Important compounds — KMnO₄, K₂Cr₂O₇ — preparation, oxidizing reactions

3.3.7 · Chemistry › d-Block (Transition Metals) & f-Block

1. Starting ores aur preparation ki logic

1.1 Potassium permanganate KMnO₄ (pyrolusite MnO₂ se)

Step 1 — Mn(+4) → Mn(+6) (manganate) ko alkaline fusion se oxidise karo:

$2 M n O_{2} + 4 K O H + O_{2} fuse 2 K_{2} M n O_{4} + 2 H_{2} O$

Hawa ka O₂ (ya KNO₃) oxidiser hai. Product = green manganate $M n O_{4}^{2 -}$ .

Step 2 — Mn(+6) → Mn(+7) (permanganate) electrolytic oxidation se (ya disproportionation/Cl₂/O₃ se):

$2 M n O_{4}^{2 -} electrolytic oxidation 2 M n O_{4}^{-} + 2 e^{-}$

Product = purple permanganate $M n O_{4}^{-}$ .

1.2 Potassium dichromate K₂Cr₂O₇ (chromite ore FeCr₂O₄ se)

Step 1 — Chromite ko alkali + air ke saath roast karo, Cr(+3) → Cr(+6) chromate:

$4 F e C r_{2} O_{4} + 8 N a_{2} C O_{3} + 7 O_{2} \to 8 N a_{2} C r O_{4} + 2 F e_{2} O_{3} + 8 C O_{2}$

Yellow chromate $C r O_{4}^{2 -}$ .

Step 2 — Acidify karo (chromate → dichromate):

$2 N a_{2} C r O_{4} + H_{2} S O_{4} \to N a_{2} C r_{2} O_{7} + N a_{2} S O_{4} + H_{2} O$

Step 3 — Na salt ko K salt mein convert karo (K₂Cr₂O₇ kam soluble hai, crystallise ho jaata hai):

$N a_{2} C r_{2} O_{7} + 2 K C l \to K_{2} C r_{2} O_{7} + 2 N a C l$

Figure — Important compounds — KMnO₄, K₂Cr₂O₇ — preparation, oxidizing reactions

2. KMnO₄ ek oxidiser ke roop mein — medium sab kuch decide karta hai

Medium	Mn kya banta hai	Electrons	Colour change
Acidic	$M n^{2 +}$ (+2)	5	purple → colourless
Neutral/weak alkali	$M n O_{2}$ (+4)	3	purple → brown ppt
Strong alkali	$M n O_{4}^{2 -}$ (+6)	1	purple → green

3. K₂Cr₂O₇ ek oxidiser ke roop mein (sirf acidic medium mein kaam karta hai)

Recall Feynman: ek 12-saal ke bacche ko samjhao

Do laalchi bacche imagine karo. Purple baccha (KMnO₄) itna laalchi hai ki woh 5 toffiyaan (electrons) grab kar sakta hai agar room mein bahut saari toffiyaan hain (acid), lekin sirf 3 ya 1 agar room thoda khali ho. Orange baccha (K₂Cr₂O₇) hamesha 6 toffiyaan grab karta hai lekin sirf acid room mein khelta hai. Jab woh toffiyaan grab karte hain toh colour badal jaata hai: purple→clear, orange→green. Unhe banane ke liye pehle hum unhe "charge up" karte hain — dull ore ko washing-soda aur hawa ke saath pakate hain (aur purple wale ke liye thodi electricity bhi) jab tak metal super-laalchi na ho jaye.

Flashcards

KMnO₄ — Mn ki oxidation state

K₂Cr₂O₇ — Cr ki oxidation state

KMnO₄ prep ke liye manganese ka ore

Pyrolusite, MnO₂

K₂Cr₂O₇ prep ke liye chromium ka ore

Chromite, FeCr₂O₄

KMnO₄ prep ka Step 1 (equation)

2 M n O_{2} + 4 K O H + O_{2} \to 2 K_{2} M n O_{4} + 2 H_{2} O

(green manganate)

KMnO₄ prep ka Step 2

MnO₄²⁻ → MnO₄⁻ ka electrolytic oxidation (green→purple)

Manganate vs permanganate ka colour

Manganate green (+6), permanganate purple (+7)

Acidic KMnO₄ half-reaction

M n O_{4}^{-} + 8 H^{+} + 5 e^{-} \to M n^{2 +} + 4 H_{2} O

(5 e⁻)

Neutral KMnO₄ product & electrons

MnO₂ (Mn +4), 3 electrons, brown ppt

Strong-alkali KMnO₄ product & electrons

MnO₄²⁻ (green), 1 electron

Dichromate half-reaction

C r_{2} O_{7}^{2 -} + 14 H^{+} + 6 e^{-} \to 2 C r^{3 +} + 7 H_{2} O

(6 e⁻)

Chromate ⇌ dichromate equilibrium

2 C r O_{4}^{2 -} + 2 H^{+} ⇌ C r_{2} O_{7}^{2 -} + H_{2} O

(acid→orange, base→yellow)

Kya chromate→dichromate ek redox hai?

Nahi — Cr +6 rehta hai, sirf condensation/acid-base hai

KMnO₄ ke saath dilute H₂SO₄ kyun, HCl kyun nahi?

HCl ka Cl⁻ Cl₂ mein oxidise ho jaata hai, KMnO₄ consume ho jaata hai

KMnO₄ + oxalic acid balanced eqn

2 M n O_{4}^{-} + 5 C_{2} O_{4}^{2 -} + 16 H^{+} \to 2 M n^{2 +} + 10 C O_{2} + 8 H_{2} O

K₂Cr₂O₇ + Fe²⁺ balanced eqn

C r_{2} O_{7}^{2 -} + 6 F e^{2 +} + 14 H^{+} \to 2 C r^{3 +} + 6 F e^{3 +} + 7 H_{2} O

K₂Cr₂O₇ primary/secondary standard kyun hai

Pure, stable, non-hygroscopic; KMnO₄ nahi hota

Oxalic acid–KMnO₄ titration garam kyun karte hain

Shuru mein slow hoti hai; Mn²⁺ ise autocatalyse karta hai

Connections

Standard Electrode Potential & E° values — explain karta hai kyun E°(MnO₄⁻)=+1.51 > E°(Cr₂O₇²⁻)=+1.33
Oxidation States of Transition Metals — kyun +7 (Mn) aur +6 (Cr) accessible hain
Balancing Redox Reactions (ion-electron method)
Volumetric Analysis / Titrations — permanganometry & dichrometry
Le Chatelier's Principle — chromate⇌dichromate shift
Colour & d-d Transitions — kyun MnO₄⁻ itna intensely purple hai (charge transfer)