chapter 16 (midterm 2) Flashcards
what are buffers? is there a limit to them? how are they made?
Buffers are solutions that resist changes in pH when an acid or base is added
They act by neutralizing acid or base that is added to the buffered solution
there is a limit to what they can do, and eventually the pH changes
Many buffers are made by mixing a solution of a weak acid with a solution of soluble salt containing its conjugate base anion
blood has a mixture of H2CO3 and HCO3−
how do buffers work?
HA + H2O = A- + H3O+
Buffers work by applying Le Châtelier’s Principle to weak acid equilibrium
Buffer solutions contain significant amounts of the weak acid molecules, HA
These molecules react with added base to neutralize it
HA(aq) + OH−(aq) → A−(aq) + H2O(l)
you can also think of the H3O+ combining with the OH− to make H2O; the H3O+ is then replaced by the shifting equilibrium
what happens when an acid is added to the buffer?
The buffer solution also contains significant amounts of the conjugate base anion, A−
These ions combine with added acid to make more HA
H+(aq) + A−(aq) → HA(aq)
After the equilibrium shifts, the concentration of H3O+ is kept constant
what is the common ion affect?
Adding a salt containing the anion A-, which is the conjugate base of the acid (the common ion), shifts the position of equilibrium to the left
This causes the pH to be higher than the pH of the acid solution
lowering the H3O+ ion concentration
what is the henderson hasselbalch equation
Calculating the pH of a buffer solution can be simplified by using an equation derived from the Ka expression called the Henderson-Hasselbalch Equation
only used in the buffer region
The equation calculates the pH of a buffer from the pKa and initial concentrations of the weak acid and salt of the conjugate base
as long as the “x is small” approximation is valid
pH= pKa + log [A-]/[HA]
when do you use ICE vs the HH equation?
The Henderson-Hasselbalch equation is generally good enough when the “x is small” approximation is applicable
Generally, the “x is small” approximation will work when both of the following are true:
the initial concentrations of acid and salt are not very dilute
the Ka is fairly small
For most problems, this means that the initial acid and salt concentrations should be over 400x larger than the value of Ka
how much does the pH of a buffer change when an acid or base is added?
Though buffers do resist change in pH when acid or base is added to them, their pH does change
Calculating the new pH after adding acid or base requires breaking the problem into two parts
a stoichiometry calculation for the reaction of the added chemical with one of the ingredients of the buffer to reduce its initial concentration and increase the concentration of the other
added acid reacts with the A− to make more HA
added base reacts with the HA to make more A−
an equilibrium calculation of [H3O+] using the new initial values of [HA] and [A−]
what is the buffering capacity? range?
The buffering capacity is the amount of acid or base a buffer can neutralize
a concentrated buffer can neutralize more added acid or base than a dilute buffer
The buffering range is the pH range the buffer can be effective
what determines the effectiveness of a buffer depend on?
The effectiveness of a buffer depends on two factors (1) the relative amounts of acid and base, and (2) the absolute concentrations of acid and base
A buffer is most effective with equal concentrations of acid and base
A buffer is most effective when the concentrations of acid and base are largest
when is a buffer most effective?
A buffer will be most effective when the [base]:[acid] = 1
equal concentrations of acid and base
A buffer will be effective when 0.1 < [base]:[acid] < 10
A buffer will be most effective when the [acid] and the [base] are large
find buffering range. which acid should you choose for a buffer?
We have said that a buffer will be effective when
0.1 < [base]:[acid] < 10
Substituting into the Henderson-Hasselbalch equation we can calculate the maximum and minimum pH at which the buffer will be effective
Therefore, the effective pH range of a buffer is pKa ± 1
When choosing an acid to make a buffer, choose one whose is pKa closest to the pH of the buffer
3 cases of buffering capacity
Buffering capacity is the amount of acid or base that can be added to a buffer without causing a large change in pH
The buffering capacity increases with increasing absolute concentration of the buffer components
As the [base]:[acid] ratio approaches 1, the ability of the buffer to neutralize both added acid and base improves
Buffers that need to work mainly with added acid generally have [base] > [acid]
Buffers that need to work mainly with added base generally have [acid] > [base]
describe a titration
In an acid-base titration, a solution of unknown concentration (titrant) is slowly added to a solution of known concentration from a burette until the reaction is complete
when the reaction is complete we have reached the endpoint of the titration
An indicator may be added to determine the endpoint
an indicator is a chemical that changes color when the pH changes
When the moles of H3O+ = moles of OH−, the titration has reached its equivalence point
describe how pH changes throughout a titration curve
A plot of pH vs. amount of added titrant
The inflection point of the curve is the equivalence point of the titration
Prior to the equivalence point, the known solution in the flask is in excess, so the pH is closest to its pH
The pH of the equivalence point depends on the pH of the salt solution
Beyond the equivalence point, the unknown solution in the burette is in excess, so pH approaches the titrant’s pH
equivalence points for different salts
equivalence point of neutral salt, pH = 7
equivalence point of acidic salt, pH < 7
equivalence point of basic salt, pH > 7
