Some Important Compounds of Transition Elements — Core Principles

Core Principles

Transition elements form a variety of important compounds, with potassium permanganate ($KMnO_4$) and potassium dichromate ($K_2Cr_2O_7$) being two prominent examples. Both are powerful oxidizing agents due to the high oxidation states of their central metal atoms (Mn in +7, Cr in +6).

$KMnO_4$ is a dark purple solid, prepared from pyrolusite ($MnO_2$). Its oxidizing action is highly pH-dependent: it reduces to $Mn^{2+}$ (colorless) in acidic media, $MnO_2$ (brown precipitate) in neutral/weakly alkaline media, and $MnO_4^{2-}$ (green) in strongly alkaline media.

It acts as a self-indicator in titrations. The permanganate ion ($MnO_4^-$) has a tetrahedral structure. $K_2Cr_2O_7$ is an orange-red solid, prepared from chromite ($FeCr_2O_4$). It is a strong oxidizing agent, especially in acidic solutions, where it reduces to green $Cr^{3+}$ ions.

The chromate ($CrO_4^{2-}$, yellow) and dichromate ($Cr_2O_7^{2-}$, orange) ions interconvert based on pH. The dichromate ion has two $CrO_4$ tetrahedra sharing an oxygen. Unlike $KMnO_4$, $K_2Cr_2O_7$ requires an external indicator for titrations.

Both compounds are widely used in analytical chemistry and as industrial oxidants.

Important Differences

vs Potassium Permanganate ($KMnO_4$) vs. Potassium Dichromate ($K_2Cr_2O_7$)

Aspect	This Topic	Potassium Permanganate ($KMnO_4$) vs. Potassium Dichromate ($K_2Cr_2O_7$)
Chemical Formula	$KMnO_4$	$K_2Cr_2O_7$
Color	Dark purple (solution is purple)	Orange-red (solution is orange)
Oxidation State of Metal	Mn is +7	Cr is +6
Primary Ore	Pyrolusite ($MnO_2$)	Chromite ($FeCr_2O_4$)
Reduction Product (Acidic Medium)	$Mn^{2+}$ (colorless)	$Cr^{3+}$ (green)
Electrons Gained (Acidic Medium)	5 electrons per $MnO_4^-$	6 electrons per $Cr_2O_7^{2-}$
Self-Indicator	Yes	No (requires external indicator)
Stability	Decomposes on heating	Relatively stable to heat
pH Dependence of Oxidizing Power	Strongly dependent (acidic > neutral > alkaline)	Mainly used in acidic medium; less variation with pH for its primary redox action

Potassium permanganate and potassium dichromate are both powerful oxidizing agents but differ significantly in their properties and applications. $KMnO_4$ is purple with Mn in +7 state, reducing to colorless $Mn^{2+}$ in acidic media by gaining 5 electrons, and acts as a self-indicator. $K_2Cr_2O_7$ is orange with Cr in +6 state, reducing to green $Cr^{3+}$ in acidic media by gaining 6 electrons, and requires an external indicator. Their preparation methods also differ, starting from pyrolusite and chromite ores, respectively. Understanding these distinctions is vital for solving NEET problems.