1. Strong and Weak Acids and Bases
Topic 8.4

2. 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

4. 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)

5. 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
At Equilibrium HA
H A-

6. common weak acids (don’t need to know)
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)

7. 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)

8. Reaction Rate with CaCO3
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 CH3COOH(aq)
[H+(aq)]
0.1 mol dm-3
~ mol dm-3 pH
1.00
2.87
Conductivity
High
Low
Reaction Rate with Mg
Fast
Slow
Reaction Rate with CaCO3

9. strong acids weak acids have more H+ ions, hence a low pH
higher conductivity
react more vigorously with metals,
have a more (-)∆H of neutralization (more heat)
dangerous
weak acids
opposite of strong acids

10. 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 (⇌)

11. 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)

12. 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%

13. common weak bases (don’t need to know)
ammonia
NH3(g) + (aq) ⇌ NH4+(aq) + OH-(aq)
amines (contain NH1,2, or 3) :
ethylamine
C2H5NH2(g) + H2O(l) ⇌ C2H5NH3+(aq) + OH-(aq)

14. 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

15. 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+] = M: 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