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Why is HOOH such a good oxidizer?

I understand that the oxygens in HOOH are in a negative 1 oxidation state, and that oxygen generally adopts an oxidation state of negative 2.

I also know that oxidizers themselves are reduced, and that reduction entails a drop in oxidation state. This suggests to me that oxygen is more stable when found with a negative two oxidation state rather than a negative one oxidation state. This makes sense; oxygen is the second-most electronegative element on the periodic table, so it likely would be okay with a more negative oxidation state.

And from organic chemistry I understand that the O-O linkage in peroxides is relatively weak; light can rather easily homolytically cleave the O-O linkage. And HOOH is generally stored in dark bottles as to block light. This says something about thermodynamic stabilities to me - that HOOH has relatively weak bonds and that it would be enthalpically favorable for the oxygens to form other, stronger, bonds. Is this how HOOH is used in bombs? The O-O bond is weak; bond formation is exothermic, thus providing a way for the O-O bond to break and rearrange into stronger bonds releases vasts amount of energy?

I'm trying to tie together redox with thermodynamic stabilities.

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From your argument I assume the real question you want to ask is "Why HOOH is such a good oxidizer compared to molecular oxygen?".

From thermodynamical point of view, molecular oxygen is a very potent oxidizer. However when you are talking about reactivity, you should always take a look at kinetics, not only at thermodynamics.

One important part of the answer to your question is that not HOOH a good oxidizer, but O2 is a very bad oxidizer from kinetic point of view. Molecular oxygen is in the triplet state, therefore its reactions are often spin-forbidden. Especially the oxygen->superoxide->peroxide steps require often level crossings between different spin surfaces. One very important reason why transition metal complexes catalyse oxidation in biology is this: as they often have electrons to flip and large spin-orbit coupling, the level crossings became easy. There is literally tons of literature on this in bioinorganic chemistry.

Note that in flames and in high temperature burning that reaction is essentially a huge variety of radical reactions, so the mechanism is completely different from the oxidation reactions in aqueous solutions.

Molecular oxygen has also 4 electrons to take when turned to oxides. This sometimes require complicated mechanism where every single step is thermodynamically and kinetically is feasible, and there is always a oxidizable component around.

Also note, that HOOH itself need catalyst to be an effective catalyst, like in Fenton reactions and similar.

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  • $\begingroup$ What do you mean by "spin-forbidden"? Is spin the QM equivalent of momentum in classical physics? Both are conserved quantities, correct? $\endgroup$ – Dissenter Aug 12 '14 at 4:17

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