Question about buffer solutions

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In summary, the CH3COOH+OH- <==> CH3COO- +H2O equation helps to maintain a relatively constant concentration of the acetate ion, while the CH3COOH+H2O <==> CH3COO-+H3O+ equation allows for a smaller change in pH.
  • #1
sgstudent
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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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  • #2
sgstudent said:
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.
 
  • #3
epenguin said:
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?
 
  • #4
sgstudent said:
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.
 
  • #5


Your understanding of buffer solutions is correct. The addition of CH3COOH and CH3COONa creates a buffer system that helps maintain a stable pH when small amounts of acid or base are added. The first equation you mentioned, CH3COO-+H3O+ <==> CH3COOH+H2O, is known as the acid-base reaction and it helps to neutralize any added acid, preventing a large change in pH. The second equation, CH3COOH+OH- <==> CH3COO- +H2O, is known as the hydrolysis reaction and it helps to maintain the concentration of the acetate ion. However, this reaction also produces hydroxide ions, which can shift the equilibrium to the left, increasing the amount of OH- ions. This is why it is important to carefully choose the concentrations of the acidic and basic components in a buffer solution to ensure that the desired pH is maintained.
 

FAQ: Question about buffer solutions

What is a buffer solution?

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added to it. It is composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Why are buffer solutions important in scientific experiments?

Buffer solutions are important in scientific experiments because they help maintain a constant pH, which is crucial for many chemical reactions to occur. They also prevent drastic changes in pH that can be harmful to living organisms.

How do you prepare a buffer solution?

To prepare a buffer solution, you need to mix a weak acid or base with its conjugate base or acid in a specific ratio. This ratio depends on the desired pH of the buffer solution and the pKa of the weak acid or base.

What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation is a mathematical equation used to calculate the pH of a buffer solution. It is pH = pKa + log([conjugate base]/[weak acid]), where pKa is the acid dissociation constant and [conjugate base] and [weak acid] are the concentrations of the conjugate base and weak acid, respectively.

Can a buffer solution have an infinite capacity to resist changes in pH?

No, a buffer solution has a limited capacity to resist changes in pH. It works best within a pH range of ±1 unit from its pKa value. Beyond this range, the buffer solution becomes less effective in maintaining a constant pH.

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