Why can transition metals exist in various positive oxidation states?

Transition metals have outer d orbital electrons with similar energies, allowing multiple oxidation states to be stable in compounds and complexes.

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Cu2O(s) + H2SO4(aq) ➔ Cu(s) + CuSO4(aq) + H2O(l)

Explain what type of redox reaction is occuring in the reaction above.

This is a disproportionation redox reaction because copper(I) oxide is simultaneously oxidised (to form copper) and reduced (to form copper sulfate).

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How can standard electrode potentials be used to predict the direction of a redox reaction?

The E⦵ values of the half-reactions involved are compared. The reaction is likely to proceed in the direction of the more positive (or less negative) E⦵ value, indicating the species that gains electrons and undergoes reduction.

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What is the role of iodide ions in the reaction with Fe3+ ions?

Iodide ions act as reducing agents, reducing Fe3+ to Fe2+.


The reaction is: 2Fe3+(aq) + 2I-(aq) ➔ 2Fe2+(aq) + I2(aq)

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Describe, and give the equation for, the reaction between aqueous copper(II) ions and excess aqueous iodide ions.

A white precipitate of copper(I) iodide forms at the bottom of a brown solution of iodine.

2Cu2+(aq) + 4I-(aq) ➔ 2CuI(s) + I2(aq)

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[Cr(OH)6]3-(aq) + 2OH-(aq) ➔ CrO42-(aq) + 4H2O(l) + 3e-

H2O2(aq) + 2e- ➔ 2OH-(aq)

Combine the half-equations above to give the overall equation for the redox reaction between chromium(III) ions and hydrogen peroxide in alkaline conditions.

2[Cr(OH)6]3-(aq) + 3H2O2(aq) ➔ 2CrO42-(aq) + 8H2O(l) + 2OH-(aq)

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What is the colour change observed when dichromate(VI) ions (Cr2O72-) are reduced to chromium(III) ions (Cr3+)?


The solution colour changes from orange to green.

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Fe3+(aq) + e- ⇌ Fe2+(aq)                                            E⦵ = +0.77 V

MnO4-(aq) + 8H+(aq) + 5e- ⇌ Mn2+(aq) + 4H2O(l)  E⦵ = +1.33 V

Explain which species is reduced when the two half-equations above are combined.

MnO4- is reduced to Mn2+.


This is because the E⦵ value for MnO4-/Mn2+ is more positive than the E⦵ value for Fe3+/Fe2+.


This means that Fe2+ is oxidised to Fe3+.

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What two factors can influence the redox potential of transition metal ions compared to standard conditions?

  1. pH - Acidic conditions (low pH) provide excess H+ ions, facilitating the reduction of some metal ions and lowering the redox potential.
  2. Ligands - Ligands other than water can influence the redox potential by forming stronger bonds with the metal ion, stabilising particular oxidation states.

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