Pem Choki Pema Dechen Shree Jana Bomjan Presenters Pem Choki Pema Dechen Shree Jana Bomjan

Oxygen Discovery Joseph Priestley and Carl Wilhelm Scheele both independently discovered oxygen. Priestley is usually given credit for the discovery. They both produced oxygen by heating mercuric oxide (HgO). Priestley called oxygen as “Dephlogisticated air” Scheele called oxygen as “Fire air” Finally the name oxygen was created by Antoine Lavoisier who incorrectly believed that oxygen was necessary to form all acids.

A brief description. Comprises 21% of the earth’s atmosphere. Supports life and makes combustion possible. Comprises 85% of its oceans and as a component of most rocks and minerals, 46% of its solid crust. Constitutes 60% of the human body

Occurrence Oxygen occurs in the free state as a gas, to the extent of 21 per cent by volume or 23 per cent by weight in the atmosphere. Combined Oxygen also occurs in water, in vegetable and animal tissues, in nearly all rocks and in many minerals.

Laboratory preparation. Using Potassium Chlorate (KClO3) Potassium chlorate decomposes at a lower temperature if previously mixed with manganese dioxide which is a catalyst for the decomposition. Only the potassium chlorate is decomposed and no per chlorate is formed; 2KClO3 ==> 2KCl+3O2

2. Using Hydrogen Peroxide The decomposition of hydrogen peroxide using manganese dioxide as a catalyst also results in the production of oxygen gas. Very convenient method since no heating is required. When hydrogen peroxide is allowed to trickle over manganese dioxide, it rapidly decomposes to form water and oxygen gas; 2H2O2 => 2H2O+O2

3. By electrolysis of water. The electrolysis of acidified water is carried out in a Hoffman voltameter. Oxygen is evolved at the positive electrode in the electrolysis; 2H2O=> 2H2+O2 4. By chemical decomposition of water Oxygen is obtained from water by passing a mixture of chlorine through a strongly heated silica tube containing pieces of broken porcelain. 2H2O+ 2Cl2=> 4HCl+O2

5. Decomposition of oxides. 2HgO=> 2Hg+O2 2PbO2=> 6PbO+O2 6. Decomposition of salts. Some salts containing oxygen decompose and release oxygen gas on heating. Potassium nitrate melts on heating and at a slightly high temperature decomposes, giving off bubbles of oxygen and forming potassium nitrite which solidifies on cooling.   2 KNO3 ==> 2 KNO2 + O2

Physical properties of oxygen Atomic number: 8 Atomic mass: 15.9994 Melting point: -214 degC Boiling point: -183degC Density: 0.001429 g/cc Phase at room temperature: gas Element classification: non- metal Periodic number: 2 Group number: 16 Group name: Chalcogen

Chemical properties of oxygen 1) Oxidizing nature of oxygen a) oxidation of non-metals i) Hydrogen Pure hydrogen burns in oxygen with a pale blue flame producing a colourless, neutral liquid that is water. 2H20 + 02=> 2H20

II)Carbon When red hot charcoal is taken in a jar containing oxygen, it bursts into flames and burns brightly with a brilliant yellow flame to form carbon dioxide gas which turns lime water milky and moist blue litmus red. Solution of carbon dioxide is called carbonic acid. C + 02=> CO2

2) oxidation of metals a) calcium- when burning Ca is introduced into a jar of oxygen, it burns vigorously with a brick red flame forming white ash of calcium oxide. Calcium oxide formed reacts exothermically with water to form basic CaOH which turns red litmus blue. 2Ca+ O2 ==> 2CaO

When a burning magnesium ribbon is introduced into a jar of oxygen, it burns vigorously with the dazzling white light to form a white powder of magnesium oxide. Magnesium oxide is very slightly soluble in water and turns red litmus solution slightly blue. This indicates that magnesium oxide is a weak base. 2Mg+ O2 ==> 2MgO

3. Oxidation of lower oxides to higher oxides. Colourless nitric oxide combines with oxygen at room temperature to form nitrogen dioxide which is a reddish brown gas 2NO+ O2 ==> 2NO2 ii. Carbon monoxide burns in oxygen with non luminous blue flame forming carbon dioxide. 2CO+O2 ==>2CO2

4. Oxidation of hydrocarbons. Hydrocarbons burn in air to form carbondioxide and water vapour. i. CH4+2O2 ==>CO2+2H2O ii.2C6H6+7O2 ==>4CO2+6H2O

Uses of Oxygen Oxygen is essential for life and it takes part in processes of combustion, its biological functions in respiration make it important. Oxygen is sparingly soluble in water, but the small quantity of dissolved oxygen in is essential to the life of fish. Oxygen gas is used with hydrogen or coal gas in blowpipes and with acetylene in the oxy-acetylene torch for welding and cutting metals. Oxygen gas is also used in a number of industrial processes.

Medicinally, oxygen gas is used in the treatment of pneumonia and gas poisoning, and it is used as an anesthetic when mixed with nitrous oxide, ether vapour, etc.. Carbon Dioxide is often mixed with the oxygen as this stimulates breathing, and this mixture is also used in cases of poisoning and collapse for restoring respiration. Liquid oxygen mixed with powdered charcoal has been used as an explosive.

Environmental effects of oxygen Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. The fire that killed the Apollo 1 crew on a test launch pad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch. 

Test for oxygen. Colourless, odourless gas which rekindles a glowing splinter. Turns colourless nitric oxide (NO) gas into brown coloured nitrogen dioxide gas. Turns alkaline pyrogallol solution brown. No effect on litmus.

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