Transition-Metal Oxides and Oxoanions 🔥

  • All transition metals except scandium make simple oxides of the form \( \text{MO} \) when heated in air. These oxides are ionic 🧂, and the highest oxidation number matches the group number (e.g., \( \text{Sc}_2\text{O}_3 \) to \( \text{Mn}_2\text{O}_7 \)). :contentReference[oaicite:0]{index=0}
  • As the oxidation number climbs, ionic character falls. Green oily \( \text{Mn}_2\text{O}_7 \) is covalent, and low-melting \( \text{CrO}_3 \) and \( \text{V}_2\text{O}_5 \) are mainly acidic. :contentReference[oaicite:1]{index=1}
  • Acidity trend in vanadium oxides: \( \text{V}_2\text{O}_3 \) (basic) → \( \text{V}_2\text{O}_4 \) (less basic) → \( \text{V}_2\text{O}_5 \) (amphoteric). \( \text{CrO} \) is basic but \( \text{Cr}_2\text{O}_3 \) is amphoteric. 🧪 :contentReference[oaicite:2]{index=2}
  • High-oxidation oxocations include \( \text{VO}_2^{+} \), \( \text{VO}^{2+} \) and \( \text{TiO}^{2+} \). :contentReference[oaicite:3]{index=3}

Disproportionation 🌈

When one oxidation state splits into a higher and a lower state, we call it disproportionation. Example:

\( 3\text{MnO}_4^{2-} + 4\text{H}^{+} \rightarrow 2\text{MnO}_4^{-} + \text{MnO}_2 + 2\text{H}_2\text{O} \)

Here \( \text{Mn}^{VI} \) changes to \( \text{Mn}^{VII} \) and \( \text{Mn}^{IV} \) in acid. :contentReference[oaicite:4]{index=4}

Potassium Dichromate \( \mathbf{K_2Cr_2O_7} \) 🧡

Preparation

  1. Roasting chromite with soda: \( 4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \rightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2 \) :contentReference[oaicite:5]{index=5}
  2. Acidify chromate solution: \( 2\text{Na}_2\text{CrO}_4 + 2\text{H}^{+} \rightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{Na}^{+} + \text{H}_2\text{O} \) :contentReference[oaicite:6]{index=6}
  3. Precipitate orange crystals: \( \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{KCl} \rightarrow K_2\text{Cr}_2\text{O}_7 + 2\text{NaCl} \) :contentReference[oaicite:7]{index=7}

Chromate ⇌ Dichromate ⚖️

\( 2\text{CrO}_4^{2-} + 2\text{H}^{+} \leftrightarrows \text{Cr}_2\text{O}_7^{2-} + \text{H}_2\text{O} \)

\( \text{Cr}_2\text{O}_7^{2-} + 2\text{OH}^{-} \leftrightarrows 2\text{CrO}_4^{2-} + \text{H}_2\text{O} \) :contentReference[oaicite:8]{index=8}

Structure 🔷

  • \( \text{CrO}_4^{2-} \) is tetrahedral.
  • \( \text{Cr}_2\text{O}_7^{2-} \) contains two corner-sharing tetrahedra with a \(126^\circ\) Cr–O–Cr bridge. :contentReference[oaicite:9]{index=9}

Oxidising Power ⚡

Key acidic half-reaction: \( \text{Cr}_2\text{O}_7^{2-} + 14\text{H}^{+} + 6e^{-} \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}\;(E^\circ = 1.33\ \text{V}) \) :contentReference[oaicite:10]{index=10}

It changes \( \text{I}^- \rightarrow \text{I}_2 \), \( \text{H}_2\text{S} \rightarrow \text{S} \), \( \text{Sn}^{2+} \rightarrow \text{Sn}^{4+} \), and \( \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} \). :contentReference[oaicite:11]{index=11}

Uses 🎯

Leather tanning, primary standard in titrations, and a go-to oxidant in organic synthesis. :contentReference[oaicite:12]{index=12}

