What is the difference between combusting and oxidizing? Oxidation of alcohols leads to formation of aldehydes and carboxylic acid whereas the combustion of alcohols leads to the formation of carbon dioxide and water. What is going on if oxygen is reacting with the same molecule?
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5$\begingroup$ The same reagents may give different products at different conditions. Surely you must have heard this before. $\endgroup$– Ivan NeretinCommented Jun 8, 2021 at 17:02
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2 Answers
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Apart from the fact that not all combustion involves oxygen (virtually everything combusts in chlorine trifluoride and oxygen doesn't come into it) the major difference is how far the reaction goes.
In normal combustion (involving oxygen) the reaction is uncontrolled and tends to go mostly to completion (that is most things end up in their most thermodynamically stable state: hydrocarbons are fully reacted to water and carbon dioxide).
When chemists talk of other oxidation reactions they tend to talk of much more specific, much more controlled reactions. So specific conditions are used to convert an alcohol to an aldehyde or an aldehyde to a carboxylic acid. These are "oxidations", but the chemist uses conditions to control the reaction so they get a specific product.
Combustion is oxidation, but uncontrolled. Most oxidations that chemists talk about are far more specific and controlled. That's the difference.
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$\begingroup$ Fire is a kind of combustion and most of fires are controlled. $\endgroup$– PoutnikCommented Jun 9, 2021 at 10:09
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2$\begingroup$ @Poutnik You are using "controlled" in a very different sense to me. From a chemical perspective fire is very uncontrolled. Sure, most coal fires in your house don't burn down your house, but that doesn't mean the fire is as "controlled" as an alcohol to aldehyde conversion in a chemist's reaction vessel. $\endgroup$ Commented Jun 9, 2021 at 10:13
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$\begingroup$ Well, oxidation is a superset to combustion, it is not a non-overlapping different set. // Combustion still can be controlled and partial, as well as non combustive oxidation. $\endgroup$– PoutnikCommented Jun 9, 2021 at 10:19
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Combustion versus oxidation is not about what products are created, but about the way how they are produced.
Combustion is the kind of oxidation, usually using gaseous oxidant, sufficiently exothermic, with a sufficiently high reaction rate. Characteristic combustion behavious is high temperature raise with thermal IR and light emission, frequently with flame, if combustible gases are present/created.
Both combustive and non combustive oxidation may or may not involve reaction with oxygen.
E.g. during fermentation of wine to vinegar, ethanol is not combusted, but is enzymatically oxidized. No flame, no light, no significant temperature raise. Ascorbic acid and other antioxidants in food are progressively oxidized, not combusted.
In ethanol burners, ethanol is combusted, forming heat, light and flame.
