Combustion and Flame

Chemistry Combustion and Flame COMBUSTION  Introduction Combustion is simply burning of a substance. It is an exothermic process, i. e., heat is produced on combustion. During combustion, carbon present in the fuel combines with oxygen and forms carbon dioxide while hydrogen forms water vapour. Therefore, the main products of combustion are CO2 and water vapour. Now the combustion may be defined as the burning of a substance in oxygen or air to produce heat and light. The process in which a substance combines chemically with oxygen or any other supporter of combustion, with simultaneous evolution of heat and light is called combustion. (I) Classification of Combustion II. Rapid Combustion (or Burning): The process in which a substance combines chemically with oxygen at a temperature above its ignition temperature with the evolution of large amounts of heat and light in a short time is called rapid combustion, or burning. Burning of hydrocarbon fuels e.g., LPG, kerosene, petrol etc., is rapid combustion. Combustion (or burning) of some common substances are described below: (i) Combustion of carbon: Carbon (or charcoal) burns in air or oxygen to give CO2 producing heat and light. 𝑐𝑎𝑟𝑏𝑜𝑛 (𝑜𝑟 𝑐ℎ𝑎𝑟𝑐𝑜𝑎𝑙) 𝑂𝑥𝑦𝑔𝑒𝑛 + (𝑓𝑟𝑜𝑚 𝑎𝑖𝑟) → 1 𝑐𝑎𝑟𝑏𝑜𝑛 𝑑𝑖𝑜𝑥𝑖𝑑𝑒 + 𝐻𝑒𝑎𝑡 + 𝐿𝑖𝑔ℎ𝑡 Chemistry (ii) Combustion of hydrocarbons: Hydrocarbons burn to produce carbon dioxide (CO2 ), water (H2O) and heat and light. For example, burning of methane or natural gas is described by the equation. 𝑀𝑒𝑡𝑎𝑛𝑒 + 𝑂𝑥𝑦𝑔𝑒𝑛 (𝑓𝑟𝑜𝑚 𝑎𝑖𝑟) 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 → 𝑐𝑎𝑟𝑏𝑜𝑛 𝑑𝑖𝑜𝑥𝑖𝑑𝑒 + 𝑊𝑎𝑡𝑒𝑟 + 𝐻𝑒𝑎𝑡 + 𝐿𝑖𝑔ℎ𝑡 Burning of LPG (which contains mainly butane) produces carbon dioxide, water, heat and light. (iii) Combustion of magnesium: Magnesium burns in air (or oxygen) to give magnesium oxide (MgO) producing heat and light. 𝑴𝒂𝒈𝒏𝒆𝒔𝒊𝒖𝒎 + 𝑶𝒙𝒚𝒈𝒆𝒏 𝐶𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 → (𝒇𝒓𝒐𝒎 𝒂𝒊𝒓) 𝑀𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 𝑜𝑥𝑖𝑑𝑒 + 𝐻𝑒𝑎𝑡 + 𝐿𝑖𝑔ℎ𝑡 III. Slow Combustion A combustion reaction in which no light is produced and temperature of the substance remains almost unchanged is called slow or spontaneous combustion. Thus, a substance undergoes slow combustion without catching fire. Some example of slow or spontaneous combustion are: (i) Digestion of food (or respiration) (ii) Oxidation of yellow phosphorus at room temperature IV. Complete Combustion The combustion in which the substance gets completely burnt to form the highest oxide of the substance is called complete combustion. Combustion in the presence of excess (or sufficient) oxygen or air is complete combustion. For example, burning of carbon-to-carbon dioxide (CO2 ) is complete combustion. + 𝐶 𝐶𝑎𝑟𝑏𝑜𝑛 + 𝑂2 𝑂𝑥𝑦𝑔𝑒𝑛 (𝑓𝑟𝑜𝑚 𝑎𝑖𝑟) − − − − − − − − −→ 𝐶𝑜𝑚𝑝𝑙𝑒𝑡𝑒 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 → 𝐶𝑂2 𝐶𝑎𝑟𝑏𝑜𝑛 𝑑𝑖𝑜𝑥𝑖𝑑𝑒 V. Incomplete Combustion The combustion reaction that takes place in the presence of insufficient quantity of oxygen (or air) is called incomplete combustion. For example, when carbon is burnt in insufficient (limited) 2 Chemistry quantity of air, carbon monoxide is formed. 2C Carbon + + O2  Oxygen 2CO Carbon monoxide (Limited amount)  Combustible & Non-Combustible Substances The substances which burn readily are called combustible substances. For example, Petrol, LPG (cooking gas), Wax, Kerosene, Paper, Cloth, Wood, Coal etc., are combustible substances. The substances which do not burn are called non-combustible substances. For example, Water, glass, sand etc., are non-combustible substances. 3