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GATE 2021 CY – Question 37

Inorganic Chemistry · Main Group Elements: Shapes and structures of molecules using VSEPR postulates, Hydrides, halides, oxides, oxoacids, nitrides, sulfides - shapes and reactivity. Structure and bonding of boranes, carboranes, silicones, silicates, boron nitride, borazine and phosphazenes: electron counting in polyhedral boranes, isolobal analogy. Allotropes of carbon, phosphorus and sulfur. Industrial synthesis of compounds of main group elements such as NH3, H2SO4 and HNO3. Chemistry of noble gases and interhalogen compounds. Acid-base concepts and principles (Lewis, Brønsted, HSAB and acid-base catalysis). · 2 marks · Multiple choice

The yellow color of an aqueous solution of K2CrO4 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:

  1. chromium chloride, +3, d-d transition
  2. dichromate ion, +6 and +6, charge transfer
  3. perchlorate ion, +7, charge transfer
  4. chromic acid, +6, charge transfer

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Correct answer: (B) dichromate ion, +6 and +6, charge transfer

Explanation

Acid shifts 2CrO4²−+2H+⇌Cr2O7²−+H2O toward dichromate. Chromium remains +6 at both centres, so both ions are d0 and cannot show ordinary d–d transitions. Their strong colour arises from ligand-to-metal charge transfer. The orange species and assignments are **dichromate, +6/+6, charge transfer, B**.