Strong and Weak Acids and
Bases
Pg 560-578
The strength of an acid is determined by
the extent to which it ionizes, its percent
ionization, not the concentration of the
acid, the concentration of its hydronium
ions, or its ability to react with a metal
Strong Acid
An acid that nearly
completely
dissociates
All molecules of the
acid break up to form
the ions soluble in
water
If more than one proton is
being removed, not all
steps need to be
complete dissociation.
Weak Acid
An acid that only
slightly dissociates in
a water solution
Only a small percent
of acid molecules
donate their
hydrogen, and most
remain the same.
Example: CH3COOH
A strong acid essentially ionizes 100%.
An example of a strong acid is hydrochloric acid,
HCl (aq)
HCl(g) +
0.10 mol
H2O(l)  H3O+(aq) + Cl-(aq)
0.10 mol 0.10 mol
100% ionization
few molecules
many ions
An example of a weak acid is acetic acid,
CH3COOH.
CH3COOH(l) + H2O(l)  H3O+(aq) + CH3COO-(aq)
0.10 mol
<< 0.10 mol << 0.10 mol
5% ionization at 25C
many molecules
few ions
Strong Base
A base that
dissociates almost
completely into its
ions.
All oxides and
hydroxides of group 1
and 2 are strong
bases.
Ex: NaOH
Weak Base
Most bases are weak
They dissociate only
slightly in a water
solution
Example: NH3
Strong acids are strong electrolytes and
weak acids are weak electrolytes
A strong base dissociates 100%.
An example of a strong base is sodium
hydroxide, NaOH.
NaOH(s) + H2O(l)  Na +(aq) + OH-(aq)
0.10 mol
0.10 mol
0.10 mol
100% dissociation
few formula units (NaOH)
many ions
A weak base ionizes to a small extent.
An example of a weak base is NH3(g).
NH3(g) + H2O(l)  NH4+(aq) + OH-(aq)
0.10 mol
<< 0.10 mol << 0.10
mol
5% ionization at 25C
many molecules
few ions
Strong bases are strong electrolytes and
weak bases are weak electrolytes.
Examples of Strong Acids and
Bases
Strong Acids
Strong Bases
HClO4 perchloric acid
HCl hydrochloric acid
HNO3 nitric acid
H2SO4 sulfuric acid
HBr hydrobromic acid
HI hydriodic acid
LiOH lithium hydroxide
NaOH sodium hydroxide
KOH potassium hydroxide
RbOH rubidium hydroxide
CsOH cesium hydroxide
Ca(OH)2 calcium hydroxide
Sr(OH)2 strontium hydroxide
Ba(OH)2 barium hydroxide
To experimentally distinguish strong acids
from weak acids; and strong bases from
weak bases:
compare a strong acid to a weak acid of
equal concentration
– more hydronium ions and anions will be
present in the strong acid solution
compare a strong base with a weak base of
equal concentration
– more hydroxide ions and cations will be present
in the strong base solution
Therefore, we could compare a strong acid and a weak
acid of equal concentration by:
a) use a conductivity apparatus test (light bulb will be
brighter for a strong acid).
b) measure conductivity of solutions (strong acid will have
a higher conductivity).
c) react the two acids with a metal like magnesium
(stronger acid will react faster, more bubbling as H2 is
formed)
d) measure the pH of the solutions using a pH meter or
indicators (strong acid has a lower pH)
A strong base can be distinguished experimentally from
a weak base of equal concentration by:
a) use a conductivity apparatus test (light bulb will be
brighter for a strong base)
b) measure conductivity of solutions (strong base will have
a higher conductivity)
c) react the two bases with a chemical and observe the
rate of the reaction (stronger base will react faster)
d) measure the pH of the solutions using a pH meter or
indicators (stronger base has a higher pH)
Dissociation Equation
A balanced chemical equation showing all
ions produced when an ionic compound
dissolves
Example:
HSO4-(aq) + H2O(l)  SO42-(aq) + H3O+(aq)
Acids: Concentration vs. Strength
WEAK
STRONG
CONCENTRATED
H+ A- H+ A- H+ A- H+ A- HA
A- H+ A- H+ A- H+ A- H+ A H+ A- HA H+ A- H+ A- H+ AA- H+ A- H+ A- H+ A- H+ A- H+
H+ A - H + A - H + A - HA H + A A- H+ A- H+ A- H+ A- H+ A–
H+ A- H+ A- H+ A- H+ A- H+
A- H+ A- H+ A- H+ A- H+ AHA A- H+ A- H+ A- H+ A- H+
HA HA H+ A- HA HA
HA HA HA HA HA
H+ A- HA HA HA HA
HA HA H+ A- HA HA
HA HA HA H+ A- HA
H+ A- HA HA HA HA
HA HA HA H+ A- HA
H+ A- HA HA HA HA
HA HA H+ A- HA HA
HA
H+ AHA
HA
HA
HA
HA
HA
HA
DILUTE
H+
A-
H+
A-
HA
A-
H+
A-
H+
A–
H+
A-
H+
A-
H+
A-
H+
HA
H+
A-
HA
H+
HA
A-
HA
HA
HA
H+ A -
HA
HA
HA
HA
H+A–
A-
Dissociate nearly 100%
HA
H1+
+
A-
+
A-
H+
HA
HA
HA
H+A–
HA
STRONG ACIDS
HA
WEAK ACIDS
Dissociate very little
HA
H1+
Strong vs. Weak Acid
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 508
Comparing Strengths
Tables tend to list strong acids towards the
top, and strong bases towards the bottom
Figure 14.12 in text page 563
Strengths Of Conjugate Acid-Base Pairs
The stronger an acid, the weaker is its
conjugate base.
The stronger a base, the weaker is its
conjugate acid.
An acid-base reaction is favored in the
direction from the stronger member to the
weaker member of each conjugate acidbase pair.
Concentrated vs. Dilute
0.3 M HCl
10.0 M CH3COOH
Dilute, strong acid
Concentrated, weak acid
2.0 M HCl
Concentrated, strong acid
OR Dilute, strong, acid
12.0 M HCl
Concentrated, strong acid
Review and Practice
Page 558-559
# 1-2,4, 6-11