Important Compounds of d-Block Elements

Introduction

Potassium dichromate (K₂Cr₂O₇) and potassium permanganate (KMnO₄) are the rockstars of transition metal chemistry! These powerful oxidizing agents appear in countless JEE questions and have vital industrial applications.

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Potassium Dichromate (K₂Cr₂O₇)

Structure and Properties

Physical Properties:

• Color: Bright orange-red crystalline solid
• Melting point: 398°C
• Solubility: Soluble in water (forms orange solution)

Molecular Structure:

$$K_2Cr_2O_7 \text{ contains } Cr_2O_7^{2-} \text{ ion}$$

graph TD
    A[Dichromate Structure] --> B[Two CrO₄ tetrahedra]
    B --> C[Share one O atom]
    C --> D[Cr-O-Cr bridge]
    D --> E[Each Cr has 4 O atoms]

Key Features:

• Chromium oxidation state: +6
• Electronic configuration of Cr⁶⁺: [Ar]3d⁰ (colorless in theory)
• Orange color due to charge transfer (O²⁻ → Cr⁶⁺), NOT d-d transition

Preparation

Laboratory Method:

Step 1: Preparation of sodium chromate from chromite ore

$$4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \xrightarrow{\Delta} 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2$$

Step 2: Conversion to sodium dichromate

$$2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O$$

Step 3: Conversion to potassium dichromate

$$Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7 + 2NaCl$$

(K₂Cr₂O₇ is less soluble and crystallizes out)

Industrial Preparation:

• Large-scale roasting of chromite ore with Na₂CO₃
• Followed by acidification and salt metathesis

Chromate-Dichromate Equilibrium

pH-dependent interconversion:

$$\boxed{2CrO_4^{2-} + 2H^+ \rightleftharpoons Cr_2O_7^{2-} + H_2O}$$

graph LR
    A[CrO₄²⁻
Yellow
Basic] -->|Add H⁺| B[Cr₂O₇²⁻
Orange
Acidic]
    B -->|Add OH⁻| A

    style A fill:#ffeb3b
    style B fill:#ff9800

Key Points:

• Basic medium: Chromate (CrO₄²⁻) is yellow
• Acidic medium: Dichromate (Cr₂O₇²⁻) is orange
• Equilibrium shifts with pH change

Chemical Properties

1. Oxidizing Agent Behavior

K₂Cr₂O₇ is a strong oxidizing agent in ACIDIC medium.

Half-reaction in acidic medium:

$$\boxed{Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O}$$

Color change: Orange (Cr₂O₇²⁻) → Green (Cr³⁺)

Interactive Demo: Visualize Electrochemical Redox Reactions

Watch how dichromate acts as an oxidizing agent in electrochemical cells with electron transfer.

Important Oxidation Reactions:

a) Oxidation of Ferrous to Ferric:

$$Cr_2O_7^{2-} + 14H^+ + 6Fe^{2+} \rightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_2O$$

b) Oxidation of Iodide to Iodine:

$$Cr_2O_7^{2-} + 14H^+ + 6I^- \rightarrow 2Cr^{3+} + 3I_2 + 7H_2O$$

c) Oxidation of H₂S:

$$Cr_2O_7^{2-} + 8H^+ + 3H_2S \rightarrow 2Cr^{3+} + 3S + 7H_2O$$

d) Oxidation of Sulfite:

$$Cr_2O_7^{2-} + 8H^+ + 3SO_3^{2-} \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 4H_2O$$

e) Oxidation of Alcohols:

Primary alcohol → Aldehyde → Carboxylic acid:

$$3CH_3CH_2OH + Cr_2O_7^{2-} + 8H^+ \rightarrow 3CH_3CHO + 2Cr^{3+} + 7H_2O$$ $$3CH_3CHO + Cr_2O_7^{2-} + 8H^+ \rightarrow 3CH_3COOH + 2Cr^{3+} + 4H_2O$$

Secondary alcohol → Ketone:

$$3(CH_3)_2CHOH + Cr_2O_7^{2-} + 8H^+ \rightarrow 3(CH_3)_2CO + 2Cr^{3+} + 7H_2O$$

2. Action of Heat

Thermal decomposition:

$$\boxed{4K_2Cr_2O_7 \xrightarrow{\Delta} 4K_2CrO_4 + 2Cr_2O_3 + 3O_2}$$

• Decomposition starts at 500°C
• Oxygen gas evolved

3. Reaction with Alkali

Fusion with KOH:

$$Cr_2O_7^{2-} + 2OH^- \rightarrow 2CrO_4^{2-} + H_2O$$

Forms yellow potassium chromate.

