Write an equation to illustrate that a reaction is acidic.

[student34]

Homework Statement

Write equations to illustrate whether P₄O₁₀(s) is an Arrhenius acid or a base.

Homework Equations

P₄O₁₀(s) + H₂O(l) will eventually have the H₃O⁺ and a negative ion because it is a non-metallic oxide in water.

The Attempt at a Solution

I can think of at least 10 different ways to come up with a H₃O⁺ in the equation, but the answer has a specific answer which is,

P₄O₁₀(s) + 6H₂O → 4H₃PO₄(aq)

I don't know why they have 6 H₂O's.

I also don't know why they only use one of the 4 H₃PO₄(aq) for this equation,

H₃PO₄(aq) + H₂O(l) → H₃O⁺(aq) + H₂PO₄^-(aq) .

I can't find anywhere in the chapter that explains how I would know this.

 

[Bystander]

P4O10(s) + 6H2O → 4H3PO4(aq)
I don't know why they have 6 H2O's.

Try balancing the equation.

I also don't know why they only use one of the 4 H3PO4(aq) for this equation,
H3PO4(aq) + H2O(l) → H3O+(aq) + H2PO4-(aq) .

Did you wish to count individual molecules, or the stoichiometry?

 

[epenguin]

For the first question you might find it easier to write it as P₂O₅, write an obvious covalent structure for it with pentavalent P. See how H₂O can be added to that in two stages.

For your second question, the means to answer it may be in another chapter - on buffers, pH calculations etc. All three protons of H₃PO₄ can diissociate. But only the first one is strongly acid. Actually what they write seems to me slightly arbitrary. From the first K_a of around 0.01 M it follows (easily ) that if you dissolved the pentoxide to around 0.01 M [P] about half of it would be in form H₂PO₄^-; the greater the dilution the greater this fraction, while at higher concentrations the majority would be undissociated H₃PO₄ - e.g. at 1M about 90% is, and only 10% is H₂PO₄^-, and very little other ionised forms.

Rereading maybe that was not your question. The scheme is not meant to represent 'only one' ot the four molecules - it represents any of them equally. That reaction is extremely fast and goes back and forth something like 10⁸ times per second.

 

[student34]

Try balancing the equation.

I understand that the equation is balanced. I just have no idea why they decided to end up with 4H₃PO₄(aq) instead of anything else. Of all of the possible molecules, how would I know to try using that molecule?

Did you wish to count individual molecules, or the stoichiometry?

This is half way through grade 11 chemistry (old student who is upgrading). I think stoichiometry is in the next chapter. So I think individual molecules would be expected for this question.

 

[Borek]

I just have no idea why they decided to end up with 4H3PO4(aq)

First - P₄O₁₀ is an acid anhydride and is known to react with water producing phosphoric acid. My bet is you were told that earlier.

Second - phosphoric acid is an obvious example of an acid containing P, and being able of producing H₃O⁺.