Weak Acid Strong Base Reactions: Buffer Solutions

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In summary: The blue line is the titration curve. The green line is the pH at the end of the titration. The red line is the endpoint of the titration (i.e. when all the NaOH has been added). The blue line is the titration curve. The green line is the pH at the end of the titration. The red line is the endpoint of the titration (i.e. when all the NaOH has been added).
  • #1
EdTheHead
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First off how exactly do weak acid strong base reactions work. Let's say I have 1 mole of a weak acid HA in a litre of water and let's say for every 4 undissociated molecules 1 molecules dissociate. So I have about 0.2 moles of H+ ions in this solution then I add some KOH let's say 0.5 moles. I know that there will now be 0.5 moles OH- ions that will snatch all the H+ ions. Once the initial 0.2 moles of H+ ions are converted into water and I'm left with no H+ ions and 0.3 moles of OH- ions what happens? Will the weak acid rapidly dissociate in an attempt to regain its usual equilibrium? In other words will the remaining 0.8 moles of HA rapidly become 6.4 moles of HA and 0.16 H+?

With all that in mind would the HA in this case be buffering the solution by neutralizing any base added but maintaining a specific pH based on its Ka? Would I be right to think that even in this solution the pH will be gradually rising as I'm adding the strong base because although the Ka of the weak acid remains constant the concentration is decreasing therefore there will be less overall H+ ions getting fed into the solution?
 
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  • #2
EdTheHead said:
Will the weak acid rapidly dissociate in an attempt to regain its usual equilibrium?

Something like that. You can assume neutralization is stoichiometric and instantaneous.

In other words will the remaining 0.8 moles of HA rapidly become 6.4 moles of HA and 0.16 H+?

No, 0.8 mole of HA would never become 6.4 moles of HA. Up to this moment there was some logic in your post, but something is terribly wrong here.

With all that in mind would the HA in this case be buffering the solution by neutralizing any base added but maintaining a specific pH based on its Ka? Would I be right to think that even in this solution the pH will be gradually rising as I'm adding the strong base because although the Ka of the weak acid remains constant the concentration is decreasing therefore there will be less overall H+ ions getting fed into the solution?

Ka is constant no matter what is present in the solution. As HA is getting easily neutralized, it consumes added base. pH slowly goes up, as described by the Henderson-Hasselbalch equation.

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  • #3
Borek said:
No, 0.8 mole of HA would never become 6.4 moles of HA. Up to this moment there was some logic in your post, but something is terribly wrong here.
Haha sorry I meant 0.64 moles. As in the 4:1 ratio would mean 0.8 moles of HA dissociates into 0.64 moles of HA and 0.16 moles of its conjugate base.

Borek said:
Ka is constant no matter what is present in the solution. As HA is getting easily neutralized, it consumes added base. pH slowly goes up, as described by the Henderson-Hasselbalch equation.

I find the henderson-hasselbach equation very hard to visualize. The log of the fraction part throws me off. If I was to graph a weak acid strong base titration and plot the pH on the y-axis and amount of base added on the x-axis would I see a slow rise in pH until the amount of base added equals the initial amount of weak acid added (in other words when base has completely neutralized the acid) then the pH would begin to rise rapidly?
 
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  • #4
HAc-NaOH-01-thymol-blue.png


(taken from titrations.info/acid-base-titration-end-point-detection)

This is acetic acid titrated with NaOH. You start with a sharp rise at the very beginning, followed by a flat part (described by Henderson-Hasselbalch equation). Once the buffering effect ends, pH skyrockets.

Yellow-blue band is about indicator color change (thymol blue).
 
  • #5


I can explain the process of weak acid strong base reactions and buffer solutions. In a weak acid strong base reaction, the weak acid (HA) will react with the strong base (KOH) to form water and a salt (K+ and A-). This reaction occurs in a stepwise manner, where the weak acid initially dissociates into H+ ions and its conjugate base (A-). As more strong base is added, the H+ ions will be neutralized by the OH- ions from the base, resulting in an increase in the concentration of the conjugate base (A-).

When the initial 0.2 moles of H+ ions are consumed, there will still be a small amount of undissociated weak acid (HA) and its conjugate base (A-) in the solution. This remaining weak acid will continue to dissociate, but at a slower rate, to maintain equilibrium. This is known as the buffering capacity of the solution. The concentration of the weak acid and its conjugate base will determine the pH of the solution, and this will remain relatively constant as long as the ratio of weak acid to conjugate base remains constant.

As you correctly stated, the pH of the solution will gradually rise as more strong base is added, as the concentration of H+ ions decreases. However, this rise in pH will be gradual and controlled by the buffering capacity of the solution. The Ka of the weak acid will remain constant, but the concentration of H+ ions will decrease, resulting in a gradual rise in pH.

In summary, the weak acid in this solution is acting as a buffer, maintaining a specific pH by neutralizing any base added. The pH will gradually rise as more strong base is added, but it will remain relatively constant due to the buffering capacity of the weak acid and its conjugate base.
 

1. What is a weak acid strong base reaction?

A weak acid strong base reaction is a chemical reaction that occurs between a weak acid and a strong base. In this type of reaction, the weak acid donates a proton (H+) to the strong base, resulting in the formation of a conjugate base and an H2O molecule.

2. 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 typically made up of a weak acid and its conjugate base, or a weak base and its conjugate acid.

3. How does a buffer solution work in a weak acid strong base reaction?

In a weak acid strong base reaction, a buffer solution works by maintaining a relatively constant pH despite the addition of a strong base. This is because the weak acid in the buffer solution can neutralize the added base, while its conjugate base can neutralize any excess H+ ions.

4. What is the importance of buffer solutions in biological systems?

Buffer solutions are crucial in biological systems because they help maintain a stable pH, which is necessary for proper functioning of enzymes and other biological molecules. Without buffer solutions, small changes in pH could have damaging effects on biological processes.

5. How do you calculate the pH of a buffer solution?

The pH of a buffer solution can be calculated using the Henderson-Hasselbalch equation: pH = pKa + log([conjugate base]/[weak acid]). The pKa is the dissociation constant of the weak acid, and [conjugate base] and [weak acid] refer to the concentrations of the respective components in the buffer solution.

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