d and f Block Elements - Preparation and properties of K2Cr2O7 and KMnO4

Potassium Dichromate ($K_2Cr_2O_7$) is prepared from chromite ore ($FeCr_2O_4$) via three steps: (i) roasting with $Na_2CO_3$ to form yellow $Na_2CrO_4$, (ii) acidification with $H_2SO_4$ to form orange $Na_2Cr_2O_7$, and (iii) treatment with $KCl$ to crystallize $K_2Cr_2O_7$.

In aqueous solution, chromate ($CrO_4^{2-}$) and dichromate ($Cr_2O_7^{2-}$) ions exist in equilibrium. Chromate is stable at $pH > 7$ (yellow), while dichromate is stable at $pH < 7$ (orange).

Potassium Permanganate ($KMnO_4$) is prepared from pyrolusite ore ($MnO_2$) by fusion with alkali ($KOH$) and an oxidizing agent ($O_2$ or $KNO_3$) to form green $K_2MnO_4$, followed by electrolytic oxidation or disproportionation in acidic medium.

$KMnO_4$ acts as a powerful oxidizing agent in acidic, neutral, and alkaline media. In acidic medium ($H_2SO_4$), it is reduced to $Mn^{2+}$, involving a change of 5 electrons.

$K_2Cr_2O_7$ is used as a primary standard in volumetric analysis, whereas $KMnO_4$ is not because it can be easily reduced by impurities and is affected by light.

The structures: $CrO_4^{2-}$ is tetrahedral, while $Cr_2O_7^{2-}$ consists of two tetrahedra sharing one oxygen atom with a $Cr-O-Cr$ bond angle of $126^\circ$.