[Ben Cohen]

Hello,

For the reaction of solid sodium peroxide added to water, is it:

Na₂O₂ + H₂O  Na⁺ + HO₂^- + OH^-

or

Na₂O₂ + H₂O  Na⁺ + OH^-

I am preparing for the USNCO and the answer keys on the USNCO say the first reaction, but the first reaction includes the HO₂^- which I have never seen before.
USNCO says the first, while the internet says the second. Which is correct?

Thanks in advance.

[discodermolide]

If you are preparing for some sort of examination you need to start by balancing your reaction equations!
I would say that the second reaction is correct, the first one may just be a typographical error.

[Diamonds]

The USNCO test doesn't require equations to be balanced so the balancing isn't important for the question.

[discodermolide]

That is irrelevant. It is good scientific practice to balance your equations.

[Borek]

Start with balancing. And remember if the reaction can't be balanced, it can't happen.

HO₂^- is just what you get from H₂O₂ in a dissociation that is similar to the water ionization.

To be honest I don't like neither of these reactions, if anything I would prefer one with O₂ and NaOH being products. Not that it is excluded by the first one.

[Arkcon]

I don't know where on the internet the second one is correct.  Wikipedia says that alkali metal peroxides form hydrogen peroxide in water, you just have to memorize that fact.  The first equation seems to be a bit of a trick question, showing an ionization of hydrogen peroxide, to give an OH.  But that is the better answer than the other one.  I'm guessing the test itself offers 4 multiple choice, or I could be wrong about its layout.

[Ben Cohen]

Na₂O₂ + H₂O  Na⁺ + HO₂^- + OH^-

So basically what you are saying is that this reaction does occur, but the HO₂^- anion is really an ionized hydrogen peroxide?

So that the correct equation is really:

Na₂O₂ + 2H₂O  2NaOH + H₂O₂ ?

The test wants me to show all things as ionized, so for my answer I would be writing 2Na ⁺ + 2OH^-, but then I am still confused as to the hydrogen peroxide.

Is the hydrogen peroxide its own molecule, or does it really ionize and leave the HO₂^- anion?

[discodermolide]

Sodium peroxide, when dissolved in water, produces sodium hydroxide and oxygen.
Furthermore the equations need to be balanced. The first equation cannot have 1 Na⁺ and two other species caring a negative charge.

[Ben Cohen]

Wikipedia says Na₂O₂ + 2H₂O  2NaOH + H₂O₂

The other equation (the first one) would then be:
Na₂O₂ + H₂O  2Na⁺ + OH^- + O₂H^-

By now it is just a question of whether to write the hydrogen peroxide or the hydrogen peroxide anion; the production of O₂ is not correct.

[Borek]

Hydrogen peroxide decomposes to O₂, so in a way it is just an intermediary product - although quite stable.

But apparently oxygen is a main product of a superoxide hydrolysis, not peroxide.

Autoionization of H₂O is

H₂O  H⁺ + OH^-

hydrogen peroxide reacts in a similar manner:

H₂O₂  H⁺ + HO₂^-

[discodermolide]

I quote from this page http://www.chemicalbook.com/ChemicalProductProperty_EN_CB7853032.htm

"Sodium peroxide reacts violently with reducing agents, combustible materials and light metals. Reacts exothermically and rapidly or even explosively with water to form a strong base (NaOH) and oxygen (O₂) [Handling Chemicals Safely 1980 p. 854]."

Hydrogen peroxide decomposes rapidly in base to give oxygen and water.

[Ben Cohen]

So what do I write on the test then? Multiple sources say different things.

It's like the question I asked about a week ago, where my textbook gave both of the following reactions:

3NO₂ + H₂O  2HNO₃ + NO

and

2NO₂ + H₂O  HNO₃ + HNO₂

and so which do I write for the test if they say "Nitrogen dioxide is bubbled into water?" (Which I still have no answer for).

What to do when two sources give different products for a reaction?