Weak acid - strong base titration, buffer region

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username2648333
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#1
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#1
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For the buffer region in this, why does it stop the pH from increasing?

At the start:

CH3COOH + NaOH > CH3COONa + H2O

Which means there will be a high amount of salt in the solution, but why is this stopping the pH from rising? I thought it would actually make it rise further as the disassociated salt it reacts with water to form OH-?

What's reacting with the OH- in solution, that isn't there anyway? Seeing as, with a buffer solution, the acid is needed (CH3COOH + OH- > CH3COO- + H2O).

The acid concentration hasn't been increased?

Or is the presence of the dissasocated salt:
CH3COONa(aq) > CH3COO- + Na+

Causing the acid to reform:
CH3COO- + H2O > CH3COOH + OH- <<< but there's more OH-!!

Surely the Na+ isn't going to react with the OH-, as it will dissociate in the water.
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charco
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#2
Report 4 years ago
#2
(Original post by s33ds)
Image

For the buffer region in this, why does it stop the pH from increasing?

At the start:

CH3COOH + NaOH > CH3COONa + H2O

Which means there will be a high amount of salt in the solution, but why is this stopping the pH from rising? I thought it would actually make it rise further as the disassociated salt it reacts with water to form OH-?

What's reacting with the OH- in solution, that isn't there anyway? Seeing as, with a buffer solution, the acid is needed (CH3COOH + OH- > CH3COO- + H2O).

The acid concentration hasn't been increased?

Or is the presence of the dissasocated salt:
CH3COONa(aq) > CH3COO- + Na+

Causing the acid to reform:
CH3COO- + H2O > CH3COOH + OH- <<< but there's more OH-!!

Surely the Na+ isn't going to react with the OH-, as it will dissociate in the water.
You need to go through this little interactive I prepared on buffers
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username2648333
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#3
Report Thread starter 4 years ago
#3
(Original post by charco)
You need to go through this little interactive I prepared on buffers
THANK YOU!

I think I get it now - if the OH- concentration is increased, then this will react with any H+ ions that are in the solution (from the CH3COOH), this will then push the acetic acid equilibria to form more H+?

So, nothing new is reacting with OH-, it's the H+ that were already in solution, which are just being replaced by the weak acid, as it isn't completely dissociated in solution - the OH- effectively causes the acids to act as acids (by forcing the equilibria), therefore more H+ are in solution, meaning no pH change.
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charco
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#4
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#4
(Original post by s33ds)
THANK YOU!

I think I get it now - if the OH- concentration is increased, then this will react with any H+ ions that are in the solution (from the CH3COOH), this will then push the acetic acid equilibria to form more H+?

So, nothing new is reacting with OH-, it's the H+ that were already in solution, which are just being replaced by the weak acid, as it isn't completely dissociated in solution - the OH- effectively causes the acids to act as acids (by forcing the equilibria), therefore more H+ are in solution, meaning no pH change.
:borat:
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