What happens when you dissolve phosphpric acid in water? (1 Viewer)

[girlworld_club]

Please explain in details, thanks

 

[hayabusaboston]

do u mean, addition of water to diphosphorus pentoxide to make phosphoric acid?
P2O5(g)+3H2O(l)-->2H3PO4(aq)

sum1 add more detail, I gotta take out trash.

 

[girlworld_club]

No. the actual reaction between phosphoric acid and water.

 

[HeroicPandas]

No. the actual reaction between phosphoric acid and water.

H3PO4 + H20 < -> H2PO4- + H30+ (first donation)

Remember KING BRONSTED-LOWRY

Word equation: Phosporiv acid + water <-> dihydrogen phosphate ion + hydronium ion

Phosphoric acid donates 1 PROTON (in the first donation)

 

[Aysce]

Alright, phosphoric acid is H3PO4. It is a triprotic acid, meaning it can donate 3 hydrogen ions successively with each reaction (we'll explore more into this as we go) and is a relatively weak acid since its degree of ionisation is not 100% ie. It does NOT disassociate completely when mixed with water. Since it is a weak acid and does not disassociate completely, we have an equilibrium reaction: most of it will disassociate into hydrogen/hydronium ions whilst there are a few intact molecules left within the aqueous solution.

So let's write out the chemical equation:

H3PO4 + H20 <==> H3O^+ + H2PO4^- As seen here, a proton is transferred from H3PO4 to H2O, hence it yields H30+ (the hydronium ion) and produces the dihydrogen phosphate ion.

The process is also an equilibrium reaction as look: We have a weak acid reacting with water. In this particular case, the water acts as the base (the proton acceptor) and is amphiprotic meaning it can act as a base or acid and it can both donate and accept protons. So when the weak acid (phosphoric acid) loses a hydrogen ion, it will form on the other side the weak conjugate base ie. H2PO4^- in this case. Although it is weak, it can potentially receive a hydrogen ion from H3O^+, hence giving us the reverse reaction. If we were to take a strong acid for example, HCl, it would form a VERY WEAK conjugate base hence it is unlikely that it will receive a hydrogen ion and hence no equilibrium reaction will occur (the amount of reactants created will be negligible). If you don't understand this, don't worry it is learned later in the course.

Moving on to the second successive reaction:

The dihydrogen phosphate ion reacts with water:

H2PO4^- + H2O <----> HPO4^2- + H3O^+

Again, you see that a proton (Hydrogen ion) is transferred to the H2O molecule to form the hydronium molecule and the resulting hydrogen phosphate ion.

Just to make sure: Bronsted-Lowry defines acids as proton donators and bases as proton acceptors

Third successive reaction involving proton transferring:

HPO4^2- + H2O <----> H3O^+ + PO4^3-

Same thing applies and we are left with again hydronium ions and now a phosphate ion.

So this is what happens when we react H3PO4 with H2O. Hope this helps!

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I am open to constructive criticism - if anyone finds anything wrong with what I have said, please tell

 

[hayabusaboston]

sorry im new to acidic environment, I only am up to defining acidic and basic oxides LOL

 

[bleakarcher]

Alright, phosphoric acid is H3PO4. It is a triprotic acid, meaning it can donate 3 hydrogen ions successively with each reaction (we'll explore more into this as we go) and is a relatively weak acid since its degree of ionisation is not 100% ie. It does NOT disassociate completely when mixed with water. Since it is a weak acid and does not disassociate completely, we have an equilibrium reaction: most of it will disassociate into hydrogen/hydronium ions whilst there are a few intact molecules left within the aqueous solution.

So let's write out the chemical equation:

H3PO4 + H20 <==> H3O^+ + H2PO4^- As seen here, a proton is transferred from H3PO4 to H2O, hence it yields H30+ (the hydronium ion) and produces the dihydrogen phosphate ion.

