Strong and Weak Acids and Bases
Topic 8.3
ACIDS
• when a STRONG acid dissolves- all, or nearly all,
of the acid molecules react 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 react 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):
– acetic acid
• CH3COOH(l) + H2O(l) ⇌ CH3COO-(aq) + H3O+
• carbonic acid: (CO2 in water)
– CO2(aq) + H2O(l) ⇌ HCO3-(aq) + H+(aq)
Characteristics of strong vs. weak acids
• equal [conc] of strong and weak acids can be easily
distinguished:
– acid strength does not change when a solution is diluted,
only the concentration (molarity) does
•
DO NOT confuse strong and weak, with dilute and
concentrated
– concentrated = high molarity (M)
– 0.1 M HCl (strong) vs 0.1 M CH3COOH (weak)
• same concentration, different strength
0.1 mol dm-3 HCl(aq)
0.1 mol dm-3 CH3COOH(aq)
[H+(aq)]
0.1 mol dm-3
~0.0013 mol dm-3
pH
1.00
2.87
Conductivity
High
Low
Reaction Rate with Mg
Fast
Slow
Reaction Rate with CaCO3
Fast
Slow
– strong acids
• have more H+ ions, hence lower 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 react 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
– all group I hydroxides and barium 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 react 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
• C2N5NH2(g) + H2O(l) ⇌ C2H5NH3+(aq) + OH-(aq)
Strong Bases
LiOH
NaOH
KOH
RbOH
CsOH
Ba(OH)2
Weak Bases
NH3
Amines (ex: C2H5NH2)
strong vs. weak base characteristics
– strong bases
• has more OH- ions, hence higher pH
• higher conductivity
• have a more (-)∆H of neutralization (more heat)
• dangerous
– weak bases
• opposite of strong bases
• http://phet.colorado.edu/en/simulation/acidbase-solutions
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