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Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Kluges last link seems to be the most useful, it recognizes that these classifications are somewhat arbitrary in nature. Some are pretty well defined like acid-base and redox reactions while others may fit several definitions (combustion).
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
So what's the 'reaction' called when magnesium 'burns' in air?
(or titanium, aluminium or even steel. is 'burning' not the same thing as 'combustion'?....What about the 'infernal combusting engine' which, as has already been pointed out, burns petrol (gasoline), diesel or paraffin (kerosene), all of which contain other elements as well as carbon and hydrogen. and can also run on hydrogen alone (some even ran on coal dust). Is 'internal combustion engine' a misnomer?)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
The answer is that there is no such thing in Science (with a captial "S") as combustion. Not a thing that can be nailed down with a definition that's shorter than two lists, the first of all reactions that are deemed to be combustion, the other of all that are not deemed to be combustion. Astronomers have a similar problem with counting Sol's planets (Pluto argument anyone?), biologists have trouble with life (virus?). Combustion is a strictly "folk" idea, you know it when you see it.
Kulge's last definition runs (and I shorten and paraphrase) "redox reaction ... combustible material ... oxidiser ... oxidised products ... exothermic, usually oxygen combines with stuff containing hydrogen and carbon"
I can think of plenty of exothermic redox reactions involving an oxidiser and another material that neither I, nor anybody else for that matter, would regard as combustion, but it's true that most of those do occur in aqueous solution. I think that "combustible material" might have been in the definition above to exclude these, but if you think about it, use of that phrase in a definition of combustion is completely circular, it tells you nothing about the material.
We could try to tighten up the definition to exclude aqueous phase reactions, perhaps require one of the reactants to be gaseous, or require the mixing of two phases (so that the decomposition of for instance a single molecule like TNT is not combustion, but that gunpowder burning is).
The bottom line is that when we see a rapid evolution of heat in a mixture that was previously stable, after part of it was given some extra energy (I wonder if there's room for the concept of ignition in the definition we're struggling for?), we'll call that combustion. Given what we have around us, 99% of the time that will involve hydrogen and carbon bearing stuff combining with oxygen. But hey, fluorine is enough like oxygen and magnesium enough like carbon, that anybody observing the reaction between any pair of these as electrons get donated and accepted (redox!) would call it combustion as well. Having a significant sulphur impurity in the mix is not going to change that.
If your chemistry course involves a multiple choice exam at the end, then you might need to find the statement in the syllabus that runs "for the purposes of this course, combustion is deemed to be x and y under the conditions of z". Otherwise, just try to avoid getting burnt.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
It's a variant of the Latin word meaning to burn. They didn't, at that time, know what sort of reaction was going on. It's likely that they would have described any sort of flame as combustion. Is it sensible to (hypothetically) argue the meaning of a word with the people who invented it?
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
Hydrogen and oxygen can coexist forever until the concentration of monoatomic atoms of both reach the point where each recombine to form water in a chain reaction. That is predicted by the Le Chatellier rate of reaction calculations.
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