Biochem CH2: Acids & Bases/Titration Curves/Buffers
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52 Terms
What is an acid?
A proton (H+) donor
What is a base?
A proton (H+) acceptor
What is a conjugate base?
The species remaining after an acid donates H+
In HA ⇌ H⁺ + A⁻, what is HA?
The acid
In HA ⇌ H⁺ + A⁻, what is A⁻?
The conjugate base
What is the relationship between an acid and its conjugate base?
They differ by one proton
What is Ka
The acid dissociation constant; it describes the extent to which an acid dissociates
What is the Ka expression for HA ⇌ H⁺ + A⁻?
Ka= [HA][H+] / [A−]
What does a larger Ka indicate?
A stronger acid
What does a small Ka indicate?
A weaker acid
What is pKa
pKa = -logKa
What does a small pKa indicate?
A stronger acid
What does a large pKa indicate?
A weaker acid
How do Ka and pKa relate to acid strength?
Larger Ka = stronger acid, smaller pKa = stronger acid
What is the equation for pH?
pH = -log[H+]
What is the pH of pure water at 25°C?
7
What pH is acidic?
Below 7
What pH is neutral?
7
What pH is basic?
Above 7
What is the Henderson-Hasselbach Equation?
pH=pKa + log[HA] / [A−]
In HH Equation what is A-?
The conjugate base
In HH equation, what is HA?
The weak acid
What does the HH equation relate?
pH, pKa, and the ratio of conjugate base to weak acid
What happens when [A-] = [HA]?
pH = pKa
if pH < pKa, which form predominates?
The protonated weak-acid form, HA
If pH > pKa, which form predominates?
The deprotonated conjugate-base form, A-
Easy way to remember pH vs pKa?
Below pKa, the proton stays ON, above pKa, the proton comes OFF
If pH = pka + 1, what is the ratio [A-]:[HA]?
10:1
if pH = pKa -1, what is the ration [A-]:[HA]?
1:10
What is titration?
Gradually adding a known acid or base to a solution while monitoring changes such as pH
What happens when NaOH is added to a weak acid HA?
OH- reacts with HA, converting it to its conjugate base A-
During titration of a weak acid with strong base, what happens to HA?
HA decreases
During titration of a weak acid with strong base, what happens to A-?
A- increases
During titration of a weak acid with a strong base, what happens to pH?
pH increases
What is the buffer region of a weak acid-titration curve?
The relatively flat region near the pKa where pH changes only slightly as base is added
Why is the titration curve relatively flat near the pKa?
Significant amount of both the weak acid and conjugate base are present, so the solution buffers changes in pH
What happens at the half-equivalence point?
Half of the original weak acid has been converted into its conjugate base?
What is the relationship between HA and A- at half equivalence?
[HA] = [A-]
What is the relationship between pH and pKa at half equivalence?
pH = pKa
What is the equivalence point?
The point at which the acid has essentially been exactly neutralized
Key difference between half-equivalence and equivalence?
Half equivalence has equal HA and A- and pH = pKa
Equivalence occurs when the starting acid has been neutralized
Sequence to remember for weak-acid/strong-base titration?
Low pH → bufffer region → ½ equivalence: pH = pKa → steep rise → equivalence point → high pH
What is a buffer?
Solution that resists changes in pH when relatively small amounts of acid or base are added
What does a typical buffer contain?
A weak acid and its conjugate base
Give an example of a biological/biochemical buffer pair from CH2.
H2PO4- / HPO4 2-
What happens when H+ is added to and HA/A- buffer?
The conjugate base A- reacts with H+ to form HA
What happens when OH- is added to an HA/A- buffer?
HA reacts with OH- to form A- and water
How do buffers prevent large pH changes?
Their acid/base components consume much of the added H+ or OH-
When does a buffer work best?
When the pH is close to the pKa.
What is the approximate useful buffering range?
pKa ± 1
At pH = pKa, what is the ratio of conjugate base to weak acid?
1:1
Why is pH = pKa an especially effective buffering condition?
Equal amounts of acid and conjugate base are available to respond to an added base or acid.