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My question has to do with some combustion related definitions. When the term complete combustion is used, is it assumed that the combustion is stoichiometric (reactants combine perfectly), but dissociation does not occur, such that only carbon dioxide and water is the product? If so, does the term stoichiometric combustion mean the same thing except dissociation effects do occur?

Edit: Minor species of combustion as a result of dissociation

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Stoichiometric, theoretical or complete combustion, is the idealized process where a hydrocarbon is completely reduced to carbon dioxide and water, consuming oxygen (or, if other elements are present, they get all also oxidated, like sulphur compounds yielding sulphur dioxide, but as you can see, not to its maximum oxidation state, sulphur trioxide is not produced in real combustions)

It is called stoichiometric for obvious reasons, stoichiometric amounts of the hydrocarbon and oxygen / water carbon dioxide are consumed / generated. It is supposed to be a complete reaction.

But is also called theoretical, because it does not take place like that in real life. In practice, long hydrocarbon chains do not break down completely as efficiently as it's expected, subproducts are obtained, not only carbon dioxide is formed, but also carbon monoxide, unburned elemental carbon and like in every chemical reaction, it does not proceed to completion.

Also, if there is not enough oxygen (not at least an stoichiometric amount), incomplete combustion takes place, yielding almost exclusively carbon monoxide (toxic stuff). In some cases, such as the fuels combustion processes, another process called pyrolysis, occurs before combustion. In this incomplete combustion, products of pyrolysis remain unburnt and important contaminants are produced, like the product of partial oxidation of ethanol can produce acetaldehyde, which is really toxic (Considerably small LD50) and very very flammable (flash point under 0 Celsius degrees).

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  • $\begingroup$ From this it looks like stoichiometric and complete combustion are synonymous? Is there a special name for stoichiometric combustion where dissociation of combustion products CO2 and H2O are considered (neglecting the other real effect mentioned, assume that fuel and oxygen are all consumed)? $\endgroup$ – Yandle Nov 19 '14 at 2:31
  • $\begingroup$ Sincerely, I'm not sure what you mean when you say "dissociation of combustion products are considered". Yu mean CO2 and H2O being dissociated into its elements? That'd never happen... $\endgroup$ – Altered State Nov 19 '14 at 9:00
  • $\begingroup$ By dissociation effects, I mean minor species that form like CO, NO, OH, H, H_2, etc. as indicated on the graph I added in my question, from the thermal decomposition of CO2 and H2O. Complete combustion to my understanding assumes no decomposition, and that only CO2 and H2O are formed. $\endgroup$ – Yandle Nov 19 '14 at 15:35
  • $\begingroup$ These components are not products of a complete combustion, they can be generated in an incomplete one. CO, NO, H2... End up converted into CO2, H20, NO2, the products of complete combustion $\endgroup$ – Altered State Nov 19 '14 at 17:33
  • $\begingroup$ In the context of the textbook I'm reading, the effects of reactant decomposition is neglected and it only considers combustion product dissociation. My book considers complete combustion to be stoichiometric and free of dissociation (products are H2O and CO2 only). However, these products do dissociate to form the minor species as noted in the graph at stoichiometric (equivalence ratio = 1). I'm looking for a term that is defined as: stoichiometric combustion with product dissociation (if such a term exists) or does the term stoichiometric combustion already assume this. $\endgroup$ – Yandle Nov 19 '14 at 18:11

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