Question

$\textbf{The orange colour of } \text{K}_2\text{Cr}_2\text{O}_7 \textbf{ and purple colour of KMnO}_4 \textbf{ is due to}$

• A. d$\rightarrow$d transition in K$_2$Cr$_2$O$_7$ and charge transfer transitions in KMnO$_4$.
• B. d$\rightarrow$d transition in KMnO$_4$ and charge transfer transitions in K$_2$Cr$_2$O$_7$.
• C. Charge transfer transition in both.
• D. d$\rightarrow$d transition in both.

Answer 1

The Correct Option is C

The question asks us to identify the reason for the colors observed in K₂Cr₂O₇ (orange) and KMnO₄ (purple). The options provided suggest different types of electronic transitions. Let's explore the correct reasoning behind the colorations:

1. Charge Transfer Transition:
   • The color of a compound is often a result of the electronic transitions happening within it.
   • In inorganic compounds, two primary types of transitions can occur: d-d transitions and charge transfer transitions.
   • In a charge transfer transition, an electron is transferred between the metal and the ligands. These transitions are generally very strong as they involve a transfer of an electron across elements, and are typically seen in highly colored compounds like K₂Cr₂O₇ and KMnO₄.
2. d-d Transition:
   • This type of transition involves an electron jumping between d-orbitals of a transition metal.
   • d-d transitions are weak as they involve electrons within the same atom without a change in chemical bonding.
   • Generally observed in complexes with partially filled d orbitals, such as many transition metal complexes.
3. Chromate and Permanganate Colors:
   • K₂Cr₂O₇ (Potassium Dichromate): The orange color of potassium dichromate arises due to the charge transfer transition between the chromium atoms and the oxygen atoms that constitute its structure.
   • KMnO₄ (Potassium Permanganate): The intense purple color arises due to similar charge transfer transitions from the ligand to the manganese metal.

Hence, the correct explanation for the coloration of both compounds is due to charge transfer transitions that occur within their molecular structures.

The correct option is: Charge transfer transition in both.

Answer 2

The colors observed in \(K_2Cr_2O_7\) and \(KMnO_4\) arise from electronic transitions within the compounds:

Charge Transfer Transitions: Charge transfer transitions involve the movement of an electron from a filled orbital (usually a ligand) to an empty orbital of a metal ion.

In the case of \(K_2Cr_2O_7\), the orange color is primarily due to the charge transfer transition associated with the \(Cr^{6+}\) ion.

In \(KMnO_4\), the purple color results from a similar charge transfer transition involving the \(Mn^{7+}\) ion.

Summary of Transitions: Both compounds exhibit charge transfer transitions rather than d → d transitions, which are more common in transition metal complexes with partially filled d orbitals.

Thus, the correct explanation for the observed colors in both \(K_2Cr_2O_7\) and \(KMnO_4\) is: Charge transfer transition in both.