OXYGEN AND ITS COMPOUNDS

OXYGEN AND ITS COMPOUNDS

Oxygen was discovered independently by Scheele in 1772 and Priestly in 1774. however, it was Lavoisier who first described the major properties of this newly discovered gas and arrived at the conclusion that the gas was essentially for the combustion of substances.

Oxygen is the most abundant element on earth. It occurs in nature in both free and combine states. Free oxygen constitutes about 21% by volume of atmospheric air and about 33% by volume of dissolved air. In the combined form, oxygen accounts for newly 50% by mass of the earth's crust, the oceans and the air.

GENERAL PROPERTIES OF OXYGEN GROUP

The oxygen family, also called the chalcogens, consists of the elements found in Group 16 of the periodic table and is considered part of the main group elements. It consists of the elements oxygen, sulphur, selenium, tellurium and polonium.

Properties and periodic trends

1.       All member of the family have six electrons in their outermost shell.

2.       As one move down the group, metallic character increases with tellurium being a metalloid and a polonium a metal.

3.       Melting point, boiling point, density, atomic radius and ionic radius all increase going down the group.

4.       Ionization energy decreases going down the group.

5.       The most common oxidation state is -2 however sulphur can also exist at a +4 and +6 state and +2, +4 oxidation states are possible for Se, Te and Po.

6.       Tends to form covalent compounds with other elements.

7.       Tends to exist as diatomic and polyatomic molecules, such as O₂, O₃, S₆, S₈ and Se₈.

THE ELECTRONIC STRUCTURE AND BINDING CAPACITY OF OXYGEN

Oxygen- 1S2 2S22P4

The oxygen atom has six valence electrons and can acquire a stable octect electronic configuration by:

1.       Accepting two electrons from electropositive elements like metal to form the negative oxide ion, O2- (i.e the formation of ionic compound) e.g Ca2+ + O2-             CaO

2.       Entering into covalent bond formation with non-metals by covalently sharing two out of its outer electrons e.g. HCOOH

3.       It form single covalent bonds with two hydrogen atoms to give water molecule, H-O-H. in oxygen molecule, a double covalent bond joins the two oxygen atoms. O=O.

Oxygen , therefore, forms a wide variety of ions and covalent compounds and it is the only element of group vi which forms multiple bonds with itself.

LABORATORY PREPARATION OF OXYGEN

Oxygen is usually prepared in the laboratory by:

1.       The decomposition of potassium trioxochlorate (v) and hydrogen peroxide.

2.       The oxidation of hydrogen peroxide

Other methods include the thermal decomposition of mercury (ii) oxide and Lead (iv) oxide and the reaction of sodium peroxide with water.

1.       From trioxochlorate(v) : Potassium trioxochlorate (v) decomposes slowly to release all its oxygen when heated above its melting point (368oC). In the presence of manganese(iv) oxide, which acts as a catalyst, the reaction occurs at a lower temperature and at a much faster rate.

2KClO3 (heat)                              2KCl  + 3O2

The oxygen gas produced is collected over water, and if required dry, it is passed through anhydrous calcium chloride or concentrated tetraoxosulphate (vi) acid and then collected over mercury.

2.       From hydrogen Peroxide: in the presence of catalyst, manganese (iv) oxide, hydrogen peroxide readily decomposes to liberate oxygen without any heating.

2H2O2                  2H2O  + O2

Hydrogen peroxide also reacts with acidified potassiumtetraoxomanganate(vii) solution in the cold to produce oxygen. This is a redox reaction in which the KMnO4 is the oxidizing agent and H2O2, the reducing agent

5H2O2 + 2KMnO4 + 3H2SO4                  K2SO4  + 2MnSO4  + 8H2O + 5O2

Ionically,

5H₂O₂ + 2MnO₄^- + 6H                   2Mn²⁺  + 8H₂O + 5O₂

INDUSTRIAL PREPARATION OF OXYGEN

In places where a cheap source of electricity is available oxygen may be prepared industrially by the electrolysis of water. However, in most places, it is prepared by the fractional distillation of liquid air. This preparation involves two main processes:

1.       The liquefaction of air.

2.       The fractional distillation of the resultant liquid air.

LIQUEFACTION OF AIR

Air, in the gaseous form, is first passed through caustic soda to remove carbon (iv) oxide. It is then compressed to a pressure of about 200 atm, cooled and allow to escape rapidly through a minute aperture. The sudden expansion of the air into a region of lower pressure cause it to cool even further (since heat energy is used up in separating the molecules). The cooled air will cool the incoming stream of air. This process is repeated until the gaseous air become a liquid at -200oC.

FRACTIONAL DISTILLATION OF LIQUID AIR

The liquid air is then led to a fractionating column, on distillation, nitrogen which has a lower boiling point of -196oC, is evolved first, leaving behind a liquid very rich in oxygen. Further heating  convert the liquid oxygen to a gas at -183oC. The oxygen gas is dried, compressed and stored in steel cylinders under a pressure of about 100 atm.

PHYSICAL PROPERTIES OF OXYGEN

1.       Pure oxygen is a colourless, odourless, and tasteless gas.

2.       It is neutral to litmus paper.

3.       It is slightly soluble in water.

4.       Gaseous oxygen liquefies at -183^oC and solidifies at -225^oC

5.       Its density is about the same as that of air.

CHEMICAL PROPERTIES OF OXYGEN

1.       Oxygen support combustion of many substances. Metals, except mercury, silver, and gold react with oxygen to form basic oxides.

4Na + O2             2Na2O3;       2Ca + O2          CaO

The oxides of very reactive metals like K, Na and Ca dissolves in water to form alkalis.

2Na2O  + 2H2O                     4NaOH

2.       When potassium and sodium are heated in a plentiful supply of oxygen, higher oxides (NaO₂, K₂O₂) are formed instead of basic oxides.    4Na + O₂               Na₂O₂

3.       Non-metals like S, C, P burns in oxygen to form acidic oxides. These are also known as acid anhydrides, as they dissolves in water to forms acids.

S +O2              SO2             +H2O              H2SO4

P4 + 3O2        P4O6           +6H2O             4H3PO3

P4 + 5O2        P4H10         +6H2O             H3PO4

4.       Most hydrocarbons burns In oxygen to form carbon(iv) oxide and water.

C2H6 +O2                 CO2 +  H2O

5.       The oxygen we breathe in oxidizes the carbohydrate we eat to release energy.

C6H12O6            6CO2  + 6H20 + Energy.

The reaction of oxygen with elements and compounds are example of oxidation reaction. An oxidation process in which heat and light are produced is called combustion.