4. Chromyl Chloride Test

Used to detect chloride ions:

Step 1: Heating with conc. H₂SO₄

$$K_2Cr_2O_7 + 4NaCl + 6H_2SO_4 \rightarrow 2CrO_2Cl_2 + 2KHSO_4 + 4NaHSO_4 + 3H_2O$$

Red vapors of chromyl chloride (CrO₂Cl₂) evolve.

Step 2: Chromyl chloride dissolves in NaOH

$$CrO_2Cl_2 + 4NaOH \rightarrow Na_2CrO_4 + 2NaCl + 2H_2O$$

Yellow solution confirms Cl⁻ presence.

Uses of K₂Cr₂O₇

1. Oxidizing agent in laboratories
2. Analytical chemistry - volumetric analysis
3. Leather tanning industry
4. Dyeing and printing textiles
5. Production of chrome alum
6. Wood preservation
7. Photography - for hardening gelatin

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Potassium Permanganate (KMnO₄)

Structure and Properties

Physical Properties:

• Color: Dark purple (almost black) crystalline solid
• Melting point: 240°C (decomposes)
• Solubility: Soluble in water (intense purple solution)

Molecular Structure:

$$KMnO_4 \text{ contains } MnO_4^- \text{ ion}$$

Tetrahedral structure:

• Mn at center
• Four O atoms at corners
• Mn oxidation state: +7
• Mn⁷⁺ configuration: [Ar]3d⁰ (no d-d transitions)
• Purple color due to charge transfer (O²⁻ → Mn⁷⁺)

Preparation

Laboratory Method (Baeyer’s Process):

Step 1: Fusion of pyrolusite (MnO₂) with KOH in presence of air/oxygen

$$2MnO_2 + 4KOH + O_2 \xrightarrow{\Delta} 2K_2MnO_4 + 2H_2O$$

Green potassium manganate (K₂MnO₄) forms.

Step 2: Electrolytic oxidation OR disproportionation

Electrolytic oxidation (at anode):

$$MnO_4^{2-} - e^- \rightarrow MnO_4^-$$

Disproportionation in acidic medium:

$$3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$$

Green solution → Purple solution

Industrial Preparation:

• MnO₂ fused with KOH and oxidized
• Followed by electrolytic oxidation

Chemical Properties

1. Oxidizing Agent Behavior

KMnO₄ is the STRONGEST common oxidizing agent.

Oxidizing power depends on pH:

a) In ACIDIC medium (H₂SO₄):

$$\boxed{MnO_4^- + 8H^+ + 5e^- \rightarrow Mn^{2+} + 4H_2O}$$

Color change: Purple (MnO₄⁻) → Colorless/Pale pink (Mn²⁺)

n-factor = 5 (most powerful)

b) In NEUTRAL/MILD ALKALINE medium:

$$\boxed{MnO_4^- + 2H_2O + 3e^- \rightarrow MnO_2 + 4OH^-}$$

Color change: Purple → Brown precipitate (MnO₂)

n-factor = 3

c) In STRONGLY ALKALINE medium:

$$\boxed{MnO_4^- + e^- \rightarrow MnO_4^{2-}}$$

Color change: Purple (MnO₄⁻) → Green (MnO₄²⁻)

n-factor = 1 (weakest)

2. Important Oxidation Reactions

In Acidic Medium:

a) Oxidation of Oxalic acid (H₂C₂O₄):

$$2MnO_4^- + 5C_2O_4^{2-} + 16H^+ \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O$$

Self-indicator titration! (Purple → Colorless at endpoint)

b) Oxidation of Ferrous to Ferric:

$$MnO_4^- + 5Fe^{2+} + 8H^+ \rightarrow Mn^{2+} + 5Fe^{3+} + 4H_2O$$

c) Oxidation of Iodide to Iodine:

$$2MnO_4^- + 10I^- + 16H^+ \rightarrow 2Mn^{2+} + 5I_2 + 8H_2O$$

d) Oxidation of H₂S to Sulfur:

$$2MnO_4^- + 5H_2S + 6H^+ \rightarrow 2Mn^{2+} + 5S + 8H_2O$$

e) Oxidation of Sulfite to Sulfate:

$$2MnO_4^- + 5SO_3^{2-} + 6H^+ \rightarrow 2Mn^{2+} + 5SO_4^{2-} + 3H_2O$$

f) Oxidation of Nitrite to Nitrate:

$$2MnO_4^- + 5NO_2^- + 6H^+ \rightarrow 2Mn^{2+} + 5NO_3^- + 3H_2O$$

g) Oxidation of Alcohols:

