why is dihydrogen phosphate stronger base than water? (1 Viewer)

[tazzyboys]

cssa 2007 question
help!

 

[tommykins]

H2PO4- + H2O -> H3PO4 + OH-

negative charge, likley to accept proton, producing h3po4 and OH.

 

[tazzyboys]

thanks for quick reply!
"negative charge, likley to accept proton" - does that apply to other combinations?

does that mean hpo4 (2-) + h2o ---> h2po4- + OH-

po4 (3-) + h2o ---> hpo4- + OH-

thanks

 

[tommykins]

chemistry has full of exceptions, but yes that should occur.

 

[gcmk]

could someone pleasee put the cssa 2006/07 papers up on the resource section?

 

[Undermyskin]

Well, you should include all the 'inverse cityrail arrow' in all of your equations.

Um, so... how about in the case of acids? Water is not positively charged but it's still considered a stronger base in regards to the acids. If you say it's because of the H-bonds water form with the acids' hydrogen and thus rip it off, why doesn't it also 'rip off' H of the dihydrogen phosphate anion? Don't say that it's negative because the reaction is reversible and hence you get the ionization reaction of dihydrogen phosphate as well!

I wonder if they love the simple answer: stick the pH meter into the solution, it indicates whatever over 7!

 

[minijumbuk]

I wonder if they love the simple answer: stick the pH meter into the solution, it indicates whatever over 7!

This was in my half yearlies. They LOVE the simple answer.

Basically what you had to do for 5 marks was:

- Define what amphiprotic substance means
- Show that dihydrogen phosphate can act as an acid or base using equations
- State that dihydrogen phosphate is more likely to act as a base (we don't need to say why...Stupid, I know...)
- Indicate that the equation for it acting as a base is more likely to occur
- Show that there are more OH^- ions, and according to pH=-log₁₀[H[⁺] and [H⁺][OH^-] = 1x10^-14, and hence pH is over 7.

 

[Undermyskin]

I know. That's the stupid point. When I look at the sample answers, I always end up writing 3 times as needed! They say 'Describe' for 3 marks and they just need exactly 3* 8-word statements. I can't believe the whole point of me studying to give them a fully detailed answers were just to be smacked at by the 'outline' type answers.

I asked my teacher this afternoon and he wonders if we actually need to know. But he suggests finding info about K of the H2O to H3O+ equation as well as H2PO4- to H3PO4 equation and compare them.

 

[impervious182]

H2PO4- + H2O -> H3PO4 + OH-

negative charge, likley to accept proton, producing h3po4 and OH.

That's not the correct reason.

Afterall the hydrogen carbonate ion ( HCO3-) will favour the formation of an acid in water, yet it also has a negative charge. In other words, it will donate not accept a proton.

----------------------------------------------------------------------------------------------

Let me explain:

The reason that dihydrogen phosphate is more likely to accept a proton and therefore act as a base, can be explained by the Bronsted-Lowry theory.

That is that when phosphoric acid (H3PO4) is put into solution such as water, it is a strong acid, and will donate a proton.

H3PO4 + H20 <----> H2PO4- + H30+
strong acid weak base weak conjugate base strong conjugate acid

Phosphoric acid is a strong acid and will therefore form a weak conjugate base. So basically, that said, H2PO4- is more inclined to act as a base, than an acid , so in water it will act as a base, by accepting a proton to go back to phosphoric acid. Though in a base, as it is amphiprotic, it will donate a proton, thereby acting as an acid.

The reason, would relate to the ionisation energies, and electronegativity (or something beyond the spectrum of the course, I'm sure exactly what). I.e. is it easier to make a base, or an acid? The way we can answer this though, without looking up the K value in a table, is by using the Bronsted-Lowry theory.

This is why carbonic acid (H2CO3), as an acid which is not strong, will form an amphiprotic substance, namely hydrogen carbonate ion (HCO3-) which favours the formation of an acid in water, rather than many of the others, including the aforementioned dihydrogen phosphate, which favour the formation of a base. Again, this relates back to the original acid being weak and so forming a strong conjugate base.

Again though, if this hydrogren carbonate were placed in water, it would act as an acid, regardless of the fact that it has a negative charge. In a base it would act as an acid, and in an acid as a base (though this last sentence, is the definition of amphiprotic.)

 

[Undermyskin]

Can I remind you? H3PO4 is NOT a strong acid. A strong acid has an absolutely feeble conjugate base which is either neutral (CO3 2-) or slightly acidic (HSO4 -). AND, its ionisation reaction goes to completion. (in HSC context) though I know factor of concentration must be taken into account in more concentrated solution.

Then I think what you mean is to be flexible when answering a question to bend the amphiprotic property of H2PO4 - in either the acidic or basic direction depending on what knowledge the markers favour us to demonstrate?

 

[minijumbuk]

Guys, don't worry about this question. This was a lame question in the CSSA, and it is very unlikely to come out in the HSC. What I have said before was the marking outline. To explain it the proper way, you'd need extra-curricular knowledge (along with most other topics as well).

For now, just stick with what I typed before. That guaranteed the 5 marks you needed. Even worse still, if you explain it using the proper method, the marker might not accept it, and you'll waste both your time and your marks.

 

[impervious182]

Can I remind you? H3PO4 is NOT a strong acid...

Then I think what you mean is to be flexible when answering a question to bend the amphiprotic property of H2PO4 - in either the acidic or basic direction depending on what knowledge the markers favour us to demonstrate?

Yeah actually you're right.

H3PO4 is not strong, as I said. Though, I think we have to relate the answer back to Bronsted-Lowry. So we could say, dihydrogen phosphate is a stronger base than water, because it is made from a relatively weak acid, which means that, it is a relatively strong conjugate base. However water dissociates at 1 x 10^-14 ... lalala... I think.

 

[kooltrainer]

you think... you think!? i thought u were so certain, i'd took ur word for it.. zzz