anoymous1111
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A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H+ (aq) ions from water.

A weak acid will, however, only partially dissociate into ions, leaving a high percentage of unreacted molecules in the solution. An equilibrium is established, and so when some of the H3O+ ions produced by a weak acid react, Le Chatelier's principle means that more of the acid will dissociate to form more H+ ions. This means that, given an equal number of moles of acid, they will be neutralized by the same amount of strong base, but their solutions will have different pH values. '

Can anyone explain why the solutions would have different pH values? Surely if in both cases the acids react fully, then they would be equally neutralised by a base?


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RMNDK
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(Original post by anoymous1111)
'
A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H+ (aq) ions from water.

A weak acid will, however, only partially dissociate into ions, leaving a high percentage of unreacted molecules in the solution. An equilibrium is established, and so when some of the H3O+ ions produced by a weak acid react, Le Chatelier's principle means that more of the acid will dissociate to form more H+ ions. This means that, given an equal number of moles of acid, they will be neutralized by the same amount of strong base, but their solutions will have different pH values. '

Can anyone explain why the solutions would have different pH values? Surely if in both cases the acids react fully, then they would be equally neutralised by a base?


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Ah. Neutralisation does NOT mean the same pH. Water at different temperatures will have different pH values, but it's neutralised because there is an equal concentration of H+ ions and OH- ions.

The pH of the solution when an acid and base come together in a neutralisation reaction will be determined by the salt that is formed.
There's like a general rule of thumb for salts.

The salt of a strong acid and strong base produces a solution of pH 7.
The salt of a weak acid and a strong base produces a basic solution.

The reason is that the salt of a weak acid strongly dissociates, such as sodium methanoate. That gives rise to hydroxide ions and also reforms your original acid. That's why the pH is different.
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anoymous1111
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(Original post by RMNDK)
Ah. Neutralisation does NOT mean the same pH. Water at different temperatures will have different pH values, but it's neutralised because there is an equal concentration of H+ ions and OH- ions.

The pH of the solution when an acid and base come together in a neutralisation reaction will be determined by the salt that is formed.
There's like a general rule of thumb for salts.

The salt of a strong acid and strong base produces a solution of pH 7.
The salt of a weak acid and a strong base produces a basic solution.

The reason is that the salt of a weak acid strongly dissociates, such as sodium methanoate. That gives rise to hydroxide ions and also reforms your original acid. That's why the pH is different.
Oh ok thank you! I just don't understand why a weak acid would produce a salt that strongly dissociates? Surely if the H+ ion is strongly attracted to the HCOO- (hence weak acid) then the Na+ would also be strongly attracted to the HCOO-?


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anoymous1111
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(Original post by RMNDK)
Ah. Neutralisation does NOT mean the same pH. Water at different temperatures will have different pH values, but it's neutralised because there is an equal concentration of H+ ions and OH- ions.

The pH of the solution when an acid and base come together in a neutralisation reaction will be determined by the salt that is formed.
There's like a general rule of thumb for salts.

The salt of a strong acid and strong base produces a solution of pH 7.
The salt of a weak acid and a strong base produces a basic solution.

The reason is that the salt of a weak acid strongly dissociates, such as sodium methanoate. That gives rise to hydroxide ions and also reforms your original acid. That's why the pH is different.
Also, how is this all related to le chanteliers principle etc? What I quoted in the question bases the idea that a weak acid will form a stronger salt on this principle?


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Maker
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LC talks about equilibrium.

At equilibrium, a strong acid dissociates more than a weak one. When you add a strong base to a weak acid solution, the acid reacts with the base and removes the protons from the solution. To restore the equilibrium, more acid will dissociate to protons which will then react with the base.

If there is excess base, the equilibrium will continue to be restored by dissociating more acid until all the acid has reacted with the base.
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anoymous1111
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Thanks everyone!


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RMNDK
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(Original post by anoymous1111)
Oh ok thank you! I just don't understand why a weak acid would produce a salt that strongly dissociates? Surely if the H+ ion is strongly attracted to the HCOO- (hence weak acid) then the Na+ would also be strongly attracted to the HCOO-?


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I think the idea is that the salt is in solution, and the H+ ion from the water itself will protonate the salt. It would rather have the H+ than the Na+ because it's more stable and the fact that protons have a greater charge density.

As for your other query. It relates to le Chatelier's principle because as more of the salt is produced, more of the dissociation occurs which forces the equilbrium of the weak acid in the direction with less dissociation (if that makes sense...).
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anoymous1111
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(Original post by RMNDK)
I think the idea is that the salt is in solution, and the H+ ion from the water itself will protonate the salt. It would rather have the H+ than the Na+ because it's more stable and the fact that protons have a greater charge density.

