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Chemical properties of oxygen

OXYGEN. STANDARD ELECTRODE POTENTIALS, EO, V

		-1		-2
Acid medium
O2		H2O		O3		O2
	H2O2
Alkaline medium
O2		OH-		O3		O2

Oxygen as a ligand

Oxygen has a high chemical activity. It enters reactions with most elements, most frequently forming oxides:

4Аl + 3О₂ = 2Аl₂О₃ DG_f° = -1582.3 kJ/mol

4P + 5О₂ = 2P₂О₅ DG_f° = -1371.7kJ/mol

Significantly negative DG_f° values are an evidence of a considerable affinity of elements to oxygen.

Dioxygen does not react only with noble gases, some metals (Au, Pt) and elemental halogens. Gibbs energy change of the mentioned elements reaction with dioxygen is positive (DG_f°>0), for example, DG_f° (Аu₂О₃) = 79 kJ/mol.

Nevertheless the oxides of these elements, except for He, Ne and Ar, can be obtained indirectly:

НСlО₄ + Р₂О₅ = Сl₂О₇ + 2НРО₃

2Аu(ОН)₃ = Аu₂О₃ + ЗН₂О

Dioxygen also interacts with a lot of compounds. Oxides, sometimes oxides with elemental substances, are formed:

СН₄ + 2О₂ = СО₂ + 2Н₂О;

SiН₄ + 2О₂ = SiО₂ + 2Н₂О;

4NН₃ + 3О₂ = 6Н₂О + 2N₂.

In most cases oxidation reactions proceed at a sufficient rate only at

high temperatures. Initial activation of О₂ molecules is usually required.

If oxidation of compounds proceeds vigorously with the formation of large quantity of heat and light it is named burning. It takes place at the excess of air or pure О₂ and is completed with the formation of the final products of oxidation (СО₂, Н₂О, N₂ and others). For example, the flame temperature can reach 3200°C at burning of acetylene С₂Н₂ in oxygen medium.

A smoldering splinter lights up at oxygen content in a mixture with N₂ not less than 28%. If a mixture contains less than 16%, a burning splint extincts.

Relatively slow reactions of oxidation depending on the character of substance appear as corrosion (oxidation of metals), and decay (oxidation of organic residues), more generally they are simply called oxidation.

In the reactions of oxidation there can be present the products of incomplete oxidation. So, burning Н₂S in air (in excess of О₂) gives the product of complete oxidation:

2Н₂S + 3О₂ = 2Н₂O + 2SO₂

and at gradual oxidation H₂S in aqueous solution with the dissolved O₂ (the lack of dioxygen) elementary S forms (appearance of H₂S solutions opalescence at their storage):

2Н₂S + О₂ = 2Н₂O + 2S

The high standard E° value of the half-reaction

О₂ + 4Н⁺ + 4е = 2Н₂О; E° = 1,23 V (at pH=0)

is the evidence of considerable oxidising activity of oxygen:

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