Chapter 17: Aqueous Ionic Equilibrium Flashcards
What are the two types of buffers?
- A buffer with significant amounts of both a weak acid and its conjugate base.
- A buffer with significant amounts of both a weak base and its conjugate acid.
When is [H3O+] equal to Ka?
In a buffer solution in which the acid and conjugate base concentrations are equal.
What is the Henderson-Hasselbalch equation used for?
It is used to calculate the ph of a buffer solution from the initial concentrations of the buffer components as long as the x is small approximation is valid.
In general, when is the x is small approximation valid?
- The initial concentrations of acids (and/or bases) are not too dilute
- The equilibrium constant is fairly small.
When calculating the pH change in a buffer solution, why do you have to break the problem into two parts?
When we add acid or base to a buffer, the buffer resists a pH change. Nonetheless, the pH does change by a small amount.
When calculating the pH change in a buffer solution, what is the first part of the problem?
The stoichiometry calculation in which we calculate how the addition changes the relative amounts of acid and conjugate base.
When calculating the pH change in a buffer solution, what is the second part of the problem?
The equilibrium calculation in which we calculate the pH based on the new amounts of acid and conjugate base.
When is the pH of a buffer equal to pKa?
When the concentrations of the weak acid and the conjugate base are equal.
When calculating the pH change in a buffer solution, what is the basic formula of the stoichiometry calculation when adding acid?
H+ + A- –> HA
Added acid + weak base in buffer –> HA
(All are aqueous)
When calculating the pH change in a buffer solution, what is the basic formula of the equilibrium calculation?
HA + H2O <–> H3O+ + A-
When calculating the pH change in a buffer solution, what is the basic formula of the stoichiometry calculation when adding base?
OH- + HA –> H2O + A-
Added base + weak acid in buffer –> water and A-
(repeat question) Give the two parts of calculating the pH change in a buffer after small amounts of strong acid or strong base are added.
- Use the stoichiometry of the neutralization equation to calculate the changes in the amounts (in moles) of the buffer components upon addition of the acid or base.
- Use the new amounts of buffer components to work an equilibrium problem to find pH. (For most buffers, this can be done with the HH equation)
When is a buffer most resistant to pH changes (most effective)?
When the concentrations of acid and conjugate base are equal. A buffer becomes less effective as the difference in the relative amounts of acid and conjugate base increases.
What [base]/[acid] ratio must a buffer have to be considered effective?
The ratio must be in the range of 0.10 to 10. In order for a buffer to be reasonably effective, the relative concentrations of acid and conjugate base should not differ by more than a factor of 10.
How do the concentrations of acid and conjugate base in a buffer affect its effectiveness?
A buffer is most resistant to pH changes when the concentrations of acid and conjugate base are high. The more dilute the buffer components, the less effective the buffer.
How can the effective range for a buffering system be expressed in relation to pKa?
The effective range for a buffering system is one pH unit on either side of pKa.
What is buffer capacity?
The amount of acid or base that we can add to a buffer without causing a large change in pH.
How is buffer capacity related to the absolute buffer concentrations?
Buffer capacity increases with increasing absolute concentrations of the buffer components. The more concentrated the weak acid and conjugate base that compose the buffer, the higher the buffer capacity.
How is buffer capacity related to the relative buffer concentrations?
Overall buffer capacity increases as the relative concentrations of the buffer components become more similar to each other. As the ratio of the buffer components gets closer to 1, the overall capacity of the buffer (the ability to neutralize added acid and added bae) becomes greater.
What is the equivalence point of a titration?
The point in the titration when the number of moles of base is stoichiometrically equal to the number of moles of acid.
During a titration, when is the pH changing most rapidly?
The pH changes very quickly near the equivalence point (small amounts of added base cause large changes in pH).
What is the pH at the equivalence point of a strong acid-strong base titration?
It is always 7.00 at 25˚ C. At the equivalence point, the strong base has completely neutralized the strong acid. The only source of hydronium ions is the ionization of water.
When titrating a strong acid with a strong base, what is the initial pH?
The initial pH is the pH of the strong acid solution to be titrated.
