverse applications, from enhancing the properties of alloys to providing vibrant pigments, make it a crucial element in various industrial and technological sectors. Understanding its chemical properties, particularly its variable oxidation states and redox behavior, is essential for competitive examinations.

CBSE/JEE Quick Revision Notes

• Symbol: Cr
• Atomic Number: 24
• Atomic Mass: 51.996 u
• Electronic Configuration: [Ar] 3d⁵ 4s¹ (an exception due to the stability of half-filled d-orbitals)
• Group: 6
• Period: 4
• Block: d-block (Transition Metal)
• Common Oxidation States: +2, +3, +6 (Most stable: +3)
• Nature: Hard, lustrous, silvery-white metal. Exhibits paramagnetism in many compounds.
• Density: 7.19 g/cm³
• Melting Point: 1907 °C
• Boiling Point: 2671 °C
• Reactivity: Forms a passive oxide layer upon exposure to air, making it resistant to corrosion.

Electron Configuration & Bonding Behavior

Chromium’s electron configuration, [Ar] 3d⁵ 4s¹, is an exception to Aufbau principle, where an electron from the 4s orbital promotes to the 3d orbital to achieve a more stable half-filled d-subshell (3d⁵) and a half-filled s-subshell (4s¹).

Chromium exhibits a variety of oxidation states, with +2, +3, and +6 being the most common and significant:

• +2 Oxidation State: (e.g., CrCl₂, CrSO₄). Generally reducing and unstable in air.
• +3 Oxidation State: (e.g., Cr₂O₃, CrCl₃). The most stable and common oxidation state. Compounds are often green or violet and amphoteric.
• +6 Oxidation State: (e.g., K₂CrO₄, K₂Cr₂O₇, CrO₃). Highly oxidizing, especially in acidic media. Compounds are typically yellow (chromates) or orange (dichromates) and toxic.

Chromium compounds, particularly in the +3 oxidation state, are known for forming numerous coordination complexes due to the availability of d-orbitals for bonding.

Crucial Chemical Reactions

1. Reactions with Oxygen

• At high temperatures, chromium reacts with oxygen to form chromium(III) oxide. 4 Cr (s) + 3 O₂ (g) → 2 Cr₂O₃ (s)

2. Reactions with Acids

• Non-oxidizing Acids (e.g., HCl, H₂SO₄ (dilute)): Chromium reacts slowly to liberate hydrogen, forming Cr(II) salts. Cr (s) + 2 HCl (aq) → CrCl₂ (aq) + H₂ (g)
• Oxidizing Acids (e.g., HNO₃, H₂SO₄ (conc.)): Chromium becomes passive due to the formation of a protective, impervious layer of chromium(III) oxide on its surface.

3. Preparation of Potassium Dichromate (K₂Cr₂O₇) from Chromite Ore (FeCr₂O₄)

Potassium dichromate is a vital oxidizing agent and its preparation involves three main steps:

• Step 1: Roasting of chromite ore with soda ash (Na₂CO₃) and air. 4 FeCr₂O₄ (s) + 8 Na₂CO₃ (s) + 7 O₂ (g) → 8 Na₂CrO₄ (aq) + 2 Fe₂O₃ (s) + 8 CO₂ (g) (Yellow solution of sodium chromate is formed)

• Step 2: Conversion of sodium chromate to sodium dichromate by acidification. 2 Na₂CrO₄ (aq) + H₂SO₄ (aq) → Na₂Cr₂O₇ (aq) + Na₂SO₄ (aq) + H₂O (l) (Yellow chromate solution turns orange due to dichromate formation)

• Step 3: Conversion of sodium dichromate to potassium dichromate. Na₂Cr₂O₇ (aq) + 2 KCl (aq) → K₂Cr₂O₇ (s) + 2 NaCl (aq) (Potassium dichromate, being less soluble, crystallizes out.)

4. Interconversion of Chromate (CrO₄²⁻) and Dichromate (Cr₂O₇²⁻) Ions

Chromate and dichromate ions exist in equilibrium, which is pH-dependent.