As for your other query. It relates to le Chatelier's principle because as more of the salt is produced, more of the dissociation occurs which forces the equilbrium of the weak acid in the direction with less dissociation (if that makes sense...).
Thank you. What exactly do you mean the direction with less dissociation? And why as more of the salt is produced would more dissociation occur?


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RMNDK
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(Original post by anoymous1111)
Thank you. What exactly do you mean the direction with less dissociation? And why as more of the salt is produced would more dissociation occur?


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For instance, consider Sodium Methanoate. You make this when methanoic acid is neturalised with Sodium Hydroxide. As more of the salt is made, more dissociation occurs because the equilibrium shifts to remove the salt.

HCOONa <---> HCOO- + Na+
(The equilibrium is highly to the right)

Now, for methanoic acid itself:
HCOOH <---> HCOO- + H+
(The equilibrium is highly to the left)

As more of the salt is made, more of the methanoate ions are produced. And as more of this is produced, more of the methanoic acid is produced, i.e. the equilibirium shifts in the direction with less dissociation.

If you've learnt about buffers then this might make more sense.
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theRaichu
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Conjugate Acid/Bases

A weak acid can disassociate, but the result will be a strong base and a proton.

E.g.

HCOOH (weak acid) <-> HCOO- (strong base) + H+ (proton)

So, suppose you add another base to neutralise the weak acid. By LC's principle, the weak acid will continue to disassociate as the base is removing H+ ions. What is left is the strong base, which will have a pH > 7. That is why the pH's are different.
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anoymous1111
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(Original post by theRaichu)
Conjugate Acid/Bases

A weak acid can disassociate, but the result will be a strong base and a proton.

E.g.

HCOOH (weak acid) <-> HCOO- (strong base) + H+ (proton)

So, suppose you add another base to neutralise the weak acid. By LC's principle, the weak acid will continue to disassociate as the base is removing H+ ions. What is left is the strong base, which will have a pH > 7. That is why the pH's are different.
Why don't the HCOO- and H+ just join together again for neutralisation? Why would the dissociated acid favour a different base for neutralisation?


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theRaichu
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(Original post by anoymous1111)
Why don't the HCOO- and H+ just join together again for neutralisation? Why would the dissociated acid favour a different base for neutralisation?


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The HCOO- and H+ could rejoin. However, the H+ is being used up by the base. In reality, the solution of HCOOH is dynamic, and is constantly be disociated and recombining. In the time that they are dissociated, the base will take away the H+, and thus HCOOH would need to continuously dissociate.

Of course, your question was why the acid would favour a different base, and my guess is relating to activation energies - recombining H+ and HCOO- would require more energy than the H+ reacting with the base. This is just my suggestion however.
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anoymous1111
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(Original post by anoymous1111)
Why don't the HCOO- and H+ just join together again for neutralisation? Why would the dissociated acid favour a different base for neutralisation?


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Oh ok thank you so much!


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anoymous1111
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(Original post by theRaichu)
The HCOO- and H+ could rejoin. However, the H+ is being used up by the base. In reality, the solution of HCOOH is dynamic, and is constantly be disociated and recombining. In the time that they are dissociated, the base will take away the H+, and thus HCOOH would need to continuously dissociate.

Of course, your question was why the acid would favour a different base, and my guess is relating to activation energies - recombining H+ and HCOO- would require more energy than the H+ reacting with the base. This is just my suggestion however.
In a buffer solution why would the salt of a weak acid completely dissociate?


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Maker
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(Original post by anoymous1111)
In a buffer solution why would the salt of a weak acid completely dissociate?


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It will dissociate as long as its below its solubility constant.
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anoymous1111
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(Original post by Maker)
It will dissociate as long as its below its solubility constant.
I don't know what that means


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theRaichu
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(Original post by anoymous1111)
In a buffer solution why would the salt of a weak acid completely dissociate?


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It won´t - weak acid's only approximately 5% disosicate.
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(Original post by anoymous1111)
I don't know what that means


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When the solution becomes saturated with the salt, it will no longer dissolve and disassociate although that is also a dynamic equalibrium as demostrated by a radioactive lead ethanoate experiment.
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(Original post by theRaichu)
It won´t - weak acid's only approximately 5% disosicate.
He/she is referring to the salt of a weak acid, not the acid itself. Weak acids vary as to the degree of dissociation.
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