In the reaction of potassium dichromate, potassium chloride and sulfuric acid (conc.), the oxidation state of the chromium in the product is (+) _________.

Solution

Chemistry ma’am · 4 steps

This is the chromyl chloride test, and chromium never changes its oxidation state in it.

1. Name the reaction. Heating a chloride with K₂Cr₂O₇ and concentrated H₂SO₄ is the chromyl chloride test. It gives orange-red fumes of chromyl chloride, CrO₂Cl₂.

   K₂Cr₂O₇ + 4KCl + 6H₂SO₄ → 2CrO₂Cl₂ + 6KHSO₄ + 3H₂O

2. Oxidation state of Cr. In CrO₂Cl₂, take Cr as x, each O as -2 and each Cl as -1. The molecule is neutral, so x + 2(-2) + 2(-1) = 0, which gives x = +6.
3. Check the start. In K₂Cr₂O₇, 2(+1) + 2x + 7(-2) = 0, so Cr is also +6. Chromium is neither oxidised nor reduced, so this is not a redox reaction.
4. So. Chromium in the product CrO₂Cl₂ is in the +6 oxidation state.

The common slip: 3. Treating dichromate as an oxidising agent that is reduced to Cr³⁺ gives 3. Chloride is not oxidised here, and the product is CrO₂Cl₂.

The idea to keep. Not every reaction of dichromate is a redox reaction. Write the actual product first, then assign oxidation states. In the chromyl chloride test, Cr stays at +6.

Answer from the NTA final answer key. Solution by PrepWiser.