## Introduction to Oxygen

Subject: Chemistry

#### Overview

Oxygen is placed in the group VIA of the periodic table. All the elements of this group have ns2np4 outermost orbit electronic configuration. 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. 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. When coke or coal is burnt with limited supply of oxygen, carbon monoxide is formed. But with sufficient oxygen supply, carbon dioxide is produced.

### Introduction of Oxygen

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.

#### Periodic Position

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:

 Element Symbol Atomic number Oxygen O 8 Sulphur S 16 Selenium Se 34 Tellurium Te 52 Polonium Po 84

#### Preparation of Oxygen

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.

$$2KClO_3\xrightarrow\Delta{2KCl\;+3O_2}$$

$$2KMnO_4\xrightarrow\Delta{K_2MnO_4\;+MnO_2\;+O_2}$$

$$2K_2Cr_2O_7\;+8H_2SO_4\xrightarrow\Delta{2K_2SO_4\;+2Cr_2(SO_4)_3\;+8H_2O\;+O_2}$$

$$2KNO_3\xrightarrow\Delta{2KNO_2\;+O_2}$$

$$3MnO_2\xrightarrow\Delta{Mn_3O_4+O_2}$$

#### Laboratory Preparation of Oxygen Gas

Principle

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.

$$2KClO_3\xrightarrow{\Delta\; MnO_2}{2KCl+3O_2}$$

In the absence of the catalyst, KClO3 decomposes only at high temperature in the following two stages.

1. Potash per chloride is first formed.
$$4KclO_3\xrightarrow{653K}{KCl+3KClO_4}$$
2. When temperature rises about 883K it decomposes into KCl and O2
$$KClO_4\xrightarrow{883K}{KCl+2O_2}$$
This addition of manganese dioxide brings the decomposition of KClO3 at 420K and at a greatly accelerated rate. Thus formed gas is collected by downward displacement of water.

Process

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.

Test

1. A glowing chip of wood continues to glow with bright colored flame when it is introduced into the gas jar containing oxygen gas.
2. When oxygen gas is passed through alkaline pyrogallate solution, the solution turns brown.
3. It is absorbed by ammonium cuprous chloride solution giving deep blue solution and it produces brown fumes with nitric oxide.

#### Laboratory Preparation of Oxygen Gas without Heating

Principle

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.

$$2KMnO_4+3H_2SO_4\longrightarrow{K_2SO_4+2MnSO_4+3H_2O+5O}$$

$$Na_2O_2+H_2SO_4\longrightarrow{Na_2SO_4+H_2O_2]\times5}$$

$$H_2O_2+O\longrightarrow{H_2O+O_2]\times5}$$

$$2KMnO_4+5Na_2O_2+8H_2SO_4\longrightarrow{K_2SO_4+2MnSO_4+5Na_2SO_4+8H_2O+5O_2}$$

Process

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.

#### Properties of Oxygen

Physical Properties

1. Oxygen is a colorless, odorless and tasteless gas.
2. It can be liquefied to pale yellow liquid under high pressure below the critical temperature.
3. It has the higher density than air. Its vapour density is 16.
4. Oxygen is slightly soluble in water. It is 3-4% soluble at 25°C. Due to this, water is useful to sustain the sea animals.
5. Its melting point is -210°Cand boiling point is -183°C.
6. It is paramagnetic in nature.
7. It is neither acidic nor basic.

#### Chemical Properties

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.

$$4Na+O_2\longrightarrow{2Na_2O}$$

$$4K+O_2\longrightarrow{2K_2O}$$

$$2Ca+O_2\longrightarrow{2CaO}$$

b) With Mg, Aland Fe:These metals react with oxygen in presence of heat.

$$2Mg+O_2\longrightarrow{2MgO}$$

$$4Al+3O_2\longrightarrow{2Al_2O_3}$$

$$2Fe+O_2\longrightarrow{2FeO}$$

$$4Fe+3O_2\longrightarrow{2Fe_2O_3}$$

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.

$$2H_2+O_2\xrightarrow{electric\;spark\;1073K}{2H_2O}$$

b) With sulphur:On heating with sulphur, sulphur dioxide is produced.

$$S_8+8O_2\xrightarrow\Delta{8SO_2}$$

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.

$$2C+O_2\xrightarrow\Delta{2CO}$$

$$C+O_2\xrightarrow\Delta{CO_2}$$

d) With nitrogen:At high temperature about 3273K, nitrogen reacts with oxygen to produce nitric oxide (NO).

$$N_2+O_2\rightleftharpoons{2NO}$$

e) With phosphorous:Phosphorous burns with oxygen to produce tetraphosphorous decaoxide (P4O10) solid.

$$P_4+5O_2\xrightarrow\Delta{P_4O_{10}}$$

3. Action with other compounds:under suitable conditions, oxygen oxidies different compounds since it is an oxidising agent.

