I understand that oxygen and halogens are both very electronegative elements and it would makes sense that they don't want to associate with each other at all. However, I know that oxides of halogens ($\ce{Cl2O, ClO2, ClO, Cl2O2}$ etc.) do exist. When reading about halogens, it says that chlorine gas doesn't react with oxygen gas. I tried to find the intuitive reasoning behind this phenomenon online, but to no avail.

I know that chlorine and fluorine gas can react to produce chlorine trifluoride (fluorination of chlorine), proving that two diatomic, covalent and non-polar gases of electronegative elements can react. $$\ce{Cl2 + 3F2 -> 2ClF3}$$

Also, drawing comparison with the reaction between nitrogen and oxygen gas: $$\ce{N2 + O2 -> NO}$$ $$\ce{N2 + 2O2 -> NO2}$$ Nitrogen's triple bond is loads stronger than the covalent bonds from oxygen or chlorine, and it can still react with oxygen gas, albeit only under extreme conditions (lightning). Is this the case with chlorine? Even so, it only makes sense that it requires a lower temperature, as the energy needed to atomize them is lower (I haven't done the calculations with enthalpy and entropy yet). I can't find any reference to a reaction such as this online, though.

Most oxides of chlorine can only be made from compounds like chlorites and chlorates or by other novel methods. Is there a reason oxygen and chlorine (or other halogens, but chlorine in particular) can't react with oxygen gas? Is there something stopping them from reacting that I don't know about? Is it due to undesirable entropy changes? Or are the reactions I mentioned special, allowing two relatively nonreactive gases to react?

  • 1
    $\begingroup$ All chlorine oxides are endothermic, to begin with. $\endgroup$ Commented Aug 23, 2020 at 7:41
  • $\begingroup$ Oxygen and fluorine reacts directly with each other to form dioxygen difluoride, $\ce{O2F2}$. $\endgroup$ Commented Aug 23, 2020 at 8:29
  • $\begingroup$ You cannot get chlorine and oxygen to react directly. Just like oxygen and nitrogen. They simply won't do it, except in an electrical arc (which is terribly inefficient). You need to take detours: chlorine + water + electrolysis, or make ammonia from nitrogen, and burn that afterwards. $\endgroup$
    – Karl
    Commented Aug 23, 2020 at 8:53
  • $\begingroup$ Yeah, that goes with what I read. But why do they not want to react, though? What's keeping them from reacting directly? Is it the high activation energy? $\endgroup$
    – chematwork
    Commented Aug 23, 2020 at 9:45
  • $\begingroup$ @chematwork You already answered that in your first sentence above! See also Ivan's comment. $\endgroup$
    – Karl
    Commented Aug 23, 2020 at 10:32

1 Answer 1


Your on the right track: in order to form a new compound, old bonds must be broken. This requires energy, even though the reaction may, overall , be exothermic. This is called the activation energy.

Think of it as pushing a car a short distance up one side of a hill before it reaches the peak and then rolls a long distance down the other side.

For example, in the dark, at room temperature, $\ce{Cl2}$ and $\ce{H2}$ coexist quietly in a flask... until light activates the chlorine (wait for the bang).

On the other hand, add enough energy, and even endothermic reactions take place, and, if the product is comparatively stable, or metastable, some can be left afterwards. For example $\ce{O2}$ is fairly stable, but given some energy input, e.g. from an electric discharge or from shortwave ultraviolet radiation, $\ce{O3}$, ozone, is produced, though that reaction is not thermodynamically favorable. The ozone is waiting to decompose... once it's activation energy is achieved.

  • $\begingroup$ So, my suspicions were right? $\ce{Cl2}$ and $\ce{O2}$ can react, but it requires very high temperatures normally unachievable. Drawing comparison to the reaction of $\ce{N2}$ and $\ce{O2}$ in the presence of lightning, $\ce{N2}$ has a much stronger triple bond. It just doesn't make sense how oxides of nitrogen $\ce{NO2}$ and $\ce{NO}$ are known to be formed from their elemental gases but not oxides of chloride. Even after radicalization, chlorine is said to react with ozone, not oxygen gas. $\endgroup$
    – chematwork
    Commented Aug 23, 2020 at 8:07
  • $\begingroup$ @chematwork Not exactly. Chlorine oxides are even more unstable at higher temperatures, just like e.g. mercury sulfide: you heat it, it decomposes into the elements (or other, more stable compounds). Chlorine oxides are usually produced electrochemically. $\endgroup$
    – Karl
    Commented Aug 23, 2020 at 8:46
  • $\begingroup$ @Karl So nitrogen oxides are more thermostable? Or is lightning more akin to an electrochemical mechanism? $\endgroup$
    – chematwork
    Commented Aug 23, 2020 at 9:48
  • $\begingroup$ @chematwork In a lightning, you have radical reactions, and the product cools extremely fast afterwards.Similar thing is done in industry: You burn ammonia (at a vanadium catalyst), and cool down the product NO as fast as possible. NO can then react with more oxygen, then add water and get HNO3. $\endgroup$
    – Karl
    Commented Aug 23, 2020 at 10:26
  • $\begingroup$ @Karl Hypothetically speaking, if chlorine and oxygen were subject to the same thing, then will oxides of chlorine form? $\endgroup$
    – chematwork
    Commented Aug 23, 2020 at 12:47

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