M(II) Redox properties

The oxidation state +2 is an intermediate state between characteristic 0 and +3, that is why E(II)compounds can be reducing and oxidizing agents simultaneously.

Oxidition oj M behavior is not typical expert the action of strong oxidants:

NiO + C = Ni + CO,

Ni2+ + 2e = Ni.

Reducing properties in the series Fe(ІІ), Co(ІІ) and Ni(ІІ) decrease significantly due to reducing stability of +3 oxidation state in the series. For instance, Fe(OH)2 moistened with water reacts with air oxygen rapidly:

4Fe(OH)2 + O2 + 2H2O = Fe(OH)3.

Reaction of Со(ОН)2 proceeds very slowly. Even with pure O2 and under increased pressure it is completed for several days. Ni(OH)2 has almost no reaction with O2.

Red-Ox potentials are increased from Fe tо Ni in alkaline medium:

Half-reaction Fe(OH)3 + e = Fe(OH)2 + OH- Co(OH)3 + e = Co(OH)2 + OH- Ni(OH)3 + e = Ni(OH)2 + OH-
Е0, V -1.76 -1.43 -1.57

Halogens (Cl2, Br2) and hypohalites actively oxidize all Е(ОН)2 in alkaline medium:

2Co(OH)2 + NaBrO + H2O = 2Co(OH)3 + NaBr;

2Ni(OH)2 + Cl2 + 2NaOH = 2Ni(OH)3 + 2NaCl.

In acid medium, Ео (Fe3+/Fe2+) = +0.77 V, thus many oxidants can oxidizeFe(ІІ) salts:

10FeSO4 + 2KMnO4 + 8H2SO4 = 2MnSO4 + 5Fe2(SO4)3 + K2SO4 + 8H2O;

6FeSO4 + K2Cr2O7 + 7H2SO4 = Cr2(SO4)3 + 3Fe2(SO4)3 + K2SO4 + 7H2O.

These reactions are used to determine quantitively Fe2+-ions content in aqueous solutions.

Fe(ІІ) salts(as well as Fe(OH)2) are also oxidized by air oxygen:

4FeSO4 + O2 + 2H2O = 4(FeOH)SO4.

But, unlike Fe(OH)2 oxidation, this reaction proceeds moderately.

Со2+-ions are less capable to oxidize in acid medium: Е° Со3+/Со2+ = +1,92 V. Thus such oxidants as KMnO4, K2Cr2O7, O2, Cl2, Br2 don’t oxidize Со2+. To realize oxidation, F2, O3 or anode process are applied.

Even the strongest oxidants will not oxidise Ni2+-ions in acid medium.

Compounds + 3


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