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Oxygen is the most abundant element in the earth crust. Oxygen was first prepared by Kari Scheele, a Swedish chemist, in 1772 by heating mercuric oxide (HgO) and he named it as vital air. However, Lavoisier studied its essential nature and named it oxygen, taking two Greek words: Oxys = sour, genus = producer. (i.e, acid producer). It is mainly found in the form of silicates, aluminates, and oxides of metals as well as non-metals. It occurs in the atmosphere to the extent of 21.04% by volume.
Oxygen is placed in the group VIA of the periodic table. All the elements of this group have ns2np4 outermost orbit electronic configuration. All these elements exhibit allotropy. Oxygen is a diatomic molecule (O2). All the elements are polyatomic. Oxygen and sulphur are non-metals, selenium and tellurium are metalloids and polonium is a metal. This means metallic character goes on increasing as we move down the group. The atomic number and symbols of the elements in this group are given below:
The main sources of oxygen are water, air, oxides, and oxy-salts. Among them, oxy-salt are the most important sources which contain the higher percentage of oxygen. And, oxygen is prepared from these salts either heating them or treating them with certain reagents.
Oxygen gas is prepared in the laboratory by heating potassium chlorate (KClO3) at about 420K in the presence of MnO2 as a catalyst in a hard glass test tube.
In the absence of the catalyst, KClO3 decomposes only at high temperature in the following two stages.
A mixture of KClO3 and MnO2 in the ratio of 4:1 is taken in a hard glass test-tube. The mixture is heated and thus produced oxygen gas as is collected in a gas jar by downward displacement of water. The delivery tube should be taken out of water before heating of the mixture is stopped, otherwise, water rushes into the hot tube and breaks it.
Oxygen gas can be prepared in the laboratory without application of heat from sodium peroxide (Na2O2). When acidified KMnO4 is dropped into sodium peroxide, oxygen is produced.
Sodium peroxide is taken in a flat bottom flask fitted with dropping funnel. As acidified KMnO4 is dropped over Sodium peroxide (Na2O2), oxygen is produced. Thus produced oxygen is collected in the gas jar by downward displacement of water.
Oxygen reacts with metals, non-metals and other compounds under suitable conditions.
1. Action with metals:
a) With reactive metals like Na, K and Ca:These metals react with oxygen at normal conditions.
b) With Mg, Aland Fe:These metals react with oxygen in presence of heat.
c) There is no action of oxygen with less reactive metals like Ag, Au, Pt etc.
2. Action with non-metals:
a) With hydrogen:When electric discharge is passed over hydrogen and oxygen at 1073K, water is produced.
b) With sulphur:On heating with sulphur, sulphur dioxide is produced.
c) With carbon:When coke or coal is burnt with limited supply of oxygen, carbon monoxide is formed. But with sufficient oxygen supply, carbon dioxide is produced.
d) With nitrogen:At high temperature about 3273K, nitrogen reacts with oxygen to produce nitric oxide (NO).
e) With phosphorous:Phosphorous burns with oxygen to produce tetraphosphorous decaoxide (P4O10) solid.
3. Action with other compounds:under suitable conditions, oxygen oxidies different compounds since it is an oxidising agent.
Oxygen is used:
Oxides are the compounds formed by any other element with oxygen. This means oxides are the binary compounds of the elements with oxygen. Oxygen , being highly reactive, combines directly with all the elements except the noble gases and the noble metals (Au, Pd, Pt) to form the oxides. On the basis of acidic or basic behaviour and structural considerations, oxides are classified into the followings:
Describing the main oxides below:
Gewali, Dr. Mohan Bikram and Dr. Pradyumna Wagley. Principles of Chemistry. Kathmandu: Buddha Academic Publishers and Distributors Pvt. Ltd., 2009.
At high temperature about 3273K, nitrogen reacts with oxygen to produce nitric oxide (NO).