Sciencemadness Discussion Board

Reduction potentials of polyatomic ions

Upsilon - 10-10-2013 at 13:25

I have found a plentitude of charts (conflicting, but existant at the very least) showing the reduction potentials of monatomic ions in single replacement reactions, but nowhere have I found any reference to the reduction potential of polyatomic ions. For example, is ammonium commonly replaced or rarely replaced? What cations may replace it? What anions may replace hydroxide? If said replacements occurs, will it create free ammonium/hydroxide?

Another question regarding potential of monatomic ions, I can't find a reliable, full chart of reduction potentials. Some say lithium can replace cesium, others vice-versa. Some say oxygen can replace chlorine, others do not. Does anyone have recent reference of high accuracy for reduction potentials?

[Edited on 10-10-2013 by Upsilon]

DraconicAcid - 10-10-2013 at 13:55

Whether or not lithium will be able to replace cesium depends on a number of factors. Most references will provide reduction potentials for standard aqueous solutions, and lithium is never going to replace cesium in an aqueous solution (either metal will react with the water). If you're reacting an anhydrous alkali metal compound with another alkali metal, the reaction (if any) will depend on the counterion present and the temperature. Aqueous reduction potentials will be irrelevant in this case.

Upsilon - 10-10-2013 at 14:20

I see. So, for single replacement reactions involving pure alkali metals, I assume it occurs in some inert liquid? Instead of using dozens of tables for different metals in different solutions, is there any way to calculate reduction potential based on quantitative properties of the metals and solvents?

Also, my question regarding polyatomic ions still stands.

[Edited on 10-10-2013 by Upsilon]

Upsilon - 19-10-2013 at 15:04

Anyone? I cannot find any information anywhere.

DraconicAcid - 21-10-2013 at 12:03

Quote: Originally posted by Upsilon  
I have found a plentitude of charts (conflicting, but existant at the very least) showing the reduction potentials of monatomic ions in single replacement reactions, but nowhere have I found any reference to the reduction potential of polyatomic ions. For example, is ammonium commonly replaced or rarely replaced? What cations may replace it? What anions may replace hydroxide? If said replacements occurs, will it create free ammonium/hydroxide?


You're not going to create free ammonium (assuming you mean NH4 radicals) or free hydroxyl radicals under normal conditions. If you react a metal hydroxide with a very reactive nonmetal (such as fluorine), you may very well get a replacement reaction to get the metal fluoride, but the other products would be oxygen and hydrofluoric acid.

If you were to react an ammonium compound with a very reactive metal in hopes of getting neutral NH4, you're much more likely to get NH3 and H2(g) being given off (e.g., 2 Na + 2 NH4Cl -> 2 NaCl + 2 NH3 + H2).

Upsilon - 21-10-2013 at 14:56

Is there like some kind of standardized chart that shows what is capable of replacing these ions in aqueous single replacement?

elementcollector1 - 21-10-2013 at 15:01

Quote: Originally posted by Upsilon  
Is there like some kind of standardized chart that shows what is capable of replacing these ions in aqueous single replacement?

I wouldn't look for reactivity so much as solubility: http://en.wikipedia.org/wiki/Solubility_chart
Ex. Mix AgNO3 (soluble) with NaCl (soluble) and get NaNO3 (soluble) and AgCl (insoluble). The reaction is driven to the right because of the low solubility of AgCl.
As Zan Divine has demonstrated in the 'Cesium from CsCl' thread, lithium can replace cesium - but only because cesium boils off, driving the reaction towards production of cesium. However, I would make a guess that cesium can replace lithium given an appropriate setup (LiCl + Cs -> Li + CsCl at lower temperatures than the BP of Cs).

DraconicAcid - 21-10-2013 at 15:24

Quote: Originally posted by elementcollector1  
Quote: Originally posted by Upsilon  
Is there like some kind of standardized chart that shows what is capable of replacing these ions in aqueous single replacement?

I wouldn't look for reactivity so much as solubility: http://en.wikipedia.org/wiki/Solubility_chart
Ex. Mix AgNO3 (soluble) with NaCl (soluble) and get NaNO3 (soluble) and AgCl (insoluble). The reaction is driven to the right because of the low solubility of AgCl.

But that's double replacement (metathesis), not single replacement.

chornedsnorkack - 23-10-2013 at 03:30

Quote: Originally posted by DraconicAcid  
Quote: Originally posted by Upsilon  
I have found a plentitude of charts (conflicting, but existant at the very least) showing the reduction potentials of monatomic ions in single replacement reactions, but nowhere have I found any reference to the reduction potential of polyatomic ions. For example, is ammonium commonly replaced or rarely replaced? What cations may replace it? What anions may replace hydroxide? If said replacements occurs, will it create free ammonium/hydroxide?


You're not going to create free ammonium (assuming you mean NH4 radicals) or free hydroxyl radicals under normal conditions. If you react a metal hydroxide with a very reactive nonmetal (such as fluorine), you may very well get a replacement reaction to get the metal fluoride, but the other products would be oxygen and hydrofluoric acid.


Free hydroxyl radicals are not longlived.

But this does not mean that producing these is not important or does not happen!

If you oxidize OH- by a strong oxidant, or anode, the lowest oxidation potential is for the reaction

4OH- -> O2+2H2O+4e

But this is a slow and difficult reaction, taking 4 electrons, 4 OH- ions et cetera.

You could get a much easier and faster reaction if you are able to spend enough energy and a higher oxidation potential for the reaction

4OH- -> H2O2+2e

H2O2 is fairly stable in dilute solutions.

Use even higher potential, and you might force through an even simpler reaction

OH- -> OH+e

Unlike H2O2, OH rapidly reacts with each other, but at least you get it, and spend the extra energy to do so. Whereas at a lower potential, production of OH would be impossible (not enough energy) so only direct production of H2O2 is possible (at a low rate).

There is a large number of short-lived oxidized oxygen species. The stable one, as stated, is triplet O2. But the rest... These include:
H2O2 and its anion HOO-
Atomic oxygen O
Ozone O3
Hyperoxide anion O2- and its acid HO2
Hydroxyl radical HO
Ozonide anion O3- and its acid HO3
Singlet dioxygen

With a sufficiently strong oxidant, all of the above should be competing reaction products.

Of course, most of them are shortlived - but they may be important reagents in side reactions. The long-lived products are H2O2 and O3. So how to predict the yield of O2, O3 and H2O2, and which of the other products will be forming?

Also, you may get products from your oxidant. Like fluorine - reaction of F2 with OH- is for some reason the standard route for OF2. Is OF2 stable in water? How liable is it for reaction
OF2+H2O -> O2+2HF?

[Edited on 23-10-2013 by chornedsnorkack]

Upsilon - 23-10-2013 at 19:05

So essentially, if the hydroxide ion is replaced by a stronger oxidant in single replacement, it will readily decompose into H2O and O2?