Question

The yellow color of an aqueous solution of K\(_2\)CrO\(_4\) changes to red-orange upon the addition of a few drops of HCl. The red-orange complex, the oxidation state of its central element(s), and the origin of its color, respectively, are:

• A. chromium chloride, +3, d-d transition
• B. dichromate ion, +6 and +6, charge transfer
• C. perchlorate ion, +7, charge transfer
• D. chromic acid, +6, charge transfer

Hint:

The color change from yellow to red-orange upon the addition of HCl is due to the reduction of the chromium ion from +6 to +3.

Answer 1

The Correct Option is B

Step 1: Identify the Yellow and Red-Orange Complexes.
The yellow solution of K\(_2\)CrO\(_4\) contains the dichromate ion (CrO\(_4^{2-}\)), which changes to a red-orange color when HCl is added, forming the chromic ion (Cr\(^{3+}\)) in the presence of chloride.

Step 2: Oxidation States and Color Origin.
The chromate ion (\( \text{Cr}^{6+} \)) is reduced to the chromic ion (\( \text{Cr}^{3+} \)) in the reaction. The color change originates from a charge transfer transition between the metal and the ligand.

Step 3: Conclusion.
The correct answer is the dichromate ion, with an oxidation state of +6 for both chromium and oxygen, and the color change is due to charge transfer.

Final Answer: \[ \text{Dichromate ion, +6 and +6, charge transfer.} \]