chemical reactions

Pb(s) + CrF3(aq)  no reaction

11. Compounds containing one or more atoms in high oxidation states often act as oxidizing agents; compounds containing atoms in low oxidation states often act as reducing agents. For most elements, the (old) group number of the atom in the periodic table gives the highest oxidation state possible for that element. For nonmetals, the lowest oxidation state possible is given by the (old) group number minus eight. Elemental metals most often act as reducing agents (they are oxidized); nonmetals frequently act as oxidizing agents (they are reduced).

For the representative elements (i.e., those in the first two and last six columns of the periodic table), oxidation states most often are two units apart. For example, Sn forms Sn(II) and Sn(IV); Br forms Br1-, Br(I), Br(III), Br(V), and Br(VII). For the transition elements, (i.e., those in the “center” ten columns of the periodic table), oxidation states are often one unit apart, but can be in almost any relationship to one another. For the transition elements, the common oxidation states (charges on their ions) must be memorized. For example, Fe forms Fe2+ and Fe3+; Cu forms Cu+ and Cu2+, etc. Some of the transition elements form oxyanions as well as cations. For example, Mn forms Mn2+, Mn3+, MnO42-, and MnO4; Cr forms Cr2+, Cr3+, CrO42-, and Cr2O72-.

Any atom in its highest possible oxidation state can only act as an oxidizing agent; any atom in its lowest possible oxidation state can only act as a reducing agent. Atoms in intermediate oxidation states can be either oxidized or reduced; that is, they can act as either reducing or oxidizing agents. Some of the oxidizing agents most commonly encountered are MnO4, CrO42-, Cr2O72-, HNO3, H2O2, and the halogens. Some of the more common reducing agents are elemental H2, metals, carbon, and I.

In predicting products of oxidation-reduction reactions, don’t forget their name–oxidation and reduction must occur simultaneously! It is impossible for oxidation to occur without reduction or vice versa.

Examples:

Sn2+(aq) + F2(g)  Sn4+(aq) + F(aq)

Mn2+(aq) + BiO3(aq)  Bi3+(aq) + MnO4(aq)

(note that the Bi is in its highest possible oxidation state in BiO3)

K(s) + P4O10(s)  K3PO3(s)

(note that P is reduced from P(V) to P(III))

MnO4(aq) + I(aq)  Mn2+(aq) + I2(aq)

CuS(s) + HNO3(aq)  Cu(NO3)2(aq) + S8(s) + NO2(g)

(note S2- S and N(V)  N(IV))

Fe2O3(s) + C(s)  CO2(g) + Fe(s)

[REACT101.S94/AJP1]

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