The process is also an equilibrium reaction as look: We have a weak acid reacting with water. In this particular case, the water acts as the base (the proton acceptor) and is amphiprotic meaning it can act as a base or acid and it can both donate and accept protons. So when the weak acid (phosphoric acid) loses a hydrogen ion, it will form on the other side the weak conjugate base ie. H2PO4^- in this case. Although it is weak, it can potentially receive a hydrogen ion from H3O^+, hence giving us the reverse reaction. If we were to take a strong acid for example, HCl, it would form a VERY WEAK conjugate base hence it is unlikely that it will receive a hydrogen ion and hence no equilibrium reaction will occur (the amount of reactants created will be negligible). If you don't understand this, don't worry it is learned later in the course.

Moving on to the second successive reaction:

The dihydrogen phosphate ion reacts with water:

H2PO4^- + H2O <----> HPO4^2- + H3O^+

Again, you see that a proton (Hydrogen ion) is transferred to the H2O molecule to form the hydronium molecule and the resulting hydrogen phosphate ion.

Just to make sure: Bronsted-Lowry defines acids as proton donators and bases as proton acceptors

Third successive reaction involving proton transferring:

HPO4^2- + H2O <----> H3O^+ + PO4^3-

Same thing applies and we are left with again hydronium ions and now a phosphate ion.

So this is what happens when we react H3PO4 with H2O. Hope this helps!

===========================

I am open to constructive criticism - if anyone finds anything wrong with what I have said, please tell

Hey Aysce, with the part I bolded don't you mean strong conjugate base? Don't weak acids, in general, have strong conjugate bases.

 

[Aysce]

Hey Aysce, with the part I bolded don't you mean strong conjugate base? Don't weak acids, in general, have strong conjugate bases.

No, I am certain that weak acids create relatively weak conjugate bases, not strong bases.

HCOOH + NH3 -----> HCOO^- + NH4^+

We know NH3 is a weak base and hence it yields a weak acid. But since it is a weak acid, it is very unlikely that it will donate a proton to HCOO^-. If HCOO^- was strong, it would be able to attain/attract a proton but since it is unable to do so, it is weak. (Sorry for the dodgy explanation, I just know for sure that weak acids --> Weak conjugate bases, I've checked).

Admittedly, I don't fully understand/know the technicalities behind this so yeah, would be nice to be confirmed.

 

[golgo13]

I'm quite sure that a weak acid dissociated creates a strong conjugate base. But to an extent it can be argued that they don't fully dissociate so it's sorta a blur tbh

 

[Aysce]

I'm quite sure that a weak acid dissociated creates a strong conjugate base. But to an extent it can be argued that they don't fully dissociate so it's sorta a blur tbh

I'm not entirely sure man, like the textbook says that it produces a weak (not too weak) conjugate base. This is what I have found though I think the intuition behind a weak acid producing a weak conjugate base is that the conjugate base is a poor proton acceptor.



http://en.wikipedia.org/wiki/Acid_strength#Conjugate_acid.2Fbase_pair

 

[golgo13]

I don't think in an exam scenario they'll throw something like the moderates it'll be more clear cut with the strong and weak, because the moderates can be argued either they produce a moderate/ strong or weak depending

 

[Aysce]

Yeah I agree with you on the exam part. But I still stick with weak acids producing moderately weak conjugate bases. They're not very weak nor are they strong to the extent of ionising completely but they're probably in the middle - higher end of the weak spectrum.

 

[ZombieApocalypse]

holy shit aysce



*bows down*

 

[Aysce]

holy shit aysce



*bows down*

You give me too much credit.

 

[someth1ng]

Yeah I agree with you on the exam part. But I still stick with weak acids producing moderately weak conjugate bases. They're not very weak nor are they strong to the extent of ionising completely but they're probably in the middle - higher end of the weak spectrum.

The weaker the acid, the stronger the conjugate base. Therefore, a weak acid does not necessarily produce moderately weak conjugate base - a weak acid is any acid that does not fully ionise - very weak ones such as CH3COOH produce reasonably strong conjugate bases but moderately weak acids such as NaHSO4 produce moderately strong conjugate bases while strong acids such as HCl produce very weak conjugate bases.