The difference in oxidation state of chromium in chromate and dichromate salts is ___________.

Solution

Chemistry ma’am · 4 steps

Work out chromium's oxidation state in each ion separately, per chromium atom. They come out equal.

1. Chromate ion. Chromate is CrO₄²⁻. Each oxygen is -2, so x + 4(-2) = -2, which gives x = +6.
2. Dichromate ion. Dichromate is Cr₂O₇²⁻, with two chromium atoms. Divide the total by 2 to get the value for one atom.

   2x + 7(-2) = -2 ⇒ 2x = +12 ⇒ x = +6

3. Why they match. The change 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O is an acid–base change, not a redox change. That is why chromium stays at +6 in both ions.
4. So. Both ions have chromium in the +6 state, so the difference is 6 - 6 = 0.

The common slip: 6. Using the total charge on both chromiums in dichromate, +12, instead of the value for one atom gives 12 - 6 = 6, not 0.

The idea to keep. Chromate and dichromate change into each other when only the pH changes. No electrons move, so chromium stays at +6 in both. Always find the oxidation state of one atom, not the total.

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