Monday March 11, 2024 Day 40
Buffers: Qualitative Operation
Textbook Readings
17.1: The Danger of Antifreeze
17.2: Buffers: Solutions That Resist pH Change
14.6 Buffers
Course Lectures
14.1 pdf Video Qualitative Buffer Operation
1. Identify the two key components of a buffer.
2. Describe how the weak acid and conjugate base levels affect the pH of a buffering system
(i.e. more weak acid lowers the pH. More conjugate base increases pH)
3. Describe how the addition of a strong acid to a buffering solution
changes the concentrations of weak acid and its conjugate base.
4. Describe how the addition of a strong base to a buffering solution
changes the concentrations of weak acid and its conjugate base.
Homework Problems
Answer these questions. Click and drag over the space below for answers.
32.1 Buffers are solutions that contain significant concentrations of a weak acid and its
conjugate base. In the diagram below, vertical bar graphs are used to show the starting
concentrations of a buffer's components:
After the addition of a small amount of strong base the buffer maintains the H3O+ levels
and the corresponding pH near their original levels.
However, the buffer's weak acid and conjugate base components shift during the process:
List all changes that take place after the strong base has been added.
32.2 If a strong acid is added to the buffer in 32.1, again the H3O+ levels and the corresponding
pH's are near their original levels. However the buffer components have shifted.
List all changes that take place after the strong base has been added.
32.3 NaHCO3 and K2CO3 solutions can be combined to form a buffer solution.
However, HNO3 and KNO3 cannot be combined to form a buffer solution.
Why?
32.4. Carefully adding NaOH to a weak acid solution will create a chemical buffer.
Explain how this is possible.
32.5 Buffers have "capacity." Capacity is a measure of how much strong acid or base they
can neutralize before one of the buffer's components is used up.
Below are two buffers, both with equal concentrations of weak acid and conjugate base.
Which has the greatest "capacity?" Explain your answer.
32. 6 Buffering systems can be "exhausted" when too much strong acid or strong base has been
added and one of the buffer's components has been completely used up.
Consider the following three buffers and decide which is better prepared to protect against
pH changes if a strong acid is added.
Click and drag the region below for correct answers
33.1 The addional strong base is neutralized by the buffer's weak acid component.
Changes: weak acid levels decrease
weak acid is converted into conjugate base
conjugate base levels increase in a 1:1 manner
The truth regarding [H3O+] and pH:
The buffer does maintain [H3O+] and pH at relatively constant levels.
That's what a buffer is supposed to do!
However, the addition of a strong base to a buffer DOES produce a slight
increase in pH and slightly lower H3O+ levels. Our bar graphs do not
show this small change.
33.2 The addional strong acid is neutralized by the buffer's conjugate base component.
Changes: weak acid levels increase
conjugate base is converted into weak acid
weak acid levels increase in a 1:1 manner
The truth regarding [H3O+] and pH:
The buffer does maintain [H3O+] and pH at relatively constant levels.
That's what a buffer is supposed to do!
However, the addition of a strong acid to a buffer DOES produce a slight
decrease in pH and slightly higher H3O+ levels. Our bar graphs do not
show this small change.
33.3 Mixing NaHCO3 and K2CO3 solutions releases the weak acid, HCO3- and it's conjugate
base CO32- into the solution in significant amounts. That's all you need for a buffer. The
K+ and Na+ ions are spectators and in no way affect the pH of the solution.
However, HNO3 is a STRONG acid and aqueous KNO3 is a mixture of ions that
cannot affect pH. Morale: you must have a weak acid and it's conjugate base to create
a buffer.
33.4 Initially, a weak acid solution contains mostly the acid component in equilibrium with
small amounts of H3O+ and the conjugate base. Adding NaOH carefully to the weak acid
neutralizes the acid and CREATES the conjugate base as part of the process.
Once conjugate base levels are significant, your solution is a buffer....
....as long as you haven't completely neutralized the acid....you need that too!
33.5 "B" has the greater buffer "capacity" since there are greater levels (concentrations) of
weak acid and conjugate base. Simply put, there is more acid/base to cope with strong
acids or bases that may attempt to change the pH.
33.6 Additional strong acid will be neutralized by the buffer's conjugate base component.
Thus, the buffer that has the greatest amount of conjugate base is best positioned for
strong acid additions. This would be "A"
Note that all three buffers have the same "capacity". Buffer "B" is best positioned to
guard against strong acid OR strong base additions. It is actually at the 1/2 equivalence
or pKa point on it's titration curves since concentrations of weak acid and conjugate base
are equal.