Oxidizer Strength: Potassium Nitrate vs Sodium Nitrate

[samblohm]

What makes something like Potassium nitrate stronger than Sodium Nitrate as an oxidizer?

 

[Borek]

Are you sure that's indeed the case? That sodium nitrate is a weaker oxidizer?

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[samblohm]

I suppose I should have used Chlorate instead as an example because I'm very sure that it's a stronger oxidizer.

 

[Cesium]

This is an excellent question samblohm! I don't have an answer, but will add to the puzzle.

I've read things that seem to say that molten KOH is a stronger oxidizer than NaOH. Any idea on why this would be?

...the oxidation of MnO2 in a NaOH melt does not lead to the required Na2MnO4 (with hexavalent Mn) but only to Na3MnO4 with pentavalent Mn.

NaOH only takes Mn(IV) to Mn(V) in this case whereas KOH takes it to Mn(VI). Why?? This seems so strange to me.

I'd be very interesting in anybody who can reference some information pointing to the reason why it seems that potassium salts are sometimes better oxidizers than their sodium counterparts!

 

[Borek]

I've read things that seem to say that molten KOH is a stronger oxidizer than NaOH.

Be careful - neither is an oxidizer.

...the oxidation of MnO2 in a NaOH melt does not lead to the required Na2MnO4 (with hexavalent Mn) but only to Na3MnO4 with pentavalent Mn.

NaOH only takes Mn(IV) to Mn(V) in this case whereas KOH takes it to Mn(VI). Why?? This seems so strange to me.

From what I understand, it is not NaOH that 'takes' Mn(IV) to higher oxidation state, oxidation takes place in molten NaOH, but NaOH is not an oxidizer.

I'd be very interesting in anybody who can reference some information pointing to the reason why it seems that potassium salts are sometimes better oxidizers than their sodium counterparts!

The only thing I know for sure is that KNO₃ was used instead of NaNO₃ because of their physical properties - NaNO₃ is much more hygroscopic, which makes its use more difficut in practice (black powder with sodium nitrate tends to clump).

Also - can you explain what you mean by "better oxidizer"?

I guess different outcome of the reactions can be attributed to the cation size - while neither K⁺ nor Na⁺ are directly involved in the oxidation, they can stabilize some byproducts or intermediates, shifting equilibrium, hence different outcome. It could be also interesting to compare standard enthalpies of products (like Na₂CO₃ and K₂CO₃ for example) to see where the energy goes.

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[Cesium]

From what I understand, it is not NaOH that 'takes' Mn(IV) to higher oxidation state, oxidation takes place in molten NaOH, but NaOH is not an oxidizer.

Yeah good point. O2 from air is the oxidizer in this case. So I should have asked: Why is O2 a more potent oxidizer in molten KOH than NaOH? Or alternatively, as you hint at, there might be some difference in the stability of the Na2MnO4 and K2MnO4.

 

[Borek]

Or alternatively, as you hint at, there might be some difference in the stability of the Na2MnO4 and K2MnO4.

That's not exactly what I am hinting at. Even if both salts have similar stability, could be there are other possible (by)products which differ in stability. These can modify the finall effect.

Sodium cation is smaller and its charge is more "concentrated" - thus its attraction with negative ions can be stronger than in the case of potassium. This can stabilize some substances.

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[Cesium]

Sodium cation is smaller and its charge is more "concentrated" - thus its attraction with negative ions can be stronger than in the case of potassium. This can stabilize some substances.

Isn't this entirely dependent upon the anion in question? I would think, in general, when you are comparing cations of similar electronegativity, ion pairs of similar size would be stabler because this would allow for better crystal packing.

If we use melting point as a metric for stability (we could also use enthalpies but these values were easier to quickly look up), it's clear in the example of alkali chlorides that sodium chloride is the most stable because Na+ is similar to size to Cl-

LiCl:605C
NaCl:801C
KCl: 770C
RbCl: 718C
CsCl: 645C

In this case Li+ is smaller than Na+, but NaCl has the higher melting point.

 

[Borek]

Yes, it depends on the anion - but I have a feeling that you are thinking only about the final product. As I wrote before, to change the reaction outcome it is enough that some intermediate is stabilized better.

MnO₄ (regardless of the charge) seems to be bulky, so could be that your idea (similar sizes ions making stabler compounds) could work here - while both sodium and potassium cations are much smaller, potassium is a little bit closer in size. But that's a speculation which can get us banned

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