Chemical properties of oxygen



  -1   -2  
Acid medium  
O2 H2O   O3 O2  
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:

4l + 32 = 2l23 DGf = -1582.3 kJ/mol

4P + 52 = 2P25 DGf = -1371.7kJ/mol

Significantly negative DGf 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 (DGf>0), for example, DGf (u23) = 79 kJ/mol.

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

l4 + 25 = l27 + 23

2u()3 = u23 + 2


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

4 + 22 = 2 + 22;

Si4 + 22 = Si2 + 22;

4N3 + 32 = 62 + 2N2.

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

high temperatures. Initial activation of 2 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 2 and is completed with the formation of the final products of oxidation (2, 2, N2 and others). For example, the flame temperature can reach 3200C at burning of acetylene 22 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.

Content of oxygen Conditions of burning combustible compounds and life of man
17% 2 An oily miner's flashlight goes out; breathing remains normal
14% A candle extincts
12% A flame of acetylene extincts; breathing is deeper than normal one
10% A man begins to feel strangled
6% A Hydrogen flame extincts
5% A mans noticeable convulsions and sometimes fatal
2% Sudden fainting fit less than in a 1 minute. During 10 minutes life can be rescued by means of artificial respiration.


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 2S in air (in excess of 2) gives the product of complete oxidation:

22S + 32 = 22O + 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):

22S + 2 = 22O + 2S

The high standard E value of the half-reaction

2 + 4+ + 4 = 22; E = 1,23 V (at pH=0)

is the evidence of considerable oxidising activity of oxygen:



  1. Compare the meanings of the given words. Define what semantic features are shared by all the members of the group and what semantic properties distinguish them from each other.

: 600

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