Howitworks
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Howitworks
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The chemistry of combustion reactions are extremely complicated.
Although you might have seen a combustion reaction such as: CH4
+ 2O2 The simplest combustion reaction is the combination of hydrogen with
pure oxygen, leading to the production of water. The familiar global
reaction is fairly simple: 2H2 + O2 Here are two links to short videos of combustion without a match: http://www.chem.leeds.ac.uk/delights/texts/expt_2.html : (click on the links on the right hand side named "Animation 1" and "Animation 2", no video player is required) There is a strong oxidizer and a flammable alcohol, which together spontaneously react in the first reaction. The heat generated from the exothermic reaction of water with potassium, provides the energy to ignite the hydrogen evolved, which in turn ignites the diethyl ether, which has a clearly visible flame. In the case of most familiar combustion reactions involving carbon, such as a candle, the yellow outermost part is due to the hydrocarbons reacting with oxygen in the air not being 100% efficient, so some unreacted carbon is present. The carbon is heated to incandescence and emits yellow light accompanying the relaxation of electrons from an excited state to a lower state. The inner core, which is a bit darker is unburned fuel, which is rising to react with oxygen. At the base of the flame is a thin blue mantle, which is where there is sufficient oxygen to allow complete combustion of carbon and is the hottest part of the flame. Combustion produces heat by unlocking the energy trapped in chemical bonds of a substance. once initiated, it is self-sustaining, as some of the energy heats the material around the combustion zone, initiating further reactions. Reactive chemicals created in the flame also promote combustion by a process called chain branching. For example, one energetic hydrogen atom can react with an oxygen molecule to form two reactive species- a hydroxyl radical (OH) and an oxygen atom (O). These species can then react, with other molecules, quickly creating more and more radicals. Below is a picture of propane burning, demonstrating the effect of the chain reaction:
From: http://www.combustion.me.vt.edu/images/propane_fire.JPG This effect can be seen spectacularly in the case of dynamite, where chain branching proceeds very rapidly. Here is a link to a short video of a dynamite explosion: http://www.ncl.ox.ac.uk/quicktime/bang.html (Quicktime or Windows Media Player required) |
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CO2
+ 2H2O , this is in fact the overall reaction, which is also
known as the global reaction. Global reactions are actually composed of a
large number of elemental reactions, which often involve the formation or
reaction of free radical species, in the mechanism. 
2H2O , however the full reaction mechanism has eight steps. As
the atoms and molecules in the global reaction become more complex, so the
reaction mechanism also becomes more so. The reaction mechanism for the
simplest hydrocarbon fuel, methane (CH4), in air has over 80 steps! It is
hardly surprising that few of the full reaction mechanisms for heavier
hydrocarbons are completely known.