Primary alcohol → Aldehyde → Carboxylic acid

Secondary alcohol → Ketone

Tertiary alcohol → No oxidation

h) Oxidation of Alkenes (Baeyer’s Test):

$$3CH_2=CH_2 + 2MnO_4^- + 4H_2O \rightarrow 3CH_2OH-CH_2OH + 2MnO_2 + 2OH^-$$

Purple → Brown (positive test for unsaturation)

In Neutral Medium:

i) Oxidation of Iodide:

$$2MnO_4^- + I^- + H_2O \rightarrow 2MnO_2 + IO_3^- + 2OH^-$$

j) Oxidation of Thiosulfate:

$$8MnO_4^- + 3S_2O_3^{2-} + H_2O \rightarrow 8MnO_2 + 6SO_4^{2-} + 2OH^-$$

3. Action of Heat

Thermal decomposition:

$$\boxed{2KMnO_4 \xrightarrow{\Delta} K_2MnO_4 + MnO_2 + O_2}$$

• Starts at 240°C
• Oxygen gas evolved (lab preparation of O₂!)
• Purple → Green + Brown

4. Action of Conc. Acids

With conc. HCl:

$$2KMnO_4 + 16HCl \rightarrow 2KCl + 2MnCl_2 + 5Cl_2 + 8H_2O$$

Chlorine gas evolved (greenish-yellow)

With conc. H₂SO₄ (hot):

$$4KMnO_4 + 6H_2SO_4 \rightarrow 2K_2SO_4 + 4MnSO_4 + 6H_2O + 5O_2$$

Oxygen gas evolved

Uses of KMnO₄

1. Analytical chemistry - volumetric titrations
2. Water purification - kills bacteria
3. Disinfectant - antiseptic properties
4. Bleaching agent - wool, silk, cotton
5. Laboratory reagent - qualitative analysis
6. Medicine - treatment of skin infections
7. Baeyer’s test - detecting unsaturation in organic compounds

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Comparison: K₂Cr₂O₇ vs KMnO₄

Property	K₂Cr₂O₇	KMnO₄
Color	Orange-red	Dark purple
Oxidation State	Cr⁶⁺	Mn⁷⁺
Oxidizing Power	Moderate	Very strong
Stability	Very stable	Less stable
Storage	No special care	Store in dark bottles
Self-indicator	No (orange→green)	Yes (purple→colorless)
Medium	Best in acidic	Works in all pH
n-factor (acidic)	6 (Cr⁶⁺→Cr³⁺)	5 (Mn⁷⁺→Mn²⁺)
Thermal stability	More stable	Decomposes at 240°C
Primary standard	Yes	No (needs standardization)

For Volumetric Analysis:

K₂Cr₂O₇ Advantages:

• Primary standard (accurate concentration)
• More stable in solution
• Can be weighed accurately

KMnO₄ Advantages:

• Self-indicator (no external indicator needed)
• Stronger oxidizing agent
• More versatile (works in different pH)

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Other Important Compounds

1. Potassium Ferrocyanide - K₄[Fe(CN)₆]

Properties:

• Yellow crystalline solid
• Fe in +2 oxidation state
• Diamagnetic (low spin, d⁶)

Test for Fe³⁺:

$$Fe^{3+} + [Fe(CN)_6]^{4-} \rightarrow Fe_4[Fe(CN)_6]_3$$

Prussian blue precipitate

2. Potassium Ferricyanide - K₃[Fe(CN)₆]

Properties:

• Red crystalline solid
• Fe in +3 oxidation state
• Paramagnetic (1 unpaired electron)

Test for Fe²⁺:

$$Fe^{2+} + [Fe(CN)_6]^{3-} \rightarrow Fe_3[Fe(CN)_6]_2$$

Turnbull’s blue precipitate

3. Copper Sulfate - CuSO₄·5H₂O

Properties:

• Blue crystalline solid
• Cu²⁺ (3d⁹, 1 unpaired e⁻)
• Paramagnetic

Test for water:

• Anhydrous CuSO₄ is white
• Turns blue on adding water (hydration test)

4. Zinc Sulfate - ZnSO₄·7H₂O

Properties:

• Colorless/white crystalline solid
• Zn²⁺ (3d¹⁰, 0 unpaired e⁻)
• Diamagnetic

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Analytical Tests

1. Detection of Oxidizing Agents

Test with KI and starch:

• Add KI solution + starch
• Oxidizing agents liberate I₂
• Blue color with starch confirms

Reactions:

$$Cr_2O_7^{2-} + 14H^+ + 6I^- \rightarrow 2Cr^{3+} + 3I_2 + 7H_2O$$ $$2MnO_4^- + 10I^- + 16H^+ \rightarrow 2Mn^{2+} + 5I_2 + 8H_2O$$

2. Detection of Reducing Agents

Test with KMnO₄:

