3.5.4 Transition Metals - Variable oxidation states

Specification

Chromium - 4s¹ 3d⁵

Chromium forms three common oxidation states in its compounds, +2, +3, +6.

Chromium(VI) - 4s⁰ 3d⁰

In the +6 state it is a strong oxidising agent, particularly in acidic solution. There are two common series of salts, the chromate(VI) salts and the dichromate(VI) salts. Chromate(VI) is stable in basic solution while dichromate(VI) is stable in acidic solution:

Cr₂O₇^2- + OH- 2CrO₄^2- + H⁺

This is not a redox reaction. The equilibrium can be moved from one side to the other by addition of acid or base.

The reduction of dichromate ions turns the solution from orange to green as the chromium 3+ ions are formed.

Cr₂O₇^2- + 14H⁺ + 6e 2Cr³⁺ + 7H₂O

Potassium dichromate(VI) is the agent of choice for the oxidation of alcohols in organic chemistry.

Acidic potassium dichromate solution reacts with zinc forming chromium 3+ ions. Colour change orange to green.

Cr₂O₇^2- + 14H⁺ + 6e 2Cr³⁺ + 7H₂O
3Zn 3Zn²⁺ + 6e
------------------------------------------------ add
Cr₂O₇^2- + 14H⁺ + 3Zn 2Cr³⁺ + 7H₂O + 3Zn²⁺

and then after further reduction by the zinc, chromium 2+ ions. Colour change green to blue

2Cr³⁺ + Zn 2Cr²⁺ + Zn²⁺

Potassium dichromate reacts with iron(II) solutions oxidising them to iron(III).

Cr₂O₇^2- + 14H⁺ + 6e 2Cr³⁺ + 7H₂O
6Fe²⁺ 6Fe³⁺ + 6e
------------------------------------------------ add
Cr₂O₇^2- + 14H⁺ + 6Fe²⁺ 2Cr³⁺ + 7H₂O + 6Fe³⁺

Chromium(III) - 4s⁰ 3d³

Chromium(III) compounds are usually dark green, although chromium(III) chloride is purple when anhydrous. They are formed when potassium dichromate is reduced in acidic solution.

Cr₂O₇^2- + 14H⁺ + 3Zn 2Cr³⁺ + 7H₂O + 3Zn²⁺

Chromium(III) salts can be reduced to chromium(II) by strong reducing agents (see below).

Chromium(III) salts can be oxidised by hydrogen peroxide in basic solution:

Firstly the hexaaquachromium(III) complex ions become deprotonated by the strong base to give the hexahydroxychromate(III) complex ion:

[Cr(H2O)₆]³⁺ + 6OH^- [Cr(OH)₆]^3- + 6H₂O

This then reacts with hydrogen peroxide to form chromate(VI) ions.

2[Cr(OH)₆]^3- + 3H2O2 2CrO₄^2- + 8H₂O + 2OH^-

Chromium(II) - 4s⁰ 3d⁴

Chromium(II) compounds are usually blue. They are formed when chromium(III) solutions are reduced by strong reducing agents such as zinc in acidic solution.

2Cr³⁺ + Zn 2Cr²⁺ + Zn²⁺

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Manganese - 4s² 3d⁵

Manganese has five common oxidation states, +2, +3, +4, +6, +7.

Manganese(VI) - 4s⁰ 3d⁰