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Question about buffer solutions

by sgstudent
Tags: buffer, solutions
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sgstudent
#1
Jul4-13, 08:25 AM
P: 645
CH3COOH and CH3COONa is an acidic buffer. It retains the pH when acid is added as CH3COO-+H3O+ <==> CH3COOH+H2O. This reduces the amount of H3O+ allowing a smaller change in pH. It also increases the concentration of the CH3COOH.

However, I was thinking about this other equation CH3COOH+OH- <==> CH3COO- +H2O. So my concentration of the acetate ion remains relatively constant due to the high concentrate from the CH3COONa. So after the first reaction happens, the concentration of the acetate acid increases. So wouldn't there be an equilibrium shift to the left increasing the amount of OH- ions?
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epenguin
#2
Jul4-13, 01:10 PM
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Quote Quote by sgstudent View Post
CH3COOH and CH3COONa is an acidic buffer. It retains the pH when acid is added as CH3COO-+H3O+ <==> CH3COOH+H2O. This reduces the amount of H3O+ allowing a smaller change in pH. It also increases the concentration of the CH3COOH.

However, I was thinking about this other equation CH3COOH+OH- <==> CH3COO- +H2O. So my concentration of the acetate ion remains relatively constant due to the high concentrate from the CH3COONa. So after the first reaction happens, the concentration of the acetate acid increases. So wouldn't there be an equilibrium shift to the left increasing the amount of OH- ions?
It is quite difficult at first to think about these things in an intutive as well as or at the same time as s calculational way. I cannot really make out what your premises and question is for the colored part, so can only make sone general comments I hope will help:

In general it is best not to think of reactions happening like one thing then another in time , but of a number of things existing in solution in equilibrium, any reaction happening one way happening at the same rate the other way so no concentrations changing.

You could say as you do that the acetic acid donates a proton to OH- the same as to H2O but it really wouldn't change anything if this process were forbidden, because H3O+ also does that, and there is a constant equilibrium such that [H3O+][OH-] = Kw all the time.

Often an important part of calculation is knowing what it is convenient and safe to ignore. So in the overall equilibria in acetate buffer, water is about 55M - but this never changes and you hardly think about it, [H3O+] which can be a concentration of interest could be typically2X10-5 M while [OH-] is 5X10-10 M and you can forget about [OH-].

I hope clarity will emerge from doing your excercise calculations.
sgstudent
#3
Jul5-13, 02:02 AM
P: 645
Quote Quote by epenguin View Post
It is quite difficult at first to think about these things in an intutive as well as or at the same time as s calculational way. I cannot really make out what your premises and question is for the colored part, so can only make sone general comments I hope will help:

In general it is best not to think of reactions happening like one thing then another in time , but of a number of things existing in solution in equilibrium, any reaction happening one way happening at the same rate the other way so no concentrations changing.

You could say as you do that the acetic acid donates a proton to OH- the same as to H2O but it really wouldn't change anything if this process were forbidden, because H3O+ also does that, and there is a constant equilibrium such that [H3O+][OH-] = Kw all the time.

Often an important part of calculation is knowing what it is convenient and safe to ignore. So in the overall equilibria in acetate buffer, water is about 55M - but this never changes and you hardly think about it, [H3O+] which can be a concentration of interest could be typically2X10-5 M while [OH-] is 5X10-10 M and you can forget about [OH-].

I hope clarity will emerge from doing your excercise calculations.
Ohh I think I get what you mean. I should just focus on the CH3COO-+H3O+ <==> CH3COOH+H2O reaction instead of trying to combine so many reactions together as its impossible for a human brain (mine haha) to comprehend so many reactions at 1 time frame?

Also, regarding this question why is it that if too much acid is added, the buffer wouldn't work anymore? I would think that its because the concentration of the CH3COO-is so small that it cannot effectively decrease the added amount of H3O+?

But it still tries to reduce the concentration even though its conc is very small as the equilibrium constant holds at a given temperature (and in this case its assumed to be constant). So the concentration of the CH3COO- can never really reach 0. But it can approach it?

epenguin
#4
Jul5-13, 04:27 AM
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Question about buffer solutions

Quote Quote by sgstudent View Post
Ohh I think I get what you mean. I should just focus on the CH3COO-+H3O+ <==> CH3COOH+H2O reaction instead of trying to combine so many reactions together as its impossible for a human brain (mine haha) to comprehend so many reactions at 1 time frame?

Also, regarding this question why is it that if too much acid is added, the buffer wouldn't work anymore? I would think that its because the concentration of the CH3COO-is so small that it cannot effectively decrease the added amount of H3O+?

But it still tries to reduce the concentration even though its conc is very small as the equilibrium constant holds at a given temperature (and in this case its assumed to be constant). So the concentration of the CH3COO- can never really reach 0. But it can approach it?
That's right.


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