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As titled, must a strong reducing agent be a weak oxidising agent, and must a strong oxidising agent be a weak reducing agent? For instance, fluorine is a very strong oxidising agent, and it cannot act as a reducing agent. Is this true for all chemicals? If not, what would be the explanation to why it isn't the case?

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    $\begingroup$ This is an obvious and pretty good rule of thumb, but a compound isn't an agent, it can only be used as one, in one way or another. Also redox potentials are hardly standard ones in reality. That means some substances can be used as both reducing agent and oxidising agent, depending on circumstances. $\endgroup$
    – Mithoron
    May 25, 2023 at 14:25

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What does it mean for a substance to be strong oxidizer? Well, an oxidizer acquires electrons. For a substance to do a good job of ripping out electrons from most other materials, it needs to have unoccupied electronic states of very low energy in order to coax the electrons to move towards it. For a strong reductant, the whole picture is flipped around - the substance needs occupied electronic states of very high energy, such that the electrons will readily jump out to anything even slightly willing to accept them.

So what would a compound that is a strong oxidizer and a strong reductant look like, conceptually? Well, it would simultaneously need both a very low energy unoccupied state, and a very high energy occupied state. You can roughly imagine taking all of the allowed electronic states in a substance, ordering them by energy, and "filling in" the electronic states from the bottom (lowest energy) and going up until you've assigned all the electrons the substance has.

The problem is that for simple/small molecules in their most stable condition, you can't really do this "filling in" procedure and be left simultaneously with a hole near the bottom and an electron way up high. If this happens, the molecule is going to do whatever it can to make that high energy electron fall into the hole. It can do this either by a direct electron transfer (the molecule self-oxidizes and self-reduces at the same time), or more likely, the molecule will rearrange itself somehow such that it forms new electronic states where there is no deep hole and sky-high electron. So in general, it is true that a substance cannot be simultaneously be a strong oxidizer and a strong reductant; if it were, it would just react on its own and stop being at least one of them.

But there are ways around this.

If a molecule is not simple and small, it can be conceivably engineered and subdivided into regions which don't "communicate" electronically very well. In a sense, a strongly oxidizing end of the molecule may not "know" there is also a strongly reducing end. I don't really know of a realized example of this, but it would be a situation similar to intramolecular frustrated Lewis pairs, where molecules simultaneously contain strongly acidic and strongly basic segments which can't interact on their own, usually due to spatial constraints. An electrochemical version of this would likely be a more subtle matter, but is not physically impossible.

The real workaround though, is to realize that you don't always have to operate in the ground electronic state. For many perfectly ordinary substances, even ones with no significant oxidising or reducing power, it is possible to expose them to high energy photons (say, blue, violet or ultraviolet photons). A photon of the right energy can then excite an electron from a low energy state to a high energy one. Not only does the excited electron carry a lot more energy (potentially making it powerfully reducing), but it leaves a hole behind (which can be powerfully oxidizing) - the photon is forcefully breaking the "filling in" procedure. Now, it is possible for the excited molecule to be, in all respects, simultaneously a strong oxidizer and a strong reductant!

This is a delicate condition, which in most cases lasts around a nanosecond, but in certain situations can last substantially longer. Nevertheless, that is plenty of time to do chemical reactions, which generally happen within a few picoseconds. This is the field of photochemistry. Even here though, one typically chooses substances in order to explore only their increased oxidation or reduction power when exposed to light, not both simultaneously in a single substance.

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  • $\begingroup$ It is interesting how recently this understanding was achieved. I think looking at a chem book from, say, 1920 is a real eye-opener. One important contributor was Conant, later prez of harvard. $\endgroup$
    – releseabe
    May 27, 2023 at 4:04
  • $\begingroup$ If you have a long molecule with one reducing end and one oxidizing end, wouldn't those molecules just interact with each other, neutralizing itself? $\endgroup$ Jun 1, 2023 at 10:16
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    $\begingroup$ @ReverentLapwing In principle yes, even if individual molecules of a substance were unable to self-neutralize, it could also be possible for multiple molecules of the same substance to neutralize each other. However, in principle this could also be suppressed with proper design of the molecular structure. As a very rough analogy, imagine if the strongly reducing end were at the bottom of a deep triangular hole, and the strongly oxidizing end were a square-shaped surface. Try as they might, they simply would not be able to reach each other. $\endgroup$ Jun 1, 2023 at 13:56
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If the same material is both a strong oxidizing agent and a strong reducing agent, it will react with itself unless some kinetic barrier exists. Some of the molecules will, at random, act as oxidizers and oxidize other "reducing agent" molecules. This turns the former molecules into products with lower oxidation states and the latter molecules into other products with higher oxidation states -- a classical disproportionation.

As noted above, we may be able to stop this kinetically. With hydrogen peroxide solutions, we add stabilizers and keep them rigorously free of transition-metal bearing catalysis like $\ce{MnO2}$ or heavy metal salts to raise this kinetic barrier.

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The reciprocal strength of acids/bases or oxidants/reductants applies to respective conjugate acid/base or redox pairs.

If an entity is simultaneously acid and base like $\ce{H2PO4-}$, or oxidant and reductant, like $\ce{Cu^+(aq)}$ or $\ce{H2O2}$, the comparative strength of both is much less related. As the entity is not to itself the conjugate pair complement.

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