UNIT IX SECTION 3 COMBUSTION FURNISHES THE ENERGY NECESSARY TO TRANSFORM MANY MINERALS INTO USEFUL MATERIALS Photosynthesis, to be studied in the next Unit, stores the radiant energy of the sunHght in the forms of wood, coal, petroleum, and natural gas. The process by which this stored energy is released, in which carbon compounds react with oxygen to produce carbon dioxide, is called combustion. Combustion is not limited to the oxidation of carbon compounds, however, for any oxidation reaction which produces heat and light is called burning, or combustion. The Discovery of Oxygen Revealed the True Nature of Combustion. In 1771 Carl Wilhelm Scheele (1742-1786) first prepared oxygen by heating certain substances. Three years later Joseph Priestley in England, ignorant of Scheele's discovery, prepared oxygen by heating mercuric oxide by focusing the sun's rays on it by means of a "burning glass." Although Priestley discovered the clue to the true nature of combustion, he refused to give up the then current phlogiston theory, and it remained for the great French chemist, Antoine Laurent Lavoisier (1743-1794), when he learned of Priestley's experiments, to realize that combustion is simply the reaction of materials with the oxygen of the air. Oxygen Is the Most Abundant and Universally Active Element. Fully half of the earth's crust is oxygen. It occurs uncombined in the air to the extent of about one fifth of the total volume, as pointed out in an earlier section. The chief chemical characteristic of oxygen is its great readiness to combine with nearly all of the other elements, including both metals and nonmetals. Not All Oxidation Reactions Take Place Rapidly Enough to Produce Combustion. Most of the reactions which yield the same products as combustion produced and the reactions may take place so slowly that no light is 644
COMBUSTION 645 heat is dispersed nearly as fast as it is produced. Regardless of the rate of the reaction, the same amount of heat is evolved for the same amounts of reactants. A log of wood will gradually decay as it lies on the ground in the forest, and it is eventually consumed, giving the same products and the same amount of heat just as truly as if it had been burned. All forms of decay are oxidation reactions which are caused by living microorganisms. Combustion Sometimes Starts Spontaneously. Sometimes the heat produced by decay accumulates until a temperature is reached at which combustion takes place; this temperature is called the kindling temperature. For example, an oily rag containing some oil such as linseed oil which is easily oxidized may start a fire. Linseed oil is used in paints because it will slowly combine with oxygen to form a tough, resistant coating. Such paints dry best when plenty of oxygen is available. If an oily rag is placed in a closet or some other place where the heat produced by the oxidation of the oil will not diffuse away as rapidly as it is produced, the kindling temperature may be reached and a fire started; such a process is called spontaneous combustion. Many fires are started by the spontaneous combustion of damp hay, paper, coal, and other organic materials. An important factor in spontaneous combustion is that the speed of a chemical reaction is roughly doubled or trebled for each ten-degree rise in the centigrade temperature. Thus, as the temperature rises, heat is given off more and more rapidly until the slow oxidation becomes rapid combustion. Some substances are so active with oxygen that spontaneous combustion takes place in even the most exposed places; thus a lump of white phosphorus left on the table will start burning spontaneously. Certain other substances having a higher kindling temperature will ignite spontaneously only on unusually hot days. A hot piece of iron placed in the vapor of carbon disulfide will cause it to ignite. When grease is spilled on a hot stove lid, it ignites because its kindling temperature has been reached. The purpose of a match used in lighting a fire is to raise the temperature of a portion of a combustible material above the kindling temperature. Incendiary leaflets, dropped from airplanes during World War II, consisted of sheets of wet guncotton (nitrocellulose) containing finely divided phosphorus between them. After the guncotton dried, the phosphorus reacted with the oxygen of the air and ignited the guncotton, which burned with a hot flame.