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Course: Chemistry archive > Unit 8
Lesson 3: Buffer solutionsProperties of buffers
Buffer solutions contain high concentrations of both a weak acid and its conjugate base (or a weak base and its conjugate acid). Because these components can neutralize added H⁺ or OH⁻, buffers are highly resistant to changes in pH. Created by Jay.
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- what if we use strong acid in place of a weak acid(3 votes)
- The conjugate base of a strong acid would be an exceedingly weak base and so it would be severely limited in neutralizing additional acid. Essentially the addition of more acid would lower the pH even more so. The purpose of a buffer is to resist these drastic changes in pH so a strong acid buffer wouldn't really be achieving that.
All you'd have really is just an acidic solution capable of neutralizing additional base only.
Hope that helps.(10 votes)
- When buffers "resist" change, won't it change the concentration of reactants/products?
How does Le Chatelier's principle work into this?(3 votes)- You're correct. Let's say you add H+ into a buffer. This addition of H+ makes it so that it is more favorable for this reaction to make the weak acid form instead of the conjugate base, rather than vice versa:
HA --> H+ + A-
Le Chatelier's principle says that the reaction seeks to counteract any stresses that are put on it. If H+ is added to the container, then the system will try to counteract this by using H+ to convert the conjugate base into its acid form.(2 votes)
- Would any acid-base pair be able to act as a buffer, or do only conjugate acid-base pairs work?
If so, why?
Thanks(1 vote)- Well, all acid-base pairs (at least in the Brønsted–Lowry definition) are conjugates of each other, regardless if they are strong or weak.
As an example, if you have hydrochloric acid reacting with water:
HCl(aq) + H2O(l) → H3O^(-)(aq) + Cl^(-)(aq)
Hydrochloric acid is acting as an acid by donating a proton, and resulting in the chloride ion as the conjugate base. Equally valid would be the say that chloride is a base (since it can accept protons) and that hydrochloric acid is the conjugate acid. So, hydrochloric acid and chloride would be an acid-base conjugate pair.
To create a buffer solution, you would need significant amounts of a weak conjugate acid-base pair. Meaning that both the acid and base must be weak. Being a weak acid/base means that they only partially dissociate, so that when the solution reaches equilibrium most of the unreacted weak acid/base will remain. This means you have plenty of acid or base to react with any additional acid or base (different from the buffer’s conjugate pair) and significantly resist major pH changes to the solution.
Hope that helps.(4 votes)
- What is the exponential formula used when applying nitrate solutes to the titrations.(2 votes)
- Please, with all do respect. can you just give a summary of what all that means, based on the above video?(1 vote)
- So, what is a buffer essentially? A buffer is just a solution formed from mixing a significant amount of weak acid with its conjugate weak base. The result is a solution which resists large changes in pH and has a relatively constant pH.
Hope that helps.(2 votes)
- do buffer solution only consists of weak acid - conjugate base or it can also consist of weak base and it conjugate acid?(1 vote)
- You're correct. A buffer solution can exist for either of these combos. As long as you have a weak acid/base and its conjugate base/acid, a buffer is created due to the equilibrium between the acid and base. Both will buffer best when the pH of the solution is near the pka of the acid.
Most weak acids have weak bases as their conjugate bases, and likewise for weak bases. It's only certain really weak acids and bases that have their conjugate bases/acids be really strong and not so useful for making a buffer.(1 vote)
- So I am struggling with the Henderson-Hasselbalch equation, and it wants me to know how to find the logarithm. It gives an example which is:
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H2CO3 ←→ H+ + HCO3-
Ka = [H+][HCO3-]/[H2CO3] = 4.47*10^-7
This can be written as ...
4.47 = 10^.65 because o.65 is the logarithm of 4.47
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my question and what has stumped me the most on this is how do you find that the Ka mentioned equals 4.47*10^-7 is there some equation or PH chart to figure that out?
any help would be awesome thanks.(1 vote)- It's not entirely clear what you're asking. Can you rephrase your question?(1 vote)
- Doesn't the weak acid doesn't lower the pH?(1 vote)
- Can you reword your question? It's not clear exactly what you're asking.(2 votes)
- Why does the OH ion react with the weak acid but not the H+ ions that would be present in the water due to dissociation? For example, the dissociation of an Alka Seltzer tablet. Wouldn't adding the OH react with the H+?(1 vote)
- I'll assume you mean the H+ (or H3O+, or hydronium) which results from water autoionization.
If we add a base like hydroxide (OH-) to a buffer solution like this, it will react with any acids in the solution. So, both the weak acid and the hydronium from water will react with hydroxide. However, the concentration of hydronium from water is so small compared to that from the acid buffer so it plays no significant role in the pH. We only focus on the react of acid/base with the buffer since it predominantly decides the properties of the solution.
Hope that helps.(1 vote)
- How come H+A->HA goes to completion (charges not written)? isn't A- a weak base?1:41(1 vote)
- Well I wouldn’t describe that reaction as going to completion. That would mean the entire supply of conjugate base was converted to weak acid, but we still have leftover conjugate base.
I think what you’re trying to ask is why does the conjugate base neutralize all of the added acid if it’s only a weak acid. A buffer solution is an equilibrium between a weak acid and its conjugate base essentially. What’s happened is that a portion of the conjugate base has neutralized added acid to form new weak acid. These simultaneous changes make sure equilibrium is maintained.
