Oxidation States & Key Compounds
Decode the vast array of d-block properties. Master the thermodynamic reasons behind E° anomalies, the exact n-factors of KMnO₄, and the pH-dependent shifts of Dichromates.
1. Variable Oxidation States & Oxides
Transition elements exhibit a large number of oxidation states because the energy difference between the $ns$ and $(n-1)d$ orbitals is very small. Both sets of electrons can participate in bonding.
- Scandium (Sc): Only shows $+3$ (rarely +2). Losing 3 electrons gives it an ultra-stable Noble Gas core ($Ar$).
- Zinc (Zn): Only shows $+2$. Losing its $4s$ electrons leaves a highly stable, completely filled $3d^{10}$ core.
- The Maximum: Oxidation states increase up to the middle of the series. Manganese (Mn) shows the highest in 3d ($+7$).
The acidic/basic nature of transition metal oxides depends strictly on the oxidation state of the metal:
- Basic Low Oxidation States (+2, +3). e.g., $MnO$
- Amphoteric Intermediate States (+3, +4). e.g., $MnO_2$, $Cr_2O_3$, $V_2O_5$
- Acidic High Oxidation States (+5, +6, +7). e.g., $Mn_2O_7$, $CrO_3$
2. Standard Electrode Potentials ($E^\circ$)
The standard electrode potential ($M^{2+}/M$) reflects the thermodynamic ease of oxidizing the solid metal to aqueous ions. It is the sum of three energy terms: Sublimation Enthalpy ($\Delta_{sub}H^\circ$), Ionization Enthalpy ($IE_1 + IE_2$), and Hydration Enthalpy ($\Delta_{hyd}H^\circ$).
Almost all 3d metals have a negative $E^\circ(M^{2+}/M)$ value, meaning they act as reducing agents and can displace $H_2$ gas from acids. Copper ($Cu$) is the ONLY exception.
Why? Copper has an unusually high Enthalpy of Atomization (sublimation) and high Ionization Enthalpies. The energy released by its Hydration Enthalpy is not sufficient to compensate for the energy required to vaporize and ionize it. Therefore, $\Delta G^\circ$ is positive, making the oxidation of Cu to $Cu^{2+}$ non-spontaneous under standard conditions.
This potential tells us how easy it is to remove a third electron. A low value means the +3 state is very stable; a high value means the +2 state is preferred.
- Scandium: Very low value. It loves being $Sc^{3+}$ because it reaches a noble gas core.
- Manganese: Very HIGH value. $Mn^{2+}$ is $3d^5$ (exactly half-filled, highly stable). Removing a 3rd electron disrupts this stability, so it strongly resists becoming $Mn^{3+}$.
- Iron: Comparatively lower value. $Fe^{2+}$ is $3d^6$. Losing an electron gives $Fe^{3+}$ ($3d^5$), which is highly stable.
3. Potassium Dichromate ($K_2Cr_2O_7$)
A. The pH-Dependent Interconversion
Chromate ($CrO_4^{2-}$, Yellow) and Dichromate ($Cr_2O_7^{2-}$, Orange) ions exist in equilibrium. The predominant species depends entirely on the pH of the solution. Note: The oxidation state of Chromium is $+6$ in BOTH ions!
Dichromate (Orange) turns into Chromate (Yellow).
Chromate (Yellow) turns into Dichromate (Orange).
B. Oxidizing Action & Equivalent Weight
$K_2Cr_2O_7$ acts as a powerful oxidizing agent, primarily in acidic medium.
The total change in oxidation state for one molecule of $K_2Cr_2O_7$ (which contains two Cr atoms going from +6 to +3) is 6.
Therefore, the n-factor is 6, and its Equivalent Weight is $\mathbf{M / 6}$.
4. Potassium Permanganate ($KMnO_4$)
$KMnO_4$ is intense purple. NEET Trap: The Mn in $MnO_4^-$ is in the +7 state ($3d^0$). Since there are NO unpaired d-electrons, it is diamagnetic, and d-d transitions cannot occur. The intense purple color is strictly due to Ligand-to-Metal Charge Transfer (LMCT) from Oxygen to Manganese.
| Medium | Reduction Half-Reaction | n-factor (Change in OS) |
|---|---|---|
| Acidic ($H^+$) | $MnO_4^- + 8H^+ + \mathbf{5e^-} \rightarrow Mn^{2+} + 4H_2O$ | 5 |
| Neutral / Faintly Basic | $MnO_4^- + 2H_2O + \mathbf{3e^-} \rightarrow MnO_2\downarrow + 4OH^-$ | 3 |
| Strongly Basic | $MnO_4^- + \mathbf{1e^-} \rightarrow MnO_4^{2-}$ | 1 |
$KMnO_4$ oxidizes Iodide ($I^-$) differently depending on the pH of the medium. You must memorize this exact difference:
NEET Grand Test: d-Block Compounds
15 High-Yield Questions testing n-factors, medium-dependent reactions, and the Copper anomaly.
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