Potassium Permanganate \( \mathbf{KMnO_4} \) 💜

Preparation

  1. Fusion route: \( 2\text{MnO}_2 + 4\text{KOH} + \text{O}_2 \rightarrow 2\text{K}_2\text{MnO}_4 + 2\text{H}_2\text{O} \) :contentReference[oaicite:13]{index=13}
  2. Disproportionation: \( 3\text{MnO}_4^{2-} + 4\text{H}^{+} \rightarrow 2\text{MnO}_4^{-} + \text{MnO}_2 + 2\text{H}_2\text{O} \) :contentReference[oaicite:14]{index=14}
  3. Industrial electrolytic oxidation of manganate in alkali. :contentReference[oaicite:15]{index=15}

Properties

  • Dark-purple crystals; low solubility (~6 g / 100 g water at 20 °C). :contentReference[oaicite:16]{index=16}
  • Thermally decomposes: \( 2\text{KMnO}_4 \rightarrow K_2\text{MnO}_4 + \text{MnO}_2 + \text{O}_2 \) 🔥 :contentReference[oaicite:17]{index=17}
  • Both manganate (\( \text{MnO}_4^{2-} \)) and permanganate (\( \text{MnO}_4^- \)) are tetrahedral. Green manganate is paramagnetic (1 unpaired e⁻), purple permanganate is diamagnetic. :contentReference[oaicite:18]{index=18}

Redox Behaviour ⚡

Acidic half-reactions:

  • \( \text{MnO}_4^- + e^- \rightarrow \text{MnO}_4^{2-} \;(E^\circ = +0.56\ \text{V}) \)
  • \( \text{MnO}_4^- + 4\text{H}^{+} + 3e^- \rightarrow \text{MnO}_2 + 2\text{H}_2\text{O}\;(E^\circ = +1.69\ \text{V}) \)
  • \( \text{MnO}_4^- + 8\text{H}^{+} + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}\;(E^\circ = +1.52\ \text{V}) \)

Typical oxidations:

  • \( \text{I}^- \rightarrow \text{I}_2 \)
  • \( \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} \)
  • \( \text{C}_2\text{O}_4^{2-} \rightarrow \text{CO}_2 \)
  • \( \text{H}_2\text{S} \rightarrow \text{S} \) 🌟 :contentReference[oaicite:19]{index=19}

Neutral/alkaline solutions give brown \( \text{MnO}_2 \) but still oxidise iodide to iodate and thiosulphate to sulphate. :contentReference[oaicite:20]{index=20}

Uses 🛠️

Self-indicating titrant, versatile oxidiser in organic labs, and bleaching/deodorising agent for textiles and oils. :contentReference[oaicite:21]{index=21}

Important Concepts for NEET 🎯

  1. Basic → acidic trend in oxides: remember \( \text{V}_2\text{O}_3 \) (basic) to \( \text{V}_2\text{O}_5 \) (amphoteric). :contentReference[oaicite:22]{index=22}
  2. Chromate (yellow) ⇌ dichromate (orange) equilibrium depends on pH. ⚖️ :contentReference[oaicite:23]{index=23}
  3. Standard redox couple of dichromate in acid: \( \text{Cr}_2\text{O}_7^{2-}/\text{Cr}^{3+} \) (E° = 1.33 V). 🌟 :contentReference[oaicite:24]{index=24}
  4. Preparation of \( \text{KMnO}_4 \) via fusion → manganate → permanganate, plus its vivid self-indicator colour. 💜 :contentReference[oaicite:25]{index=25}
  5. Disproportionation concept illustrated by manganate (\( \text{Mn}^{VI} \)) → permanganate (\( \text{Mn}^{VII} \)) + \( \text{MnO}_2 \) (\( \text{Mn}^{IV} \)). 🌈 :contentReference[oaicite:26]{index=26}