Potassium Permanganate and Potassium Dichromate — Part 2: Potassium dichromate: visual lesson, ISC Class 12 Chemistry

Lesson

Orange to green: potassium dichromate

Part 2 — making K₂Cr₂O₇ from chromite, its structure, and its use as an oxidising agent.

In this lesson you will

Draw the structure of the dichromate ion
Write the equations for the preparation of K₂Cr₂O₇ from chromite ore
Write balanced equations for dichromate oxidations in acidic medium and use them in titrations
Explain the colour change when the pH of a chromate or dichromate solution is changed

Concept

Preparation from chromite ore

Step 1. Fuse chromite with sodium carbonate in air: chromium is oxidised to +6 as yellow sodium chromate.

Step 2. Filter and acidify with $\ce{H2SO4}$ : orange sodium dichromate forms.

Step 3. Add KCl to the hot concentrated solution: less soluble $\ce{K2Cr2O7}$ crystallises on cooling.

\ce{4FeCr2O4 + 8Na2CO3 + 7O2 -> 8Na2CrO4 + 2Fe2O3 + 8CO2}

\ce{2Na2CrO4 + H2SO4 -> Na2Cr2O7 + Na2SO4 + H2O}

\ce{Na2Cr2O7 + 2KCl -> K2Cr2O7 + 2NaCl}

Table of the three steps in the preparation of potassium dichromate
Step	Product	Colour	Cr state
Roasting with Na₂CO₃ in air	Na₂CrO₄	Yellow	+6
Acidification	Na₂Cr₂O₇	Orange	+6
Adding KCl	K₂Cr₂O₇	Orange crystals	+6

Worked example

Worked example: chromium through the process

Problem. Find the oxidation state of chromium in $\ce{FeCr2O4}$ , $\ce{Na2CrO4}$ and $\ce{Na2Cr2O7}$ . In which step is chromium oxidised, and which reagent oxidises it?

Step 1. $\ce{FeCr2O4}$ (Fe +2): $2 + 2x - 8 = 0$ , so $x = +3$ .

Step 2. $\ce{Na2CrO4}$ : $2 + x - 8 = 0$ , so $x = +6$ . $\ce{Na2Cr2O7}$ : $2 + 2x - 14 = 0$ , so $x = +6$ .

Step 3. Chromium is oxidised (+3 → +6) only in the roasting step, by atmospheric oxygen; acidification is not a redox change.

Table of chromium oxidation states during the preparation
Compound	Cr oxidation state
FeCr₂O₄	+3
Na₂CrO₄	+6
Na₂Cr₂O₇	+6
K₂Cr₂O₇	+6

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