🛡️ d & f-Block Elements & Lanthanides

Transition and inner-transition elements bridge electropositive s-block metals and electronegative p-block elements. Key examination highlights include standard reduction potential ($E^{c}irc$) trends, d-d transition colors, interstitial hydrides & carbides, catalytic mechanisms, and $KMn{O}_4/K_{2}C{r}_2{O}_7$ preparation and redox cycles.

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1. ⚙️ Characteristic Transition Metal Properties

1. Catalytic Activity: Due to ability to adopt variable oxidation states ($V_2{O}_5$ in Contact process, finely divided $Fe$ in Haber process, $TiC{l}_4+Al(C_2{H}_5{)}_3$ Ziegler-Natta catalyst) and provide large surface area.
2. Interstitial Compounds: Small non-metal atoms ($H,C,N,B$) trapped inside interstitial voids of transition metal lattices (e.g. $TiC,F{e}_{3}H,M{n}_{4}N$).
   • Properties: Very high melting points (higher than pure metals), extremely hard, retain metallic conductivity, chemically inert.
3. Standard Electrode Potential ($E^{c}ir{c}_{M^{2+}/M}$) Anomalies:$$E_{M^{2+}/M}^{\circ }=\mathrm{\Delta }{H}_{\text{sub}}+{\text{IE}}_1+{\text{IE}}_2+\mathrm{\Delta }{H}_{\text{hyd}}$$
   • $E^{c}ir{c}_{C{u}^{2+}/Cu}=+0.34\text{ V}$ (Positive!): High sum of sublimation enthalpy and ionization energies of Copper cannot be compensated by its hydration enthalpy. Therefore, $Cu$ does not liberate $H_2$ gas with non-oxidizing dilute acids ($HCl$).

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2. 🧪 Potassium Dichromate ($K_{2}C{r}_2{O}_7$) Chemistry

2.1 Industrial Preparation from Chromite Ore ($FeC{r}_2{O}_4$)

1. Fusion with Soda Ash:$$4FeC{r}_2{O}_4+8N{a}_{2}C{O}_3+7O_2\to 8N{a}_{2}Cr{O}_4(\text{Yellow})+2F{e}_2{O}_3+8C{O}_2\uparrow$$
2. Acidification with Sulfuric Acid:$$2N{a}_{2}Cr{O}_4+H_{2}S{O}_4\to N{a}_{2}C{r}_2{O}_7(\text{Orange})+N{a}_{2}S{O}_4+H_{2}O$$
3. Potassium Exchange:$$N{a}_{2}C{r}_2{O}_7+2KCl\to K_{2}C{r}_2{O}_7\downarrow (\text{Orange crystals})+2NaCl$$

2.2 Chromate-Dichromate pH Equilibrium

$$2Cr{O}_4^{2-}(\text{Yellow})+2H^{+}\underset{\text{Alkali}}{\stackrel{\text{Acid}}{\rightleftharpoons }}C{r}_2{O}_7^{2-}(\text{Orange})+H_{2}O$$

• In basic medium ($ ext{pH} > 7$):YellowChromate($CrO_4^{2-}$) predominates.
• In acidic medium ($ ext{pH} < 7$):OrangeDichromate($Cr_2O_7^{2-}$) predominates.
• Note: Oxidation state of Chromium remains strictly $+6$ in both species; it is an acid-base equilibrium, NOT a redox reaction!

2.3 Chromyl Chloride Test ($Cr{O}_{2}C{l}_2$)

Solid ionic chloride salt $+K_{2}C{r}_2{O}_7(s)+\text{conc. }{H}_{2}S{O}_4\stackrel{\mathrm{\Delta }}{\to }$ Deep red vapors of Chromyl Chloride ($Cr{O}_{2}C{l}_2$):

$$Cr{O}_{2}C{l}_2+2NaOH\to N{a}_{2}Cr{O}_4(\text{Yellow solution})+2NaCl+H_{2}O$$$$N{a}_{2}Cr{O}_4+(C{H}_{3}COO{)}_{2}Pb\to {\mathbf{PbCrO}}_{\mathbf{4}}\downarrow \text{ (Yellow Precipitate)}$$

(Covalent chlorides like $H{g}_{2}C{l}_2,AgCl,PbC{l}_2,SnC{l}_2$ do NOT give the Chromyl Chloride test).

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3. 🟣 Potassium Permanganate ($KMn{O}_4$) Chemistry

3.1 Industrial Preparation from Pyrolusite Ore ($Mn{O}_2$)

1. Alkaline Oxidative Fusion:$$2Mn{O}_2+4KOH+O_2\to 2K_{2}Mn{O}_4(\text{Dark Green})+2H_{2}O$$
2. Electrolytic / Acidic Disproportionation:$$3Mn{O}_4^{2-}+4H^{+}\to 2Mn{O}_4^{-}(\text{Deep Purple})+Mn{O}_2\downarrow +2H_{2}O$$

3.2 Origin of Color in $KMn{O}_4$ and $K_{2}C{r}_2{O}_7$

In $Mn{O}_4^{-}$ ($M{n}^{7+}$, $d^0$) and $C{r}_2{O}_7^{2-}$ ($C{r}^{6+}$, $d^0$), there are zero d-electrons ($d^0$), so $d-d$ transition is impossible!

$$\text{mathbf{text{Intense color is due to Ligand-to-Metal Charge Transfer (LMCT) } O^{2-} to M^{n+}}}}$$

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4. 🌌 Lanthanides & Actinides (f-Block)

4.1 Lanthanide Contraction & $+3$ Oxidation State

• General electronic configuration: $[Xe]4f^{1-14}5d^{0-1}6s^2$.
• Most stable oxidation state is $+3$.
• $E{u}^{2+}$ ($[Xe]4f^7$) and $Y{b}^{2+}$ ($[Xe]4f^{14}$) are strong reducing agents ($E{u}^{2+}\to E{u}^{3+}+e^{-}$).
• $C{e}^{4+}$ ($[Xe]4f^0$) and $T{b}^{4+}$ ($[Xe]4f^7$) are strong oxidizing agents ($C{e}^{4+}+e^{-}\to C{e}^{3+}$).

4.2 Basic Character of Lanthanide Hydroxides

$$\mathbf{La}\mathbf{(}\mathbf{OH}{\mathbf{)}}_{\mathbf{3}}\mathbf{>}\mathbf{Ce}\mathbf{(}\mathbf{OH}{\mathbf{)}}_{\mathbf{3}}\mathbf{>}\cdots \mathbf{>}\mathbf{Lu}\mathbf{(}\mathbf{OH}{\mathbf{)}}_{\mathbf{3}}$$

• Due to lanthanide contraction, size of $M^{3+}$ decreases from $L{a}^{3+}$ ($1.06\text{AA}$) to $L{u}^{3+}$ ($0.86\text{AA}$).
• Covalent character of $M-OH$ bond increases (Fajans' rule) $⟹La(OH{)}_3$ is most basic, $Lu(OH{)}_3$ is least basic.