How is potassium dichromate prepared from chromite ore? What is the effect of pH on its solution? Write ionic equations for its reaction with iodide and with iron(II) ions in acid.

How to lay out the answer. Follow the frame, then write it in your own words.

1 · Step 1: fusion. 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂ (free access of air).
2 · Step 2: acidify. 2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + 2Na⁺ + H₂O.
3 · Step 3: add KCl. Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl; the less soluble orange K₂Cr₂O₇ crystallises.
4 · Effect of pH. Acid: yellow CrO₄²⁻ → orange Cr₂O₇²⁻. Alkali: Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O.
5 · Draw the dichromate ion. Two tetrahedra sharing one O; label Cr-O-Cr 126°.
6 · Oxidising reactions.
- Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 3I₂ + 7H₂O
- Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O
7 · Close in one line. So K₂Cr₂O₇ is a strong oxidant in acid, used in leather tanning and as a primary standard.

Scoring tip: Check each ionic equation for charge: for the iodide one, −2 + 14 − 6 = +6 on the left and 2 × (+3) = +6 on the right.