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5.3.2Unit 5. Transition Metals and Organic Nitrogen Chemistry

Chromium Chemistry — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 22 of 43 Unit quiz

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Part 1 of 6Oxidation States

5.3.2 Chromium Chemistry
+6
  • Dichromate Cr₂O₇²⁻ is orange and chromate CrO₄²⁻ is yellow.
+3
  • The aqua ion [Cr(H₂O)₆]³⁺ is violet, though solutions usually look green.
+2
  • Cr²⁺ is blue and is a strong reducing agent, readily oxidised by air.
Reduction
  • Zinc and acid reduce orange Cr₂O₇²⁻ to green Cr³⁺ and then to blue Cr²⁺.

Part 2 of 6Chromium(III) Reactions

1
Adding a little sodium hydroxide gives a grey-green precipitate of Cr(OH)₃(H₂O)₃.
2
The hydroxide is amphoteric, so excess NaOH dissolves it to give a dark green solution containing [Cr(OH)₆]³⁻.
3
Adding excess ammonia also dissolves the precipitate slowly, forming a purple solution of [Cr(NH₃)₆]³⁺.
4
Hydrogen peroxide in alkaline solution oxidises the green chromium(III) complex to yellow chromate(VI).
5
2[Cr(OH)₆]³⁻ + 3H₂O₂ → 2CrO₄²⁻ + 2OH⁻ + 8H₂O

Part 3 of 6Chromate-Dichromate Equilibrium

Chromate-Dichromate Equilibrium
Equation
  • 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O, yellow on the left and orange on the right.
Adding acid
  • The equilibrium shifts right and the solution turns orange.
Adding alkali
  • H⁺ ions are removed and the equilibrium shifts left, giving a yellow solution.
Redox or not
  • The oxidation number of chromium stays at +6, so this is an acid-base process and not a redox reaction.

Part 4 of 6Dichromate as an Oxidising Agent

Dichromate as an Oxidising Agent
Half-equation
  • Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O.
Colour change
  • The solution changes from orange to green as chromium(VI) is reduced.
Use
  • Acidified dichromate is used to oxidise alcohols and to titrate iron(II).

Part 5 of 6Worked Example

Question

  • A 25.0 cm³ sample of Fe²⁺ solution is titrated with 0.0150 mol dm⁻³ K₂Cr₂O₇ and needs 20.0 cm³.
  • Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H₂O.
  • Calculate [Fe²⁺].

  1. Moles Cr₂O₇²⁻ = 0.0200 dm³ x 0.0150 mol dm⁻³ = 3.00 x 10⁻⁴ mol.
  2. Moles Fe²⁺ = 6 x 3.00 x 10⁻⁴ = 1.80 x 10⁻³ mol.
  3. Concentration = 1.80 x 10⁻³ / 0.0250 dm³ = 0.0720 mol dm⁻³.

Answer [Fe²⁺] = 0.0720 mol dm⁻³

Part 6 of 6Exam Checklist & Tips

5.3.2 – Key Points for the Exam

  • Amphoteric Able to react with both acids and bases.
  • Chromate The yellow CrO₄²⁻ ion.
  • Dichromate The orange Cr₂O₇²⁻ ion.
  • Hexaaqua ion A metal ion surrounded by six water ligands.

Common Mistakes

  • Saying the chromate-dichromate change is a redox reaction.
  • Confusing the orange dichromate with the yellow chromate.
  • Forgetting that Cr(OH)₃ dissolves in excess alkali.
  • Writing the dichromate half-equation without 14H⁺.

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