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Concept 04 / 06The d and f Block Elements

Important Compounds: Potassium Dichromate and Potassium Permanganate

Preparation of potassium dichromate from chromite ore, chromate-dichromate equilibrium and structures, oxidising reactions of acidified dichromate; preparation of potassium permanganate from pyrolusite (fusion, disproportionation or electrolytic oxidation), its structure and colour, and oxidising reactions in acidic, neutral and alkaline media.

Figure 1
Potassium dichromate, K2Cr2O7\mathrm{K_{2}Cr_{2}O_{7}}. Preparation from chromite ore (FeCr2O4\mathrm{FeCr_{2}O_{4}}): 1. Fusion with sodium carbonate in free air: 4FeCr2O4+8Na2CO3+7O2→8Na2CrO4+2Fe2O3+8CO2\mathrm{4FeCr_{2}O_{4} + 8Na_{2}CO_{3} + 7O_{2} \rightarrow 8Na_{2}CrO_{4} + 2Fe_{2}O_{3} + 8CO_{2}}. 2. The yellow sodium chromate solution is filtered and acidified with H2SO4\mathrm{H_{2}SO_{4}}: 2Na2CrO4+2H+→Na2Cr2O7+2Na++H2O\mathrm{2Na_{2}CrO_{4} + 2H^{+} \rightarrow Na_{2}Cr_{2}O_{7} + 2Na^{+} + H_{2}O} (orange). 3. Treatment with KCl gives the less soluble potassium salt: Na2Cr2O7+2KCl→K2Cr2O7+2NaCl\mathrm{Na_{2}Cr_{2}O_{7} + 2KCl \rightarrow K_{2}Cr_{2}O_{7} + 2NaCl}. Chromate-dichromate equilibrium (see figure): 2CrO42−+2H+⇌Cr2O72−+H2O\mathrm{2CrO_{4}^{2-} + 2H^{+} \rightleftharpoons Cr_{2}O_{7}^{2-} + H_{2}O}. In acid, orange dichromate dominates; in alkali, yellow chromate. Chromium is +6+6 in both (not a redox reaction). Chromate is tetrahedral; dichromate consists of two tetrahedra sharing one corner oxygen (Cr-O-Cr angle 126∘126^{\circ}). Oxidising action (acidic solution): Cr2O72−+14H++6e−→2Cr3++7H2O\mathrm{Cr_{2}O_{7}^{2-} + 14H^{+} + 6e^{-} \rightarrow 2Cr^{3+} + 7H_{2}O}, E∘=1.33 VE^{\circ} = 1.33\,\mathrm{V} (orange to green). 6I−→3I2\mathrm{6I^{-} \rightarrow 3I_{2}}; 6Fe2+→6Fe3+\mathrm{6Fe^{2+} \rightarrow 6Fe^{3+}}; 3H2S→3S\mathrm{3H_{2}S \rightarrow 3S}; 3Sn2+→3Sn4+\mathrm{3Sn^{2+} \rightarrow 3Sn^{4+}}; for example Cr2O72−+3H2S+8H+→2Cr3++3S+7H2O\mathrm{Cr_{2}O_{7}^{2-} + 3H_{2}S + 8H^{+} \rightarrow 2Cr^{3+} + 3S + 7H_{2}O}. Uses: primary standard in volumetric analysis (it is not hygroscopic), leather tanning, oxidant in organic chemistry, chrome alum. Potassium permanganate, KMnO4\mathrm{KMnO_{4}}. Preparation: pyrolusite (MnO2\mathrm{MnO_{2}}) is fused with KOH and an oxidant (air or KNO3\mathrm{KNO_{3}}) to give dark green potassium manganate: 2MnO2+4KOH+O2→2K2MnO4+2H2O\mathrm{2MnO_{2} + 4KOH + O_{2} \rightarrow 2K_{2}MnO_{4} + 2H_{2}O}. Manganate disproportionates in neutral or acidic solution: 3MnO42−+4H+→2MnO4−+MnO2+2H2O\mathrm{3MnO_{4}^{2-} + 4H^{+} \rightarrow 2MnO_{4}^{-} + MnO_{2} + 2H_{2}O}. Commercially, it is oxidised electrolytically in alkaline solution: MnO42−→MnO4−+e−\mathrm{MnO_{4}^{2-} \rightarrow MnO_{4}^{-} + e^{-}}. In the laboratory, Mn2+\mathrm{Mn^{2+}} is oxidised by peroxodisulfate: 2Mn2++5S2O82−+8H2O→2MnO4−+10SO42−+16H+\mathrm{2Mn^{2+} + 5S_{2}O_{8}^{2-} + 8H_{2}O \rightarrow 2MnO_{4}^{-} + 10SO_{4}^{2-} + 16H^{+}}. Structure and properties: permanganate is tetrahedral with Mn in +7+7 (d0d^{0}, diamagnetic); its intense purple colour is due to charge transfer; manganate (Mn6+\mathrm{Mn^{6+}}, d1d^{1}) is