Which is an example of a self sustaining chemical reaction?
IntroductionThe phenomenon of fire is a complex scientific event. It has had many simplified definitions applied to it in an attempt to provide a simple explanation for those who require a working knowledge of fire for practical purposes. From a forensic science viewpoint, this applies to those who are required to investigate and understand the ignition and propagation of fire, i.e. fire (cause) investigators. This knowledge must include a basic understanding of fire chemistry. This article outlines the basic chemical concepts that apply to combustion. It also defines numerous physical and chemical properties of gases and liquids that are necessary for their ignition and combustion and also explains their ‘performance’ on undergoing combustion. Show FireConditions for fireAir – Oxygen As mentioned in the definition of fire, the oxidizing agent is usually atmospheric oxygen. As will be explained later, the actual presence of oxygen is usually easily explained by the fact that it constitutes approximately 20% of the atmosphere. However, the amount of oxygen available is a critical parameter in the spread and intensity of a fire. Figure 1 The fire triangle. Figure 2 The fire tetrahedron. The common fuels requiring consideration in fire investigation are organic compounds usually containing significant amounts of carbon (approx. 50% and higher) and hydrogen. They include naturally occurring compounds such as wood, cotton, etc., synthetic materials such as plastics, paints, rubbers, etc. and refined fuels and solvents which include petrol, lighting kerosene and methylated spirits. Heat Energy in the form of heat is necessary to excite the molecules of both the fuel and the oxygen to an activated state required for chemicalreaction. The minimum temperature necessary to initiate the ‘self-sustaining chemical reaction’ referred to in the definition of fire, is known as the ignition temperature of the fuel. It is the source of ignition that is the primary interest of fire investigators because this can be interpreted as the cause of the fire.Fire requirements and processes When a fueland an oxidizing agent are heated to ignition temperature, either through autoignition or piloted ignition, chemical reactions occur. These exothermic (heat-producing) reactions provide additional energy to the situation and, as a consequence, further reactions proceed. When the production of excited species exceeds the decay of these species, the chemi-calreactions can be regarded as a chain or self-sustaining reaction and a fire or explosion results. The mechanism for this process is further discussed under ‘Flame Chemistry’ below. For toluene, a major component of petroleum, the equation is However, for uncontrolled combustion, where there is limited or no control over the supply of combustion reactants and removalof combustion products, the following equation more accurately defines the situation. Hydrocarbon + Oxygen + heat — Carbon Dioxide + Water + Heat + products of incomplete combustion (carbon monoxide, soot, pyrolysis products, polynuc-lear aromatic hydrocarbons, etc.) Again, for toluene, with a limited supply of air, the equation could be It is the products of incomplete combustion that form the smoke at the scene of a fire. Smoke is the cloud of soot particles, particulate matter, unburnt gases and combustion gases which, as a result of convection, rises above the fire. However, even with efficient combustion there may be gray ‘smoke’ seen in the gas plume due to condensing water vapor. The products of incomplete combustion may themselves be subsequently oxidized in a poorly ventilated fire. If such a fire is suddenly ventilated, these products may reach their explosive limits (see later) and ignite explosively in a process known as ‘flashback’ or ‘smoke explosion’. Flame ChemistryFundamentally, the investigation of fire origins and causes requires an understanding of fire-travelin-dicators such as depth of burning, lowest point of burning, etc. ‘Glowing’ fires, although extremely efficient oxidation processes, do not contribute significantly to fire spread, particularly early in the development and spread of fires and are usually associated with the middle and final stages. Most of the effect of fire can be attributed to the convection and radiation of heat from flames and, as such, an understanding of the basic chemistry of flames, which are the source of that heat, is where the science of fire investigation actually commences. ExplosionsThe mechanisms of explosions are examined in detail in other sections in this text. However, because they represent a particular form of combustion they require some consideration here. Fuel-air (diffuse) explosionsIn fires, the burning rate is controlled by the amount of oxygen available, that is the ventilation of the fire, and the fuel available for combustion. The supply of fuel is itself dependent on the rate of burning and therefore on the rate of pyrolysis. Therefore, to a certain extent, the production of flammable gases and their mixing with air is dependent on the fire itself. In an uncon-fined area and while there is adequate ventilation,products of combustion are removed from the system, air enters the system and the combustion process continues. However, in the case of explosions, the air and vaporized fuelare intimately mixed before ignition and, as a result, the ignition is extremely rapid. If this rapid combustion occurs in a confined space, then the resulting products of combustion and the existing atmospheric gases undergo a large and rapid expansion and the effects of an explosion are experienced. The pressure wave resulting from this rapid combustion is in the order of approximately a 1500 kgm”2. Dense-phase (concentrated) explosionsDense-phase or concentrated explosions differ significantly in that they are not dependent on oxygen in the air but result from an oxidation made possible because the explosive compounds contain both the fuel and the oxidant. With the input of a small energy source, the chemicalstructure of these compounds is rearranged with the simultaneous production of much more energy. This energy can be measured and is known as the heat of decomposition. Common explosives in this class include dynamite, trinitrotoluene (TNT), pentaerythritoltetranitrate (PETN) and nitroglycerine. More details concerning the structure of explosives and the mechanisms of explosions are provided in other articles. What is an example of a selfCombustion is the self-sustaining process of rapid oxidation of a fuel, which produces heat and light. The percentage of a substance (vapor) in air that will burn once it is ignited. The minimum temperature when a liquid fuel gives off sufficient vapors to form an ignitable mixture.
What is an example of a selfAn exothermic chemical reaction that is a self-sustaining process of rapid oxidation of a fuel, that produces heat and light. A change that occurs when a substance remains chemically the same but changes in size, shape or appearance. Example: Water freezing or boiling.
Which is an example of a selfCombustion is a very rapid, self-sustaining process that combines oxygen with another substance and results in the release of heat and light.
What is a selfNASA defines life as a self-sustaining chemical system capable of undergoing Darwinian evolution [1, 2]. Here, “self-sustaining” implies that a living system should not need continuous intervention by a higher entity (e.g. a graduate student or a god) to continue as life [1].
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