5.6.6Physical Chemistry and Transition Elements
Redox Reactions of Transition Elements
5 min read 4 parts Lesson 39 of 40 Virtual lab
Part 1 of 4Iron(II) and Iron(III)
Iron shows two common oxidation states, and the change is easy to follow by colour.
Oxidation of Fe²⁺
- Acidified manganate(VII) oxidises pale green Fe²⁺ to yellow-brown Fe³⁺: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺.
Dichromate with Fe²⁺
- Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺, with the colour changing from orange to green.
Reduction of Fe³⁺
- Iodide ions reduce Fe³⁺2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂, and the brown colour of iodine can mask the colour change.
Why it works
- The E⦵ of Fe³⁺/Fe²⁺ lies between the values for the strong oxidising agents and for iodide, so iron can act either way.
Part 2 of 4Chromium Chemistry
Chromium has a clear sequence of colours in its different oxidation states.
1
Dichromate(VI) ions, Cr₂O₇²⁻, are orange and become green Cr³⁺ when acidified and reduced by zinc: Cr₂O₇²⁻ + 14H⁺ + 3Zn → 2Cr³⁺ + 7H₂O + 3Zn²⁺.
2
Excess zinc reduces green Cr³⁺ further to blue Cr²⁺: 2Cr³⁺ + Zn → 2Cr²⁺ + Zn²⁺.
3
In alkaline conditions Cr(III) is more easily oxidised, and hot hydrogen peroxide converts green [Cr(OH)₆]³⁻ to yellow chromate(VI), CrO₄²⁻.
4
The balanced equation is 2[Cr(OH)₆]³⁻ + 3H₂O₂ → 2CrO₄²⁻ + 2OH⁻ + 8H₂O.
5
Chromate(VI) and dichromate(VI) interconvert without a change in oxidation state: 2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O, so acid gives orange and alkali gives yellow.
Part 3 of 4Copper Oxidation States
Copper Oxidation States
Copper(II) with iodide
- Cu²⁺ is reduced by iodide ions to white CuI, with iodine formed: 2Cu²⁺ + 4I⁻ → 2CuI + I₂.
Copper(I) is unstable
- In aqueous solution Cu⁺ disproportionates to Cu²⁺ and copper metal: 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s).
Disproportionation
- A reaction in which the same element is simultaneously oxidised and reduced, here from +1 to +2 and 0.
Example
- Copper(I) oxide in dilute sulfuric acid gives a blue solution and a brown precipitate of copper.
Part 4 of 4Exam Checklist & Tips
5.6.6 – Key Points for the Exam
- Oxidising agent A species that gains electrons and is reduced.
- Reducing agent A species that loses electrons and is oxidised.
- Disproportionation Oxidation and reduction of the same element in one reaction.
- Variable oxidation state The ability of transition elements to form ions with different charges.
Common Mistakes
- Confusing the colour of Cr₂O₇²⁻ (orange) with CrO₄²⁻ (yellow) and Cr³⁺ (green).
- Calling the chromate–dichromate change a redox reaction, when the oxidation state of chromium stays at +6.
- Writing Cu⁺ as stable in water, when it disproportionates.
- Forgetting to balance H⁺ and H₂O when writing half-equations for dichromate or manganate.