chemistry notes on chemistry of oxygen

Introduction to Oxygen

Oxygen is the most abundant element in the earth crust after nitrogen. It is represented by O. It exists in diatomic form. It is in gas state at room temperature but can be liquefied by lowering temperature and solidified by increasing pressure.
The word ‘oxygen’ is derived from the Greek word oxysgenus in which oxys meant sour and genus meant producer. It was believed that oxygen was one of the main constituent of common acid.
Oxygen is found in molecular state in free atmosphere. The atmosphere consists of 20.9 percent by volume and 23 percent by weight of oxygen in air. It combines with other elements to form oxide, nitrates, sulphate, silicates etc. It has three isotopes ¹⁶₈O, ¹⁷₈O, ¹⁸₈O, among which ¹⁸₈O s radioactive and other two are non radioactive. ¹⁶₈O is the most abundant isotopes of oxygen and is found in 99.76% among all the oxygen isotopes found in nature.

Structure of oxygen atom and molecule

Oxygen is highly electronegative element. It has 6 electrons (3 lone pairs) in its valence shell. So, it has strong tendency to gain electrons for acquiring the stable configuration state. Two oxygen atom combine to each other by a covalent bond to form a oxygen molecule as shown below.
Structure of oxygen atom and molecule
Name: Oxygen
Symbol: H
Molecular formula: O₂
Atomic number: 8
Atomic Mass: 16 amu
Electronic configuration: 1s², 2s², 2p⁴
Boiling Point: 90 Kelvin
Melting Point: 55 Kelvin
Isotopes: ¹⁶₈O, ¹⁷₈O, ¹⁸₈O

Preparation of Oxygen

1. From salt: When oxygen rich salts are heated, gets decomposed and gives oxygen.
$$\ce{2KNO3->[\Delta]2KNO2 + O2 ^ }$$
$$\ce{2KClO3->[\Delta]2KCl + 3O2 ^ }$$
2. From Potassium permanganate: When potassium permanganate is heated, it gives oxygen.
$$\ce{2KMNO4->[\Delta]K2MnO4 + MnO2 + O2 ^ }$$
3. From Potassium dichromate: On heating potassium dichromate, oxygen is obtained.
$$\ce{4K2Cr2O7->[\Delta]4K2Cr2O4 + 2Cr2O3 + 3O2 ^ }$$
4. From bleaching powder: When bleaching powder is reacted with Cobalt chloride, oxygen is obtained.
$$\ce{2CaOCl2->[\ce{CoCl2}]2CaCl2 + O2 ^ }$$
5. By heating metal oxides: Metal oxides when heated, gives oxygen.
$$\ce{2HgO->[\Delta]2Hg + O2 ^ }$$
$$\ce{2Ag2O->[\Delta]4Ag + O2 ^ }$$
$$\ce{2PbO2->[\Delta]2PbO + O2 ^ }$$
$$\ce{3MnO2->[\Delta]Mn3O4 + O2 ^ }$$
6. From acids: When concentrated sulphuric acid and concentrated nitric acid are dropped slowly in heated pumice stone, it decomposes as follows.
$$\ce{4HNO3->4NO2 + 2H2O + O2 ^ }$$
$$\ce{4H2SO4->2SO2 + 2H2O + O2 ^ }$$
Manufacture in large scale:
From air: Air mainly consists of dinitrogen and dioxygen. Both gas can be isolated from each other by fractional distillation of liquefied air. The nitrogen is distilled away first as it has lower boiling point (-196^oC) and leaves the liquid oxygen (boiling point = -183^oC).
From Water: Oxygen can be obtained from by water by the process of electrolysis either in the presence of acids or alkalis. Hydrogen is liberated at cathode and oxygen at anode.The following cell reaction takes place in electrolysis.
$$\ce{H2O->H^+ + OH^- }$$
At cathode: $$\ce{2H^+ + 2e^- ->2H2O + O2 ^ }$$
At anode: $$\ce{4OH^- – 4e^- ->2H2O + O2 ^ }$$

Preparation of oxygen

In the cell, iron sheet is used as cathode and nickel plated iron sheet acts as cathode. The two electrodes are separated from each other by a long rod called asbestos diaphram. This prevents the mixing of hydrogen and oxygen gas.

Physical properties of oxygen:
– Oxygen is colorless, odourless and tasteless gas.
– It is pale blue color in liquid and solid state.
– It is slightly soluble in water.
– It is heavier than air.

Note: Metals like gold, silver, platinum can absorb oxygen at high temperature and expels it on cooling. This phenomenon is called spitting of silver.

Chemical properties of oxygen:
Combustibility: It does not burn itself but it supports for combustion. Oxygen requires high initial heating due to its high bond dissociation energy of 493.4KJmol^– between the O=O atoms.

Note:The presence of oxygen is must for burning anything. Nothing can get burnt without oxygen. Dissociation energy is the energy required to break old bonds when any chemical species undergo chemical reaction

Action with hydrogen: Oxygen when heated with hydrogen forms water.

2H2+O2−→Δ2H2O

Action with Nitrogen: Oxygen reacts with nitrogen at high temperature to give nitrogen dioxide.

N2+O2−→−−−3000∘C2NO

NO+O2⟶2NO2

Action with carbon: Carbon when reacted with limited oxygen gives carbon monoxide and with excess oxygen, carbon gives carbon dioxide.

2C+O2limited⟶COCarbon Monoxide

2C+O2Excess⟶CO2Carbon Monoxide

Action with ammonia: Oxygen reacts with ammonia to give nitric oxide and water. It is a reaction involved in manufacture of nitric acid in Ostwald’s process.

