Why do we need to supply heat before reacting any chemical substances with oxygen?

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    $\begingroup$ Rusting of iron is a chemical reaction with oxygen that is slow, but does not require heating. $\endgroup$ Mar 13, 2021 at 16:47
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    $\begingroup$ Would Chemistry be a better home for this question? $\endgroup$
    – Qmechanic
    Mar 14, 2021 at 13:39
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    $\begingroup$ I think almost all oxidations (and indeed, all exothermic reactions) happen spontaneously; they happen just too slowly to drive your car or heat your house with them. Oxygen is a bitch. $\endgroup$ Mar 14, 2021 at 15:51
  • $\begingroup$ I am sorry for this but this question was in Modern Physics. @Qmechanic $\endgroup$
    – Erucix
    Mar 14, 2021 at 18:08

2 Answers 2


Not ALL reactions involving oxygen require heat to get them going: see pyrophoricity for example.

But many do.

Although oxygen is almost as electronegative as fluorine, the latter is much more reactive at room temperature.

This is because in $\text{F}_2$ the $\text{F}$ atoms are bound by a single bond $\text{F}-\text{F}$ while in the case of $\text{O}_2$ the $\text{O}$ atoms are bound by a double bond $\text{O}=\text{O}$.

For the $\text{F}$ or $\text{O}$ atoms to take part in reactions these bonds need first be broken. Breaking single bonds is much easier (requiring lower energy) than breaking double bonds.

This explains the generally higher reactivity at room temperature of fluorine compared to oxygen.

Biosystems like human beings owe their existence to the "sluggish reactivity" of oxygen, as a biosystem based on the aggressive reactivity of fluorine is almost unimaginable.

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    $\begingroup$ ... and we already have to have tons of mechanisms to keep oxygen from destroying our bodies. Much of the molecular damage (which causes all sorts of things from cellular death to cancer) is caused by free oxygen and other related radicals. Needless to say, these don't have the problem of having to break the dioxygen double bond. And on the other side, it's why we need so many enzymes just to "combust" food using oxygen - we're very cold, so we need to lower the activation energy another way; that's what enzymes/catalysts do. $\endgroup$
    – Luaan
    Mar 14, 2021 at 11:18

Because any reaction involving oxygen not requiring heat has already happened. So, all the reactions that have not already happened (and so you can observe them) require heat.

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    $\begingroup$ This might be true for systems exposed to Earth’s atmosphere; what about other systems? $\endgroup$ Mar 14, 2021 at 12:10
  • $\begingroup$ I rather like this answer. Other systems are not so relevant to the perception that combustion reactions require heat to get started. We don't see, for example, the reaction of sodium with oxygen because there is so little native sodium lying around! $\endgroup$
    – James K
    Mar 14, 2021 at 17:58

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