Preparation of dioxygen:
The most convenient method for preparing dioxygen in the laboratory involves the catalytic decomposition of solid potassium chlorate. Manganese dioxide is used as the catalyst in this reaction.
→ 2KCl + 3O2
The other laboratory method is the thermal decomposition of oxides of metals from the lower part of the electrochemical series. EX: The thermal decomposition of silver oxide or mercuric oxide gives dioxygen.
O → 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.
→ 2PbO + O2
Lead (IV) oxide Δ Lead (II) oxide Dioxygen
→ 2BaO + O2
Barium peroxide Δ Barium oxide Dioxygen
O + O2
Manganese Sulphuric Δ manganese Water Dioxygen
(IV) oxide acid (II) sulphate
The thermal decomposition of salts rich in oxygen, such as nitrates, and permanganates also yields dioxygen.
Potassium Δ Potassium Dioxygen
Sodium Δ Sodium Dioxygen
Potassium Δ Potassium manganese Dioxygen
permanganate manganate (IV) oxide
Commercially, dioxygen is prepared by either the fractional distillation of liquid air or by the electrolysis of water.
Properties of dioxygen:
- Dioxygen is a colourless, tasteless and odourless gas.
- It is slightly heavier than air.
- It is slightly soluble in water .This small quantity of dissolved dioxygen is just sufficient to support marine and aquatic life.
- It can be liquefied under pressure to a pale blue liquid by compressing the gas at 90K. It can also be solidified into a bluish white solid at 55K.
It has three isotopes forms with mass numbers 16, 17 and 18. Dioxygen exhibits paramagnetic behaviour.
Chemical properties of dioxygen:
Dioxygen is a very reactive element and reacts directly with nearly all metals and non-metals. It does not react directly with some metals like gold and platinum, and some noble gases like helium, neon and argon.
Reaction of dioxygen with metals:
Most metals burn in dioxygen and form oxides that are mostly basic in nature.
Metal Dioxygen Metal-oxide
4M + O2
2M + O2
4M + 3O2
Most of non-metals burn in the presence of dioxygen forms acidic oxides. EX: Sulphur burns in the presence of oxygen gives sulphur dioxide.
S + O2
Reactions of dioxygen with some compounds:
Sulphur dioxide undergoes catalytic oxidation in the presence of vanadium pentoxide to form sulphur trioxide. This is an important step in the manufacture of sulphuric acid by the contact process.
Dioxygen reacts with several organic compounds, such as hydrocarbons and carbohydrates, at elevated temperatures or on ignition, forming carbon dioxide and water.
Methane Dioxygen Carbon Water
O6 + 6O2
Glucose Dioxygen Carbon Water
Uses of dioxygen:
- Dioxygen lies in its support to vital processes such as combustion and respiration.
- The main importance of dioxygen lies in its support to vital processes such as combustion and respiration.
- Dioxygen mixed with carbon dioxide or helium is used for artificial respiration.
- It is used in the manufacture of many metals.
- It is used in oxy-acetylene welding and metal cutting.
- It is used to oxidise ammonia in the manufacture of nitric acid.
- Oxygen cylinders are widely used in hospitals, high-altitude flying and in mountaineering.
- Liquid oxygen is an important constituent of the fuel used in rockets.