Strong and Weak Acids and Bases
Topic 8.4
ACIDS
• when a STRONG acid dissolves- all, or nearly all,
of the acid molecules dissociate to produce H+ or
H3O+ ions (reaction goes to “completion”)
– have a very high Kc value
• HA  H+(aq) + A-(aq)
0%
~100%
• HA + H2O(l)  H3O+(aq) + A-(aq)
0%
~100%
– equilibrium is so far to the right for strong acids that
we use a yields symbol () instead of an equilibrium
symbol (⇌)
Initial amount
of HA
HA
H+
A-
At Equilibrium
common strong acids (need to know)
• hydrochloric acid:
– HCl(g) + H2O(l)  H3O+(aq) + Cl-(aq)
• nitric acid:
– HNO3(l) + H2O(l)  H3O+(aq) + NO3-(aq)
• sulfuric acid:
– H2SO4(l) + H2O(l)  H3O+(aq) + HSO4-(aq)
• when a WEAK acid dissolves, very few acid
molecules dissociate to produce H+ ions
(equilibrium lies to the left)
– have a very low Kc value
• HA ⇌ H+(aq) + A-(aq)
99%
~1%
• HA + H2O(l) ⇌ H3O+(aq) + A-(aq)
99%
~1%
Initial amount
of HA
HA
HA
H + A-
At Equilibrium
common weak acids (know these)
• carboxylic acids (contains one or more carboxyl
groups, COOH):
– methanoic acid (CHOOH)
– ethanoic (acetic) acid
• CH3COOH(l) + H2O(l) ⇌ CH3COO-(aq) + H3O+
– carbonic acid: (CO2 in water)
• CO2(aq) + H2O(l) ⇌ H2CO3-(aq)
Characteristics of strong vs. weak acids
• acid strength does not change when a
solution is diluted, only the concentration
(molarity) or pH does
• for example, HCl is always a strong acid
– it just may be diluted down and therefore have a
pH close to 7 and low concentration
• DO NOT confuse strong and weak, with dilute
and concentrated
– concentrated = high molarity (M)
same concentration, different strength because
the week acid (CH3COOH) does not dissociate
into H+ as much as the HCl does
0.1 mol dm-3
HCl(aq)
0.1 mol dm-3
0.1 mol dm-3
CH3COOH(aq)
~0.0013 mol dm-3
pH
Conductivity
Reaction Rate
with Mg
1.00
High
Fast
2.87
Low
Slow
Reaction Rate
with CaCO3
Fast
Slow
[H+(aq)]
– strong acids
• have more H+ ions, hence a low pH
• higher conductivity
• react more vigorously with metals, metal
oxides, metal carbonates and bicarbonates
• have a more (-)∆H of neutralization (more
heat)
• dangerous
– weak acids
• opposite of strong acids
BASES
• when a STRONG base dissolves, nearly all the
base molecules dissociate to produce
hydroxide (OH-) ions solution
– have a very high Kc value
• BOH + (aq)  OH-(aq) + B+(aq)
0%
~100%
– equilibrium is so far to the right for strong bases
that we use a yields symbol () instead of an
equilibrium symbol (⇌)
common strong bases
(know these)
– all group I hydroxides and barium (group 2)
hydroxide
• NaOH(s) + (aq)  Na+(aq) + OH-(aq)
• KOH(s) + (aq)  K+(aq) + OH-(aq)
• Ba(OH)2(aq) + (aq)  Ba2+(aq) + 2OH-(aq)
• when a WEAK base dissolves, very few base
molecules dissociate to produce OH- ions
– have a very low Kc value
• BOH + (aq) ⇌ OH-(aq) + B+(aq)
99%
~1%
common weak bases
• ammonia
– NH3(g) + (aq) ⇌ NH4+(aq) + OH-(aq)
• amines (contain NH1,2, or 3) :
– ethylamine
• C2H5NH2(g) + H2O(l) ⇌ C2H5NH3+(aq) + OH-(aq)
= group 1 on periodic table
= group 2 on periodic table
Strong Bases
LiOH
NaOH
KOH
RbOH
CsOH
Ba(OH)2
Weak Bases
NH3
Amines (example: C2H5NH2)
strong vs. weak base characteristics
– strong bases
• has more OH- ions, hence a high pH
• higher conductivity
• have a more (-)∆H of neutralization (more heat)
• dangerous
– weak bases
• opposite of strong bases
Using experimental data to determine acids and bases
• Given the following are 0.1 M solutions, determine
whether an acid or base, and strength
–
–
–
–
–
–
–
–
–
ph of 1: strong acid
poor conductor with pH of 6: weak acid
fast reaction with magnesium: strong acid
[H+] = 10 -10M: weak base
[H+] = 10 -4M: weak acid
neutralized 0.1M HCl quickly: strong base
good conductor with pH of 13: strong base
slow reaction with calcium: weak acid
has a high -ΔH when reacting with a base: strong acid
• Strong acid  Weak conjugate base
– ALL of the acid donates H+
– almost NO H+ is accepted back
• Weak acid  Strong conjugate base
• Strong base  Weak conjugate acid
• Weak base  Strong conjugate acid
Acid
Strength
Base
Strength
H2SO4
Very Strong
HSO4-
Very Weak
HCl
Cl-
HNO3
NO3-
H3O+
Fairly Strong
H2 O
HSO4-
SO42-
CH3COOH
CH3COO-
H2CO3
Weak
NH4+
HCO3H2 O
HCO3-
Weak
Less Weak
NH3
Very Weak
CO32OH-
Fairly Strong