Another way to think about is that the reaction initially exists in equilibrium, and then we add additional acid which disturbs that equilibrium. The reaction moves in a direction to counter that change and restore equilibrium according to Le Chatelier's principal.
Hope that helps.(1 vote)
Video transcript
- [Instructor] A buffer solution consists of a significant amount of a weak acid and its conjugate base. Let's say we have a generic weak acid, HA, and its conjugate base, A-. And we're gonna use some
particulate diagrams to try to understand how buffers work. So for our first particulate diagram, let's count out how many
particles we have of each. So first let's count how many HAs we have. So there's 1, 2, 3, 4, 5 HAs, and there's also, for A-,
there's 1, 2, 3, 4, 5 A minuses. And when looking at a
particulate diagram of a buffer, usually water molecules
are omitted for clarity. And also keep in mind that
this particulate diagram is just meant to represent a
small portion of the solution, so we can get an idea
about what's happening in the entire solution. Also notice that like water molecules, cations are also left out
of the particulate diagram. So we have five HA particles
and five A- particles in our aqueous solution. Having equal amounts of a weak
acid and its conjugate base is a good buffer solution. Let's see what happens
to the buffer solution if we add in a small amount of acid. So here I'm drawing in an H+ ion, and let's think about adding this H+ ion to our buffer solution. When the H+ ion is added to the solution, the base that is present
will react with the H+ ion to neutralize it. So the added H+ reacts with A- to form HA. So for the particulate diagrams, this added H+ is going to react with one of the A minuses
present in the buffer solution. So the H+ and the A- form an HA. So we're gonna go from
five HAs to six HAs. So let's look at this next
particulate diagram here. We can see there are now
six HAs in the solution, so let me write down six here. And since we started with five,
A minuses and we lost one, we should have only four
A minuses in solution now. So let's write down a four here. So you started off with
five HAs and five A minuses, and upon the addition of
a small amount of acid, the acid was neutralized by
the base that was present, and we formed six HAs and four A minuses. So a buffer solution
resists changes in pH. So the added H+ was neutralized by the presence of the base. If the buffer solution
had not been present, if we just had some water and we added some H+, the pH would have changed dramatically. One way to write the acid-base
neutralization reaction that occurred is to write
H+ plus A- goes to HA. However, since H3O+ and H+ are used interchangeably in chemistry, we could have also written
the net ionic equation as H3O+ plus A- goes to HA and H2O. Next, let's go back to our
middle particulate diagram with five HA and five A-. And this time let's try
to add some hydroxide ions to the solution. So think about, let me go ahead and draw an arrow here, so we're gonna add a small amount of base to our buffer solution. The hydroxide anion will
react with the weak acid that is present, HA, to form H2O and A-. So for the particulate diagrams, we can think about this OH-
reacting with one of the HAs to form H2O and an A-. Since we're using up one of the HAs, we're gonna go from five
HAs down to four HAs. So let me write down here, four HAs. And when this HA reacts,
it's gonna turn into an A-. So we're gonna go from
five A minuses up to six. So let's count them over here. 1, 2, 3, 4, 5, 6. So let's write in six A minuses. If there is no buffer present and we're just adding
hydroxide anions to water, the pH would change dramatically. However, with a buffer present, since there is a weak acid, HA, present to neutralize the added hydroxide anions, the buffer solution
resists a change to the pH. So let's summarize how
buffer solutions work. If we add a small amount of an acid, H+, to a buffer solution, the conjugate base that's present, A-, neutralizes the added acid. Therefore, the buffer solution
resists a change in pH. And if we add a small amount of a base, the weak acid that's present will neutralize the hydroxide anions. Therefore, the buffer solution
resists a change in pH. Let's say we have an aqueous
solution of acetic acid and an aqueous solution of sodium acetate. And let's say we have
equal moles of acetic acid and of sodium acetate. Mixing these two solutions together would form a buffer solution. To understand why this
forms a buffer solution, let's first think about acetic acid. Acetic acid is a weak acid and only partially ionizes
in aqueous solution. Therefore, in aqueous solution, we have mostly acetic acid, CH3COOH. Sodium acetate is a soluble salt and it dissociates completely
in aqueous solution. Therefore, in aqueous solution, we have sodium cations and
acetate anions, CH3COO-. The acetate anion is the
conjugate base to acetic acid. Therefore, in aqueous solution, we have a weak acid
and its conjugate base, and so we have a buffer solution. If we try adding a small amount of acid to the buffer solution, the conjugate base
that's present will react with the acid and neutralize it. So in the balanced net ionic equation, the added acid hydronium ion, H3O+, reacts with the acetate anion
to form acetic acid and water. So the added hydronium
ion was neutralized, and the buffer has resisted
a dramatic change in pH. If we try adding a small amount of base to the buffer solution, the weak acid that is present will neutralize the added base. So in the balanced net ionic equation, hydroxide anions react with acetic acid to form the acetate anion and water. So the added base was neutralized. So to summarize, the acetic acid-acetate buffer system resists dramatic changes in pH when small amounts of
acid or base are added. And I just noticed, I forgot to include the
L for liquid for water in the balanced net ionic equations.