Chemistry Class 12 – PREPARATION, PROPERTIES, AND USES OF DIOXYGEN
O2 is the most common allotrope of elemental oxygen on earth, itis also called as dioxygen, diatomic oxygen, molecular oxygen, or oxygen gas to distinguish it from the element itself and from the triatomic allotrope ozone, O3. Dioxygen is a major component (about 21% by volume) of the earth’s atmosphere. Aerobic organisms utilize atmospheric dioxygen as an oxidant during the process of cellular respiration. Dioxygen is the normal allotrope of oxygen having two atoms of oxygen in the molecule.
PREPARATION OF DIOXYGEN
The most common and convenient method used for preparing dioxygen in the laboratory is the catalytic decomposition of solid potassium chlorate. Manganese dioxide is used as the catalyst in this reaction.
2KClO3 → 2KCl +3O2
Another laboratory method is the thermal decomposition of oxides of metals from the lower part of the electrochemical series. Like,the thermal decomposition of silver oxide or mercuric oxide gives dioxygen.
2Ag2O → 4Ag + O2
Silver oxide Δ Silver Dioxygen
2HgO → 2Hg + O2
Mercuric oxide Δ Mercury Dioxygen
Dioxygen may also be obtained in the laboratory by heating the higher oxides of some metals like lead, barium, and manganese.
2PbO2 → 2PbO + O2
Lead (IV) oxide Δ Lead (II) oxide Dioxygen
2BaO2 → 2BaO + O2
Barium peroxide Δ Barium oxide Dioxygen
2MnO2 + 2H2SO4 → 2MnSO4 + 2H2O + O2
Manganese Sulphuric Δ manganese Water Dioxygen
(IV) oxide acid (II) sulphate
Salts that are rich in oxygen, such as permanganates and nitrates, when decomposed thermally also yields dioxygen.
- 2KNO3 → 2KNO2 + O2
Potassium Δ Potassium Dioxygen
- 2KMnO4 → K2MnO4 + MnO2 + O2
Potassium Δ Potassium manganese Dioxygen
permanganate manganate (IV) oxide
- 2NaNO3 → 2NaNO2 + O2
Sodium Δ Sodium Dioxygen
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Commercially, dioxygen is prepared by either the fractional distillation of liquid air or by the electrolysis of water
The Fractional Distillation of Liquid Air
An air separation unit is generally called as an oxygen or nitrogen generator, as its purpose is to extract one or both of these elements from the air. In this distillation process
1. The air is first passed through a filter that absorbs all the water vapor.
2. Then the cooling process begins, which involves the use of turbines and high-energy refrigeration systems.
3. Carbon dioxide and other trace gases settle out when the temperature reaches the sublimation or boiling points of each of the component gases. Sublimation is the change of matter from a solid-state to a gaseous state.
4. When the temperature reaches −200 °C, the liquified mixture is added through a tube into a vessel that is slightly warmer at the bottom (−185 °C) than it is at the top ( −190 °C).
5. Oxygen liquefies at −183 °C, therefore it flows out of the flask through a tube from the bottom of the vessel. The boiling point of nitrogen is −196 °C. It flows out through a tube connected to the top of the flask.
PHYSICAL PROPERTIES OF DIOXYGEN
- It is a colourless, tasteless and odourless gas.
- It is somewhat heavier than air.
- It is slightly soluble in water. This small fraction of dissolved dioxygen is just sufficient to support aquatic life.
- By compressing the oxygen gas at 90K, it can be liquefied under pressure to a pale blue liquid. It can also be solidified into a bluish-white solid at 55K.
CHEMICAL PROPERTIES OF DIOXYGEN
Dioxygen is a very reactive gas and reacts directly with almost all metals and non-metals. It does not react directly with metals like gold and platinum, and some noble gases like helium, neon, and argon.
- The reaction of dioxygen with metals
Most metals burn in oxygen to form oxides that are most basic in nature. All metal oxides are basic in nature because they react with dilute acids to form salt and water, they also dissolve in water to form their metal hydroxides which are alkaline in nature. These metal hydroxides release OH- ions, hence they are basic in nature.
Metal Dioxygen Metal-oxide
4M + O2 → 2M2O
2M + O2 → 2MO
4M + 3O2 → 2M2O3
Non-metals burn in the presence of dioxygen to form acidic oxides. For example, sulfur burns in the presence of oxygen give sulfur dioxide.
S + O2 → SO2
The non – metallic oxides are acidic in nature because they react with water to produce corresponding acids. SO2 produces sulphuric acid (H2SO4) on reaction with water. Thus, SO2 is acidic in nature.
- Reactions of dioxygen with some compounds:
Sulfur dioxide undergoes oxidation in the presence of a catalyst, vanadium pentoxide to give sulfur trioxide. It is an important step in the manufacture of sulphuric acid by the contact process.
2SO2 + O2 → 2SO3
- Dioxygen reacts with several organic compounds, such as hydrocarbons and carbohydrates, at very high temperatures or on the ignition, forming carbon dioxide and water.
CH4 + 2O2 → CO2 + 2H2O
Methane Dioxygen Carbon Water
C6H12O6 + 6O2 → 6CO2 + 6H2O
Glucose Dioxygen Carbon Water
USES OF DIOXYGEN
- Dioxygen is extremely vital for processes like combustion and respiration.
- Dioxygen is mixed with carbon dioxide or helium to be used for artificial respiration.
- It is used in the manufacture of many metals.
- It is highly used in oxy-acetylene welding and metal cutting.
- Dioxygen is used to oxidise ammonia in the manufacture of nitric acid.
- It is used in oxygen cylinders, which are widely used in hospitals, high-altitude flying and in mountaineering.
- Liquid oxygen is an important constituent of the fuel used in rockets.
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