4NH3+5O2−→−−−Pt/MO800∘C4NO+6H2O↑

Action with glucose: Glucose reacts with oxygen to give carbon dioxide, water and energy. This reaction takes place inside the human body.

Glucose + Oxygen→Carbon Dioxide + Water + Energy

i.e.,

C6H12O6+6O2⟶6CO2+6H2O+Energy

Action with metals:
Oxygen combines with many metals to form their respective oxides.

4Na+O2−→−−−−−−−−−Room temperature2NaO2

2Na+O2−→−−300∘CNa2O2

4K+O2⟶2K2O

2Mg+O2−→Δ2MgO

2Zn+O2−→Δ2ZnO

4Al+3O2−→ΔAl2O3

Action with iron: Oxygen when reacted with iron gives ferrous oxide. When excess of oxygen is passed, it gives ferrosoferic oxide and further addition of oxygen gives ferric oxide. To be discused

Fe+O2−→ΔFeOFerrousoxide

6FeO+O2−→Δ2Fe3O4Ferrosofericoxide

4Fe+3O2−→Δ2Fe3O3Ferricoxide

When oxygen is reacted with oxygen in presence of water, rust is formed.

4Fe+3O2+2H2O⟶2Fe2O3⋅XH2ORust

Uses of oxygen:
– It is used for artificial respiration in hospitals, mountaineers in high altitude, miners and sea divers in the form of oxygen mask.
– It is used as aero fuel in rocket engines and planes.
– It is used for the generation of energy inside our body.
– It is used as strong oxidizing agent in laboratory.
– It is used by the plants for the process of photosynthesis.
– It is used in preparing different explosives.
– It is used as a germicides and insecticides.
– It is the main element for the formation of ozone.

Oxides and types of oxides

The binary compounds of oxygen with other elements are called oxides.
Classification of oxides: On the basis of chemical properties and structures, there are eight types of oxides.
1. Acidic oxides
2. Basic oxides
3. Neutral oxides
4. Amphoteric oxides
5. Peroxide
6. Super oxide
7. Sub oxide
8. Mixed oxides
1. Acidic oxide: Oxide of non metal which combine with water to form acid is called acidic oxide.
Example: SO₂, CO₂, NO₂, SnO₂, CrO₃ etc.
$$\ce{SO2 + H2O->$\underset{\text{Sulphurous acid}}{\ce{H2SO3}}$}$$
$$\ce{SO3 + H2O->$\underset{\text{Sulphuric acid}}{\ce{H2SO4}}$}$$
$$\ce{SO2 + H2O->$\underset{\text{Sulphurous acid}}{\ce{H2SO3}}$}$$
$$\ce{CO2 + H2O->$\underset{\text{Carbonic acid}}{\ce{H2CO3}}$}$$
$$\ce{CrO3 + H2O->$\underset{\text{Chromic acid}}{\ce{H2CrO4}}$}$$
$$\ce{SnO2 + H2O->$\underset{\text{Stannic acid}}{\ce{H2SnO3}}$}$$
2. Basic oxides: Metallic oxide which combines with water to form bases is called basic oxide. Examples:
CaO, Na₂O, BaO etc.
$$\ce{CaO + HCl->CaCl2 + H2O}$$
$$\ce{MgO + 2HCl->MgCl2 + H2O}$$
3. Neutral oxide: The oxide which shows neither acidic nor basic nature is called neutral oxide. For examples CO, H₂O, N₂O etc.
4. Amphoteric oxides: Those oxides which shows the property of both acidic and basic oxides are called amphoteric oxides. Examples: ZnO, Al₂O₃, PbO, SnO etc.
$$\ce{$\underset{\text{Oxide}}{\ce{ZnO}}$ + $\underset{\text{Acid}}{\ce{H2SO4}}$ ->$\underset{\text{Salt}}{\ce{ZnSO4}}$ + $\underset{\text{Water}}{\ce{H2O}}$}$$
$$\ce{$\underset{\text{Oxide}}{\ce{ZnO}}$ + $\underset{\text{Base}}{\ce{2NaOH}}$ ->$\underset{\text{Salt}}{\ce{Na2ZnO2}}$ + $\underset{\text{Water}}{\ce{H2O}}$}$$
$$\ce{$\underset{\text{Oxide}}{\ce{ Al2O3}}$ + $\underset{\text{Acid}}{\ce{6HCl}}$ ->$\underset{\text{Salt}}{\ce{2Al2O3}}$ + $\underset{\text{Water}}{\ce{3H2O}}$}$$
$$\ce{$\underset{\text{Oxide}}{\ce{ Al2O3}}$ + $\underset{\text{Base}}{\ce{ Al2O3}}$ ->$\underset{\text{Salt}}{\ce{2NaAlO2}}$ + $\underset{\text{Water}}{\ce{3H2O}}$}$$
5. Peroxides: Those oxides which contains peroxy linkage –O – O– and produces hydrogen peroxide(H₂O₂) are called peroxides. Examples: BaO₂, Na₂O₂, Li₂O₂ etc.
$$\ce{BaO2 + 2HCl->BaCl2 + H2O2}$$
6. Super oxides: Those oxides which contain super oxide ions are called super oxides. Examples: KO₂, RbO₂ etc. These oxidation number of oxygen in these compounds is always 1/2.
7. Sub oxides: Those oxides which contain less number of oxygen then usual and has bonds between the same atoms of the elements besides having bonds between the elements and oxygen is called sub oxides. Examples: C₂O₃(O=C=C=C=O), Pb₂O etc.
8. Mixed oxides: Those oxides which behaves as a mixture of two simpler oxides of the same elements is called mixed oxides. Examples: Red Lead (Pb₃O₄) contains two simple oxides PbO and PbO₂ respectively.