Importance of molecular oxygen

Oxygen group - group 16

The elements oxygen, sulphur, selenium, tellurium and polonium constitute 16th group of the periodic table. The first four elements are non metals. Collectively they are called the 'chalcogens' or ore -forming elements. This is because a large number of metals are oxides or sulphides.

Oxygen is a very important element in inorganic chemistry, since it reacts with almost all the other elements. Oxygen is the most abundant of all elements. It exists in the free form as dioxygen or molecular oxygen and makes up 20.9% by volume and 23% by weight of the atmosphere.

Importance of molecular oxygen

Haemoglobin is an iron containing coordination compound in red blood cells responsible for the transport of oxygen from the lungs to various parts of the body. Myoglobin is a similar substance in muscle tissue, acting as a reservoir for the storage of oxygen and as a transport of oxygen within muscle cells.

Haemoglobin consists of heme, a complex of Fe(II) bonded to a porphyrin ligand and globin protein. The sixth position is vacant in free hemoglobin but is occupied by oxygen in oxyhaemoglobin. Hemoglobin (Hb) and O2 are in equilibrium with oxyhaemoglobin.

Hb + O_{2 -- > < --} HbO_{2 (}Oxyhaemoglobin₎

oxyhaemoglobin is formed in the lungs and carried to the cells, where it gives up its oxygen.

Haemoglobin then binds with HCO₃^- which is formed by the reaction of CO₂ (released by the cell) with water. After reaching the lungs, due to hydrolysis CO₂ is released.

Hb + O_{2 -- > < --}  HbO_{2  -- >}  Hb + O₂ Cell wall

Myoglobin -- > MbO₂ -- > O_{2 -- >} Cell

Most of the O2 has been produced by photosynthesis. The dioxygen (or) molecular oxygen is prepared by the green plants. The chlorophyll in the green parts of the plants uses the solar energy to make carbohydrate and molecular oxygen. Oxygen makes up 46.6% by weight of the earth's crust, where it is the major constituent of silicate minerals.

Practically all the elements react with dioxygen either at room temperature (or) on heating except Pt, Au, W and Noble gases. Eventhough the bond energy of oxygen is high (493 kJ mol^-1), the reactions are generally strongly exothermic and once started often continue spontaneously.

Dioxygen is also called as molecular oxygen. The molecular oxygen is essential for respiration (for the release of energy in the body) by both animals and plants. It is therefore essential for life. Hence molecular oxygen acts as a cell fuel.

The complex formed between dioxygen and haemoglobin (the red pigment in blood) is of vital importance. Since it is the method by which higher animals transport dioxygen around the body to the cells.

Nascent oxygen and molecular oxygen

Oxygen molecule is very stable. It dissociates only to a small extent when heated to a very high temperature. This reaction is endothermic

O2  -- > 2[O]     delH=+116.8 kcal

However, when an electric discharge is passed through oxygen at a very low pressure, it dissociates to the extent of about 20%.

For example when oxygen is passed at about 1 mm pressure through a discharge tube, the resulting gas is found to be chemically more reactive. Its line spectrum shows that it consists of the free atoms. Hence atomic oxygen is formed according the following endothermic reactions.

O2 -- > O + O   delH=489.6kJ

Reactions

1. Formation of molecular oxygen

When a thin platinum wire is placed in atomic oxygen, it quickly gets heated up and begins to glow due to the recombination of oxygen atoms with liberation of heat energy. The rise of temperature of platinum wire under standardised conditions is a measure of the concentration of the atomic oxygen in the gas.

2. Formation of ozone

Atomic oxygen combines with molecular oxygen to give ozone which may be condensed by means of liquid air

O₂ + [O] -- > O3

3. Oxidation

Atomic oxygen is an extremely powerful oxidizing agent and oxidises aliphatic and aromatic hydrocarbons and methyl alcohol with emission of heat and light. With nitric oxide, a characteristic greenish - white luminescence is produced. H₂S and CS₂ react with it and burst into greyish blue coloured flame.

Oxides

Generally all the elements react with dioxygen to form oxides. Oxides are binary compounds of oxygen. Oxides may be classified depending on their structure (or) their chemical properties.

i)  Acidic oxides

The oxides of non-metals are usually covalent and acidic. They have low melting and boiling points, though some B2O3 and SiO2 form infinite "giant molecules" and have high melting points. They are all acidic. Some oxides dissolve in water and thus forming acids. Hence they are called as acid anhydrides

B₂O₃ + 3H₂O  2H₃BO₃

N2O5 + H2O  2HNO3

P4O10 + 6H2O  4H3PO4

SO3 + H2O  H2SO4

others which do not react with water such as SiO2 reacts with NaOH and shows acidic properties.

ii) Basic oxides

Metallic oxides are generally basic. Most metal oxides are ionic and contain the O^2- ion. Some oxides dissolve in water and form alkaline solution.

Na₂O + H₂O  -- >  2NaOH

BaO + H2O  -- > Ba(OH)2

Many metal oxides with formula M2O3 and MO2, though ionic, do not react with water.

Examples : Tl₂O₃, Bi₂O₃ and ThO₂.

These oxides react with water to form salts and hence they are bases.

CaO + 2HCl  -- > CaCl₂ + H₂O

If a metal exists in more than one oxidation state and they form more than one oxide

eg. CrO, Cr2O3, CrO3, PbO, PbO2

iii) Amphoteric oxides

The oxides which react with both strong acids and strong bases are called as amphoteric oxides.

ZnO + 2NaOH  -- > Na2ZnO2 + H2O

Sodium zincate

ZnO + 2HCl  -- > ZnCl2 + H2O

iv) Peroxides

These oxides contain more oxygen than would be expelled from the oxidation number of M. Some are ionic and contains the peroxide ion O₂^2-. The metal belonging to the group I and II (Na₂O₂, BaO₂) contain O₂^2- ion. Others are covalently bound and contain -O-O- in the structure.

Oxides such as PbO₂ react with acids liberate Cl₂

PbO₂ + 4HCl  -- > PbCl₂ + 2H₂O + Cl₂

v) Compound oxides

Some oxides behave as if they are compounds of the two oxides.

Ex. Ferrous-ferric oxide (Fe₃O₄). This is considered to be the mixture of FeO and Fe₂O₃.

They react with acids and forms a mixture of ferrous and ferric salts.

Fe₃O₄ + 8HCl  -- >  FeCl₂ + 2FeCl₃ + 4H₂O

vi) Neutral oxides

A few covalent oxides have no acidic (or) basic properties (N₂O, NO, CO).

vii) Dioxides

They also contain higher proportion of O₂ than expected. But they do not liberate H₂O₂ with acid.

Ex. NO₂, SO₂