• Add dilute KMnO₄
• Purple color disappears if reducing agent present
• Confirms presence of reductant

3. Baeyer’s Test (Unsaturation)

Cold, dilute, neutral KMnO₄:

$$R-CH=CH-R' + [O] \rightarrow R-CH(OH)-CH(OH)-R'$$

Purple → Brown MnO₂ precipitate

Positive test for C=C or C≡C

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Common JEE Mistakes

1. Using wrong acid in KMnO₄ titrations

   • Never use HCl (gets oxidized)
   • Never use HNO₃ (itself oxidizing)
   • Always use dilute H₂SO₄

2. n-factor confusion

   • K₂Cr₂O₇ in acidic: n = 6 (Cr⁶⁺ → Cr³⁺, 2 Cr atoms)
   • KMnO₄ in acidic: n = 5 (Mn⁷⁺ → Mn²⁺)
   • KMnO₄ in neutral: n = 3 (Mn⁷⁺ → Mn⁴⁺)
   • KMnO₄ in basic: n = 1 (Mn⁷⁺ → Mn⁶⁺)

3. Color change confusion

   • K₂Cr₂O₇: Orange → Green (Cr³⁺)
   • KMnO₄ (acidic): Purple → Colorless (Mn²⁺)
   • KMnO₄ (neutral): Purple → Brown ppt (MnO₂)

4. Primary standard confusion

   • K₂Cr₂O₇ is primary standard (stable, pure)
   • KMnO₄ is NOT (needs standardization)

5. Chromyl chloride test

   • Works ONLY for chlorides
   • Doesn’t work for fluorides, bromides, iodides

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Practice Problems

Level 1: Basic Concepts

1. Write balanced equations for:

   • K₂Cr₂O₇ oxidizing FeSO₄ in acidic medium
   • KMnO₄ oxidizing H₂C₂O₄ in acidic medium

2. Why is K₂Cr₂O₇ orange but Cr³⁺ compounds are green?

3. Complete and balance:

   $$MnO_4^- + I^- + H^+ \rightarrow$$

Level 2: Application

4. Calculate the equivalent weight of:

   • K₂Cr₂O₇ in acidic medium
   • KMnO₄ in acidic medium
   • KMnO₄ in neutral medium

5. Explain why:

   • KMnO₄ solution is stored in dark bottles
   • We use H₂SO₄ (not HCl) in KMnO₄ titrations
   • K₂Cr₂O₇ is preferred over KMnO₄ as primary standard

6. A compound decolorizes KMnO₄ in cold, neutral solution. What does this indicate?

Level 3: JEE Advanced

7. 10 mL of K₂Cr₂O₇ solution oxidizes 50 mL of 0.1 M Fe²⁺ in acidic medium. Calculate the molarity of K₂Cr₂O₇.

8. 100 mL of 0.02 M KMnO₄ oxidizes 100 mL of H₂C₂O₄ in acidic medium. Find the molarity of oxalic acid.

9. A mixture of K₂Cr₂O₇ and KMnO₄ (1:1 molar ratio) oxidizes FeSO₄ in acidic medium. Calculate the effective n-factor of the mixture.

10. Assertion (A): Purple color of KMnO₄ is due to charge transfer. Reason (R): Mn⁷⁺ has d⁰ configuration, so no d-d transition is possible.

    • (a) Both A and R true, R explains A
    • (b) Both true, R doesn’t explain A
    • (c) A true, R false
    • (d) Both false

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Memory Tricks

“DIANA” for K₂Cr₂O₇ vs KMnO₄

K₂Cr₂O₇:

• Dichromate
• Is
• Always
• Nice for
• Accurate work (primary standard)

KMnO₄:

• Permanganate
• Is
• More
• Powerful (oxidizer)

n-factor Quick Reference

“5-3-1 Down the Line” (KMnO₄)

• Acidic: 5
• Neutral: 3
• Basic: 1

“Chromium Gives 6” (K₂Cr₂O₇)

• In acidic: always 6

Color Changes

“Orange to Green, Purple to Clean”

• K₂Cr₂O₇ (orange) → Cr³⁺ (green)
• KMnO₄ (purple) → Mn²⁺ (colorless/clean)

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Related Topics

Within d-f Block Elements

• Transition Elements - General characteristics
• Properties of d-Block - Oxidation states, color
• Lanthanoids - f-block elements
• Actinoids - Radioactive elements

Other Chemistry Topics

• Redox Reactions - Oxidation-reduction
• Chemical Equilibrium - Chromate-dichromate
• Coordination Compounds - Ferrocyanide complexes

Practical Chemistry

• Volumetric Analysis - Titrations
• Qualitative Analysis - Detection tests

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