1. Carbon monoxide is oxidized to carbon dioxide:
$$2CO+O_2\xrightarrow\Delta{2CO_2}$$
2. Oxidation of sulphur dioxide:sulphur trioxide takes place at 723K and in the presence of finely divided nickel and V2O5 (Vanadium pentoxide).
$$2SO_2+O_2\rightleftharpoons{2SO_3}$$
3. Oxidation of ammonia:Oxygen reacts with ammonia at two different conditions.
i) In the absence of the catalyst, ammonia is oxidized to nitrogen.
$$4NH_3+2O_2\xrightarrow\Delta{2N_2+6H_2O}$$
ii) When ammonia is treated with oxygen in presence Pt gauze at 823K, it is oxidized to nitric oxide.
$$4NH_3+5O_2\xrightarrow{pt\;823K}{4NO+6H_2O}$$
4. Vapour of HCl reacts with oxygen : produce chlorine gas in the presence of CuCl2 at 700K.
$$2HCl+O_2\xrightarrow{CuCl_2\;700K}{H_2O+Cl_2}$$
5. Action with hydrocarbons:Hydrocarbons like alkane, alkene etc. react with oxygen to produce water and carbon dioxide.
$$CH_4+2O_2\longrightarrow{2H_2O+CO_2+heat}$$
6. Action with glucose:Glucose goes on oxidation with oxygen in the presence of biocatalysts to give CO2, H2O, and energy.
$$C_6H_{12}O_6+6O_2\longrightarrow{6CO_2+6H_2O+energy}$$
7. Action on electric discharge:Silent electric discharge changes oxygen into Ozone at low temperature.
$$3O_2\xrightarrow{silent\;electric\;\\discharge}{2O_3+energy}$$

#### Uses of Oxygen

Oxygen is used:

1. As a liquid fuel in rocket engines.
2. For the preparation of various compounds like CO2, HNO2, H2SO4, SO2 etc.
3. For artificial respiration in the case of surgery, heart ailments by pilots, for mountaineers at high altitude, for miners and sea divers.
4. As an oxidising agent in various chemical reactions.
5. For the manufacture of steel and metal fabrications.
6. To produce oxy-acetylene and oxy-hydrogen flame for cutting and welding purposes.
7. As a germicide.
8. To produce an explosive.

#### Oxides

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:

1. Acidic oxide
2. Basic oxides
3. Neutral oxides
4. Amphoteric oxides
5. Peroxides
6. Mixed oxides
7. Super oxides
8. Sub oxides

Describing the main oxides below:

1. Acidic oxides:Acidic oxides are also called anhydride of an inorganic acid. Oxides of non-metals are called acidic oxides. Many of these oxides combine with water to form acids and these oxides are neutralized by bases or basic oxides. For example; CO2, N2O5, SO3, SO2, N2O3, P2O5, Cl2O7 etc.
$$a)\;CO_2+H_2O\longrightarrow{H_2CO_3}$$
$$b)\;N_2O_5+H_2O\longrightarrow{2HNO_3}$$
$$c)\;SO_3+H_2O\longrightarrow{H_2SO_4}$$
$$d)\;CO_2+2NaOH\longrightarrow{Na_2CO_3+H_2O}$$
$$e)\;CO_2+MgO\longrightarrow{MgCO_3}$$
The acidic oxides, combining with water, gives a ternary acid (acid containing O, H, and another element).
2. Basic oxides:Oxides of metals are called basic oxides. Many of these oxides combine with water to form bases. These oxides are neutralized by acids or acidic oxides. Oxides of group IA metals and some of the group IIA metals dissolve in water to give alkali-solution whereas many other metal oxides are water-insoluble. For example; Na2O, CaO, MgO, FeO, CuO, BaO etc.
$$a)\;Na_2O+H_2O\longrightarrow{2NaOH}$$
$$b)\;CaO+H_2O\longrightarrow{Ca(OH)_2}$$
$$c)\;Na_2O+2HCl\longrightarrow{2NaCl+H_2O}$$
$$d)\;BaO+CO_2\longrightarrow{BaCO_3}$$
3. Neutral oxides:A neutral oxide is the one which neither forms a salt with an acid nor with a base. i.e. neutral oxides do not form acids or bases with water. H2O, CO, NO, N2O are neutral oxides.
4. Peroxides:These oxides contains the O2-2 ion (O - O2-), in which the oxidation number of oxygen is -1. Examples:
H-O-O-H (hydrogen peroxide)
BaO2 (barium peroxide)
Na2O2 (sodium peroxide)
Pure peroxide must be derived from hydrogen peroxide. Peroxides on treatment with dilute acids yield hydrogen peroxide.
$$BaO_2+H_2SO_4\longrightarrow{BaSO_4+H_2O_2}$$

Bibliography:

Gewali, Dr. Mohan Bikram and Dr. Pradyumna Wagley. Principles of Chemistry. Kathmandu: Buddha Academic Publishers and Distributors Pvt. Ltd., 2009.

##### Things to remember
• Oxygen reacts with ammonia at two different conditions.
•  In the absence of the catalyst, ammonia is oxidized to nitrogen.
• $$4NH_3+2O_2\xrightarrow\Delta{2N_2+6H_2O}$$
• Glucose goes on oxidation with oxygen in presence of biocatalysts to give CO2, H2O and energy.
• $$C_6H_{12}O_6+6O_2\longrightarrow{6CO_2+6H_2O+energy}$$
• Silent electric discharge changes oxygen into Ozone at low temperature.

$$3O_2\xrightarrow{silent\;electric\;\\discharge}{2O_3+energy}$$

At high temperature about 3273K, nitrogen reacts with oxygen to produce nitric oxide (NO).

$$N_2+O_2\rightleftharpoons{2NO}$$

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