Transition Metals - Examples of Redox Reactions in Transition Metals (A-Level Chemistry)

Examples of Redox Reactions in Transition Metals

Reduction of Vanadate (V) ions

Metallic zinc is a good reducing agent.

It reacts with dilute acid to form Zn²⁺ ions and releases electrons for reduction.

Zn(s) → Zn²⁺(aq) + 2e⁻

Addition of zinc to vanadium(V) ions from a solution of ammonium vanadate (V) (NH₄VO₃) in acidic solution, will reduce the vanadium through each successive oxidation state. Each oxidation state produces a different colour.

Vanadium (V) ions are first reduced to vanadium (IV) ions:

2[VO₂(H₂O)₄]⁺(aq) + Zn(s) + 4H⁺(aq) → 2[VO(H₂O)₅]²⁺(aq) + Zn²⁺(aq)
Yellow                                                                Blue

Vanadium (IV) ions are then reduced to vanadium (III) ions:

2[VO(H₂O)₅]²⁺(aq) + Zn(s) +4H⁺(aq) → 2[V(H₂O)₆]³⁺+(aq) + Zn²⁺(aq)
Blue                                                                           Green

Vanadium (III) ions are finally reduced to vanadium (II) ions:

Until now, the reactions have been feasible because their redox potentials have been positive.

Examples of Redox in Transition Metals
Examples of Redox in Transition Metals

Vanadium (II) ions will however not be further reduced by zinc to vanadium metal because the redox potential for the reaction is negative, which means that the reaction is not feasible under standard conditions.

2[V(H₂O)₆]²⁺ (aq) + Zn(s) → V(s) + Zn²⁺(aq) + 6H₂O(l)                             E⦵ = -0.42V

Reduction of Silver Diammine [Ag(NH₃)₂]⁺

Silver diammine [Ag(NH₃)₂]⁺ is used in Tollen’s reagent to distinguish between aldehydes and ketones.

Aldehydes are strong enough reducing agents to reduce the silver (I) to metallic silver.

2[Ag(NH₃)₂]⁺ + CH₃CHO + H₂O → 2Ag(s) + 4NH₃ + CH₃COOH + 2H⁺

Ketones are unable to do this and have no reaction.

Reduction and Oxidation of Chromium Ions

The large number of oxidation states that can be taken by chromium, allow its to part-take in a wide variety of redox reactions.

When added to an acidic solution of zinc metal, dichromate (VI) ions are reduced to chromium (III) ions (E⦵ = +2.09V):

Cr₂O₇²⁻(aq) + 3Zn(s) + 14H⁺(aq) → 2Cr³⁺(aq) + 3Zn²⁺(aq) + 7H₂O(l)
Orange                                                         Green

Zinc will further reduce Cr³⁺ to Cr²⁺ (E⦵ = +0.35V):

2Cr³⁺(aq) + 3Zn(s) → 2Cr²⁺(aq) + Zn²⁺(aq)
Green                               Yellow

When added to an alkaline solution of hydrogen peroxide, chromium (III) ions are oxidised to chromate (IV) ions (E⦵ = +1.08V):

2Cr³⁺(aq) + 3H₂O₂(s) + 10OH⁻ (aq) → CrO₄²⁻(aq) + 8H₂O(l)
Green                                                                Yellow

If we then add some dilute sulphuric acid to the solution, Cr₂O₇²⁻ ions form turning the solution orange.

CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l)
Yellow                                  Orange

This reaction is reversible and an equilibrium between CrO₄²⁻ and Cr₂O₇²⁻ ions sets up.

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