Buffers are a common term in the english language, but what do they mean in Chemistry?
Buffers are a common term in the english language, but what do they mean in Chemistry (0:39)? Episode 38 describes the chemical set up of buffers (1:31) and takes a closer look at how the buffer works on a particle level (2:03). To calculate the pH of a buffer the episode describes the Henderson-Hasselbalch equation (3:04). Using the theme of equal concentration of conjugate acid/base pair, two approaches to prepare a buffer solution are described (5:39) and the concept of buffer capacity is being discussed (6:36).
Question: How does a smaller concentration of conjugate base than weak acid affect the pH of a buffer?
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Hi and welcome to the APsolute Recap: Chemistry Edition. Today’s episode will recap buffers
Lets Zoom Out:
Unit 8 - Acids and Bases
Topic 8.8 - 8.10 - Buffers
Big idea - Structure and Properties
Introduction:
The word “buffer” is a somewhat common term in the English language. Depending on your use of it, it usually means that the buffer lessens the impact of something. This can be used in a variety of ways, like when it comes to money: If you have a buffer and suffer some monetary loss, the buffer helps to soften the blow and make sure that you are not completely broke. People can also be buffers, especially between two other people who don’t get along. The buffer person often absorbs the antagonistic behavior between two people and prevents greater damage in the relationship. But what does that have to do with chemical buffers? It actually means the same, a buffer solution resists changes in pH when a strong acid or base is added. So let’s recap buffers!
Let’s zoom in:
Buffer solutions are solutions in which you have large, but approximately equal amounts of weak acid and its conjugate base. For example, you could have hydrofluoric acid as a weak acid and its conjugate base F-. These solutions, as mentioned in the introduction, do not change the pH much when a small amount of strong acid or base is added. Why is that? Let’s take a closer look!
When adding a small amount of strong acid, H+, to the buffer solution the H+ will react with the conjugate base, F- to form the weak acid, HF. This reaction lessens the impact on pH, because, even though we are increasing the concentration of HF and decreasing the concentration of F-, it doesn't really change the concentration of H+ - or H3O+ ions - in the solution. If the conjugate base wasn’t there and wouldn’t be able to react with the added H+, then the pH would change much more, because then you would directly increase the concentration of H+ ions and therefore lower the pH. When adding a strong base to a buffer, the OH- ions will react with the weak acid and form water plus the conjugate base. Now the weak acid is sacrificing itself, neutralizing the added OH- and therefore not significantly increasing the pH.
The pH of a buffer itself is related to the pKa of the acid and the ratio of the concentrations of weak acid and its conjugate base - or, of course, weak base and its conjugate acid. The buffer solution has approximately the same concentration of the conjugate acid-base pair. We can calculate the pH of the buffer solution by taking into account the equilibrium expression of the weak acid or base, its pKa or pKb and deriving the Henderson-Hasselbalch Equation: pH = pKa + logarithm of the concentration of conjugate base over acid. When the concentration of conjugate base and conjugate acid are equal, the log term falls away and the pH equals the pKa. That should sound familiar: This is the half-equivalence point in titrations of weak acids/bases with strong bases/acids! Now, when adding a bit of strong acid or base to the buffer system, the amount of HF and F- remains approximately the same and therefore, the pH doesn’t change much. Good news: the Henderson-Hasselbalch equation is given on the equation sheet. You do not have to derive or memorize it, but you should be able to use it. For example, when calculating the pH of a buffer or when selecting a buffer with a specific pH, use the idea of pH= pKa! Good news on this end: you will NOT have to calculate the change in pH when adding an acid or base to the buffer on the AP exam!
Speaking of the half-equivalence point: That gives us some indication on how we can prepare buffer solutions: There are two - or actually four if you count acid and base - approaches. Have in mind, the goal is to have equal concentrations of the conjugate acid/base pair. We reach that when we are partially neutralizing a weak acid with a strong base - or vice versa. In the titration curve, we can identify this point and the connected buffer region as the area where the slope more or less remains constant - and the half-equivalence point is obviously a point within that buffer area.
We can also prepare a buffer by combining a weak acid/base with the salt of its conjugate base/acid. For example: We can combine acetic acid with sodium acetate or ammonia with ammonium chloride.
As you’ve heard throughout this episode, the key to buffers working and resisting the change in pH is having a significant concentration of the conjugate acid/base pair. If we increase this concentration, but keep the ratio the same, we increase the buffer capacity, which is the amount of acid/base you can add to the buffer before changing its pH significantly. Having more weak acid/base and its conjugate means you have more particles that can sacrifice themselves for the greater good. That also applies to the ratio between conjugate acid/base: If you have more conjugate acid, the buffer will resist the addition of OH- better and if you have more conjugate base, the buffer will resist the addition of a strong acid better.
To recap:
Buffer solutions are those that resist the change in pH when a small amount of strong acids/bases are added. They contain large, but approximately equal amounts of weak acid and its conjugate base. The weak acid neutralizes the effect of added OH- ions and the conjugate base neutralizes the effect of added H+ ions. The Henderson-Hasselbalch equation can be used to calculate the pH of a buffer solution. Buffers can be prepared in weak acid/base titrations as well as by combining a weak acid/base with the salt of its conjugate. Increasing the concentration of the conjugate acid/base pair increases the buffer capacity.
Coming up next on the APsolute RecAP Chemistry Edition: entropy.
Today’s Question of the day is about the pH of buffers
Question: How does the concentration of conjugate bases smaller than weak acid affect the pH of a buffer?