BIOL 2112 Chapter Notes - Chapter 4: Lysine, Sodium Lactate, Partial Pressure
10 May 2018
School
Department
Course
Professor
Lecture 5 - Supplement and Study Guide – Acids, Bases, and Buffers
What is a conjugate acid-base pair? A conjugate acid is the species formed by the
reception of a proton H+ by a base. A conjugate base is the species that is left over after
an acid has donated a proton. In the following reaction, which is the acid and which is
the conjugate base? HA is acid, A- is conjugate base
HA H+ + A-
The tendency of an acid to form its conjugate base is shown in the following equation:
[H+] [A-]
Keq = = Ka
[HA]
Analogous to pH is the pKa, defined by the following equation:
pKa = log 1/Ka = -log Ka
The pKa is the measure of a tendency of a functional group to donate a proton. In a
manner analogous to pH, that tendency decreases 10-fold as pKa increases by 1 unit.
Does the dissociation constant, Ka, become larger or smaller with the increasing strength
of an acid? Larger How is the pKa related to the dissociation constant, Ka? pKa = -log Ka
Formic acid has a pKa of 3.74; acetic acid has a pKa of 4.76. Which is the stronger acid?
Formic Acid What is the difference between a monoprotic and a triprotic acid? Give an
example of each. Monoprotic releases one proton into the solution, triprotic releases 3
protons into the solution. Titration is the means to determine the amount of acid in a
given solution. In a titration, a base, usually sodium hydroxide (NaOH), is added in small
increments to a measured volume of acid solution. As NaOH is added, OH- dissociates
from the base to combine with H+ from acid to form H2O. A change in pH, as measured
by a pH meter, reveals the point when the acid is consumed and all free H+ is
incorporated into H2O. Each conjugate acid-base pair has a characteristic zone of
buffering – usually ~1 pH value on either side of the pKa. For example, acetic acid, with
a pKa of 4.76, buffers best in the pH range of 3.76 to 5.76. On the titration curve shown
on slide 4, what is shown in the flat zone on the curve? Keq
Buffers resist changes in pH most effectively at the
midpoint of a titration. The reason for this is explained
in the following question. At the midpoint of a titration,
how are the concentrations of an acid and its conjugate
base related? Equal. When does pH=pKa? At the
midpoint of a titration. Biological systems are very
sensitive to pH because amino acids and nucleic acids
contain ionizable groups. Where on an amino acid are
the ionizable groups? Carboxyl, Amino Where on a
nucleic acid is the ionizable group? Phosphate Slide 5
demonstrates how a buffer system works; there are two
reversible chemical reactions happening simultaneously. One reaction donates protons
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture 5 - supplement and study guide acids, bases, and buffers. A conjugate acid is the species formed by the reception of a proton h+ by a base. A conjugate base is the species that is left over after an acid has donated a proton. The tendency of an acid to form its conjugate base is shown in the following equation: Analogous to ph is the pka, defined by the following equation: pka = log 1/ka = -log ka. The pka is the measure of a tendency of a functional group to donate a proton. In a manner analogous to ph, that tendency decreases 10-fold as pka increases by 1 unit. Larger how is the pka related to the dissociation constant, ka? pka = -log ka. Formic acid has a pka of 3. 74; acetic acid has a pka of 4. 76. Monoprotic releases one proton into the solution, triprotic releases 3 protons into the solution.