• In acidic medium (pH < 7): Yellow chromate converts to orange dichromate. 2 CrO₄²⁻ (aq, yellow) + 2 H⁺ (aq) ⇌ Cr₂O₇²⁻ (aq, orange) + H₂O (l)

• In alkaline medium (pH > 7): Orange dichromate converts to yellow chromate. Cr₂O₇²⁻ (aq, orange) + 2 OH⁻ (aq) ⇌ 2 CrO₄²⁻ (aq, yellow) + H₂O (l)

5. Oxidizing Properties of Potassium Dichromate (K₂Cr₂O₇)

Potassium dichromate is a strong oxidizing agent in acidic medium, where the Cr(VI) is reduced to Cr(III).

• Ionic half-reaction: Cr₂O₇²⁻ (aq) + 14 H⁺ (aq) + 6 e⁻ → 2 Cr³⁺ (aq) + 7 H₂O (l)

• Oxidation of Iodide to Iodine: 6 I⁻ (aq) + Cr₂O₇²⁻ (aq) + 14 H⁺ (aq) → 3 I₂ (s) + 2 Cr³⁺ (aq) + 7 H₂O (l)

• Oxidation of Ferrous ions to Ferric ions: 6 Fe²⁺ (aq) + Cr₂O₇²⁻ (aq) + 14 H⁺ (aq) → 6 Fe³⁺ (aq) + 2 Cr³⁺ (aq) + 7 H₂O (l)

• Oxidation of Sulfite/Sulfur dioxide to Sulfate: 3 SO₂ (g) + Cr₂O₇²⁻ (aq) + 2 H⁺ (aq) → 3 SO₄²⁻ (aq) + 2 Cr³⁺ (aq) + H₂O (l)

6. Chromyl Chloride Test (for Chloride ions)

This is a specific test for chloride ions, which forms volatile chromyl chloride.

• Reaction with solid metal chloride (e.g., NaCl) and conc. H₂SO₄ with K₂Cr₂O₇: K₂Cr₂O₇ (s) + 4 NaCl (s) + 6 H₂SO₄ (conc.) → 2 CrO₂Cl₂ (g, red-orange vapours) + 2 KHSO₄ (aq) + 4 NaHSO₄ (aq) + 3 H₂O (l)

• Confirmation: The red-orange vapours of chromyl chloride are passed into a NaOH solution, forming a yellow solution of sodium chromate. CrO₂Cl₂ (g) + 4 NaOH (aq) → Na₂CrO₄ (aq, yellow) + 2 NaCl (aq) + 2 H₂O (l) Adding lead acetate to the yellow solution gives a yellow precipitate of lead chromate (PbCrO₄).

Industrial and Biological Importance

Industrial Importance

• Metallurgy: Chromium is a key component in stainless steel (e.g., 18% Cr, 8% Ni for 18/8 stainless steel) and other alloys (e.g., nichrome) due to its hardness, corrosion resistance, and high-temperature strength.
• Electroplating: Chromium plating (chrome plating) provides a hard, shiny, corrosion-resistant finish to metal objects.
• Pigments: Chromium compounds are widely used as pigments.
  • Chromium(III) oxide (Cr₂O₃) is “chrome green.”
  • Lead chromate (PbCrO₄) is “chrome yellow.”
• Catalysis: Chromium compounds are used as catalysts in various organic and inorganic reactions.
• Tanning: Chromium(III) salts (e.g., basic chromium sulfate) are used in the tanning of leather to make it more flexible and durable.

Biological Importance

• Essential Trace Element (Cr(III)): Trivalent chromium (Cr(III)) is considered an essential trace element in humans, playing a role in glucose and lipid metabolism, particularly in enhancing the action of insulin.
• Toxicity (Cr(VI)): Hexavalent chromium (Cr(VI)) compounds are highly toxic and carcinogenic. They are potent oxidizing agents and can cause severe health problems, including respiratory issues, skin ulcers, and increased cancer risk. This difference in toxicity between Cr(III) and Cr(VI) is crucial.