Easy Practice — Set 2

Quick Recap — Compounds, Colour & Contraction

• $KMnO_4$ (from pyrolusite $MnO_2$) is a strong oxidiser; in acid Mn($+7$) $\to$ Mn($+2$) gaining 5 electrons.
• $K_2Cr_2O_7$ (from chromite): $Cr_2O_7^{2-}$ (orange, acidic) $\rightleftharpoons2CrO_4^{2-}$ (yellow, basic); oxidises $Fe^{2+}\to Fe^{3+}$.
• Colour needs partially filled d-orbitals: $d^0$ (Sc$^{3+}$, Ti$^{4+}$) and $d^{10}$ (Zn$^{2+}$, Cu$^+$) ions are colourless.
• Lanthanoid contraction ($\Rightarrow$ Zr $\approx$ Hf) arises from poor 4f shielding; there are 14 lanthanoids and 14 actinoids.