green and paramagnetic. On heating to 513 K513\,\mathrm{K}: 2KMnO4→K2MnO4+MnO2+O2\mathrm{2KMnO_{4} \rightarrow K_{2}MnO_{4} + MnO_{2} + O_{2}}. Oxidising action: acidic (MnO4−+8H++5e−→Mn2++4H2O\mathrm{MnO_{4}^{-} + 8H^{+} + 5e^{-} \rightarrow Mn^{2+} + 4H_{2}O}, E∘=1.52 VE^{\circ} = 1.52\,\mathrm{V}): I−→I2\mathrm{I^{-} \rightarrow I_{2}}, Fe2+→Fe3+\mathrm{Fe^{2+} \rightarrow Fe^{3+}}, C2O42−→CO2\mathrm{C_{2}O_{4}^{2-} \rightarrow CO_{2}} (at 333 K333\,\mathrm{K}), H2S→S\mathrm{H_{2}S \rightarrow S}, SO32−→SO42−\mathrm{SO_{3}^{2-} \rightarrow SO_{4}^{2-}}, NO2−→NO3−\mathrm{NO_{2}^{-} \rightarrow NO_{3}^{-}}; neutral or faintly alkaline (MnO4−+2H2O+3e−→MnO2+4OH−\mathrm{MnO_{4}^{-} + 2H_{2}O + 3e^{-} \rightarrow MnO_{2} + 4OH^{-}}): I−→IO3−\mathrm{I^{-} \rightarrow IO_{3}^{-}}, S2O32−→SO42−\mathrm{S_{2}O_{3}^{2-} \rightarrow SO_{4}^{2-}}, Mn2+→MnO2\mathrm{Mn^{2+} \rightarrow MnO_{2}} (in presence of zinc sulfate or zinc oxide). Uses: oxidant in analysis (self-indicator), bleaching of wool, cotton and silk, disinfectant, decolourisation of oils. Key Takeaways: - Chromite + Na2CO3\mathrm{Na_{2}CO_{3}}/air → Na2CrO4\mathrm{Na_{2}CrO_{4}} → (acid) Na2Cr2O7\mathrm{Na_{2}Cr_{2}O_{7}} → (KCl) K2Cr2O7\mathrm{K_{2}Cr_{2}O_{7}}. - Chromate (yellow, alkali) ⇌ dichromate (orange, acid); both Cr +6+6. - Cr2O72−\mathrm{Cr_{2}O_{7}^{2-}}: 66 electrons, orange to green Cr3+\mathrm{Cr^{3+}}. - MnO2\mathrm{MnO_{2}} → K2MnO4\mathrm{K_{2}MnO_{4}} (green) → KMnO4\mathrm{KMnO_{4}} (purple) by disproportionation or electrolytic oxidation. - In neutral medium, permanganate gives MnO2\mathrm{MnO_{2}} and oxidises iodide to iodate. - Pitfall: the chromate-dichromate interconversion changes colour but is not a redox reaction. Brain-Teaser 1: How many moles of K2Cr2O7\mathrm{K_{2}Cr_{2}O_{7}} are needed to oxidise 3 mol3\,\mathrm{mol} of H2S\mathrm{H_{2}S} in acid, and what is the colour change? Solution: Cr2O72−+3H2S+8H+→2Cr3++3S+7H2O\mathrm{Cr_{2}O_{7}^{2-} + 3H_{2}S + 8H^{+} \rightarrow 2Cr^{3+} + 3S + 7H_{2}O}: 1 mol1\,\mathrm{mol} dichromate for 3 mol3\,\mathrm{mol} H2S\mathrm{H_{2}S} (each S loses 22 electrons; dichromate gains 66). The solution turns from orange to green, with a pale yellow precipitate of sulfur. Brain-Teaser 2: Why is KMnO4\mathrm{KMnO_{4}} purple although manganese is in the +7+7 (d0d^{0}) state? Solution: With no d electrons, d-d transitions are impossible. The colour arises from ligand-to-metal charge transfer: an electron from an oxygen p orbital is excited into an empty Mn d orbital by absorption of green-yellow light, so the transmitted light appears purple. Brain-Teaser 3: What happens when potassium manganate is dissolved in dilute acid? Write the equation and identify the type of reaction. Solution: Green manganate disproportionates: 3MnO42−+4H+→2MnO4−+MnO2+2H2O\mathrm{3MnO_{4}^{2-} + 4H^{+} \rightarrow 2MnO_{4}^{-} + MnO_{2} + 2H_{2}O}. Mn(+6+6) is both oxidised to +7+7 (purple permanganate) and reduced to +4+4 (brown MnO2\mathrm{MnO_{2}} precipitate).

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Important Compounds: Potassium Dichromate and Potassium Permanganate — DPP

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