p-Block Elements: Group 16
Dioxygen − preparation, properties and uses
Preparation: In the laboratory, dioxygen is prepared in the following ways:
(i) By heating salts such as chlorates, nitrates and permanganates. The most efficient technique for producing oxygen in the laboratory uses the catalytic decay of potassium chlorate in the solid form where manganese dioxide works as a catalyst.
(ii) By the thermal decomposition of the oxides of metals low in the electrochemical series and higher oxides of some metals.
2Ag2O(s) → 4Ag(s) + O2(g)
2HgO(s) → 2Hg(l) + O2(g)
Thermal treatment of huger oxides may also be employed
2Pb3O4(s) → 6PbO(s) + O2(g)
2PbO2(s) → 2PbO(s) + O2(g)
(iii) Hydrogen peroxide can be readily decomposed into water and dioxygen by using catalysts such as finely divided metals along with manganese dioxide.
(iv) In large quantities, it can be prepared from water or air. Electrolysis of water leads to the release of hydrogen at the cathode and oxygenate the anode. Dioxygen can also be obtained from air by first removing carbon dioxide and water vapour and then, the remaining gases are liquefied and fractionally distilled to give dinitrogen and dioxygen.
Fig 1: Fractional distillation of liquid air.
Dioxygen is a colourless and odourless gas. Its solubility in water is to the extent of 3.08 cm3 in 100 cm3 water at 293 K which is just enough for the vital support of marine and aquatic life. Under pressure, it can be condensed into a light blue fluid by pressing the gas at 90K. It can likewise be solidified into a bluish white solid at 55K. It is slightly heavier than nitrogen and air. It has three stable isotopes: 16O, 17O and 18O. Some characteristics of O2 and related species, for example, the peroxide and superoxide particles, are shown below.
Some compounds are catalytically oxidized. For example:Molecular oxygen, O2 is unique in being paramagnetic despite having even number of electrons. It contains two unpaired electrons and the structure is linear. Dioxygen directly reacts with nearly all metals and non-metals except some metals (e.g., Au, Pt) and some noble gases. Most reactions are exothermic. However, to initiate the reaction, some activation energy is required because the bond dissociation enthalpy of the oxygen-oxygen double bond is high (493.4 kJ mol–1). Some important reactions with oxygen as a reagent are stated below:
Oxygen is an important component for combustion to occur. The main use of oxygen is respiration al life forms. Any reaction that required heat and combustion needs oxygen. For example, manufacturing of iron, steel and other metals. Cutting and oxyacetylene welding also require oxygen. Water treatment, oxidation processes, petroleum refining medicinal processes also require oxygen. Rocket fuel oxidizers and mountaineering equipment also require stored oxygen.