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Free elements

Table 1. Some properties of iron triad elements | Iron triad trends | Cobalt (II) compounds | M(II) Redox properties | Iron (III) compounds | Cobalt (III) compounds | COMPLEXES OF IRON | COMPLEXES OF COBALT | COORDINATION COMPOUNDS OF NICKEL | TESTS FOR IRON |


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Iron. Iron combines with many non-metals at when heated forming oxides: Fe2O3 and Fe3O4 (when heated in air above 430 K). Steam produces with iron Fe3O4 and hydrogen. Iron is dissolved in dilute non-oxidant acids, giving iron(II) solutions, e. g.:

Fe + 2H+(aq) Fe2+(aq) + H2.

This follows from the value of E0 for the half-reaction:

Fe2+(aq) + 2e Fe(s), E0 = –0.44 V.

The impurities in ordinary iron provide fast dissolution of iron in acids. These impurities are responsible for the characteristic smell of hydrogen from this source.

In dilute nitric acid, ammonium nitrate is formed:

4Fe + 10H+ + NO3- 4Fe2+ + NH4+ + 3H2O.

Concentrated nitric acid renders the metal passive, i.e. chemically inactive, due to formation of a thin oxide surface film (which can be removed by scratching or heating in hydrogen). Iron is a good reducing agent (see the value of E° above): it reduces some cations to the metal (for example, copper) in aqueous solution, giving iron (II). Iron absorbs hydrogen readily and is a hydrogenation catalyst.

Hydrochloric acid of any concentration dissolves iron:

Fe + 2HC1 = FeCl2 + H2↑.

It is dissolved in dilute sulfuric acid similarly:

Fe + H2SO4 = FeSO4+H2.

In concentrated sulfuric acid solutions, iron is oxidized to iron(III):

2Fe + 6H2SO4 = Fe2(SO4)3 + 3SO2 ↑ + 6H2O.

Although in sulfuric acid, whose concentration is close to 100%, iron becomes passive and virtually no reaction occurs.

Iron dissolves in dilute and moderately concentrated solutions of nitric acid:

Fe + 4HNO3 = Fe(NO3)3 + NO ↑ + 2H2O.

At high concentrations of HNO3 dissolving retards and iron becomes passive.

Cobalt is similar to iron chemically; when heated in the air it gives oxides Co3O4 and CoO, but it is less actively attacked by dilute acids. Cobalt(II) halides are formed with halogens, excepting fluorine (CoF3 is obtained). Like iron and the next transition element, nickel, cobalt is not generally found in any oxidation state above +3, and this and +2 are the usual states.

Nickel is attacked by dilute aqueous acids but not by alkalis; it combines readily with many non-metals when heated.

 

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