1. 11.3 Strengths of Acids and Bases
Weak acids only partially dissociate in water.
Hydrofluoric acid, HF, is the only halogen that forms a weak acid.
Learning Goal Write equations for the dissociation of strong and weak acids; identify the direction of reaction.

2. Strong and Weak Acids
A strong acid completely ionizes (100%) in aqueous solutions.
HCl(g) + H2O(l) H3O+(aq) + Cl−(aq)
A weak acid dissociates only slightly in water to form a few ions in aqueous solutions.
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3− (aq)

3. Strong Acids In water, the dissolved molecules of HA, a strong acid,
dissociate into ions 100%.
produce large concentrations of H3O+ and the anion (A−).
The strong acid HCl dissociates completely into ions:
HCl(g) + H2O(l)
H3O+(aq) + Cl−(aq)

4. Weak Acids In weak acids, only a few molecules dissociate.
Most of the weak acid remains as the undissociated (molecular) form of the acid.
The concentrations of H3O+ and the anion (A−) are small.
H2CO3 is a weak acid:
H2CO3(aq) + H2O(l)
H3O+(aq) + HCO3−(aq)

5. Relative Strength of Acids and Bases

6. Strong and Weak Acid Dissociation
In an HCl solution, the strong acid HCl dissociates 100% to form H+ and Cl−.
A solution of the weak acid HC2H3O2 contains mostly molecules of HC2H3O2 and a few ions of H+ and C2H3O2−.

7. Strong and Weak Acid Dissociation
Figure 11.2 ▶ After dissociation in water, (a) the strong acid HI has high
concentrations of H3O+ and I–, and (b) the weak acid HF has a high concentration of HF and low concentrations of H3O+ and F–.

8. Diprotic Acids: Carbonic Acid
Some weak acids, such as carbonic acid, are diprotic acids that have two H+, which dissociate one at a time.
H2CO3(aq) + H2O(l) H3O+(aq) + HCO3−(aq)
Because HCO3− is also a weak acid, a second dissociation can take place to produce another hydronium ion and the carbonate ion, CO32−.
HCO3−(aq) + H2O(l) H3O+(aq) + CO32−(aq)

9. Diprotic Acids: Sulfuric Acid
Some strong acids, such as sulfuric acid, are diprotic acids that have two H+, which dissociate one at a time.
H2SO4(aq) + H2O(l) H3O+(aq) + HSO4−(aq)
Because HSO4− is a weak acid, a second dissociation can take place to produce another H+ and the sulfate ion, SO42− .
HSO4−(aq) + H2O(l) H3O+(aq) + SO42−(aq)

10. KOH(s) K+(aq) + OH−(aq)
Strong Bases
Strong bases as strong electrolytes
are formed from metals of Groups 1A (1) and 2A (2).
include LiOH, NaOH, KOH, Ba(OH)2, Sr(OH)2, and Ca(OH)2.
dissociate completely in water.
KOH(s) K+(aq) + OH−(aq)
are found in household products used to remove grease and unclog drains.

11. Weak Bases Weak bases are weak electrolytes
that are poor acceptors of H+ ions.
produce very few ions in solution.
include ammonia.
NH3(g) + H2O(l) NH4+(aq) + OH−(aq)
Ammonia Ammonium hydroxide

12. Strong and Weak Bases Strong Bases Lithium hydroxide LiOH
Sodium hydroxide NaOH
Potassium hydroxide KOH
Rubidium hydroxide RbOH
Cesium hydroxide CsOH
Calcium hydroxide Ca(OH) 2*
Strontium hydroxide Sr(OH) 2*
Barium hydroxide Ba(OH)2*
*Low solubility, but they dissociate completely
Bases in Household Products
Weak Bases
Window cleaner, ammonia, NH3
Bleach, NaOCl
Laundry detergent, Na2CO3, Na3PO4
Toothpaste and baking soda, NaHC3
Baking powder, scouring powder, Na2CO3
Lime for lawns and agriculture, CaCO3
Laxatives, antacids, Mg(OH) 2, Al(OH)3
Strong Bases
Drain cleaner, oven cleaner, NaOH

13. Direction of Reaction
Strong acids have weak conjugate bases that do not readily accept H+.
As the strength of the acid decreases, the strength of its conjugate base increases.
In any acid–base reaction, there are two acids and two bases.
However, one acid is stronger than the other acid, and one base is stronger than the other base.
By comparing their relative strengths, we can determine the direction of the reaction.

14. Direction of Reaction: H2SO4
Sulfuric acid, H2SO4, is a strong acid that readily gives up H+ to water.
H2SO4(aq) + H2O(l) H3O+(aq) + HSO4−(aq)
Stronger Stronger Weaker Weaker
acid base acid base
The hydronium ion H3O+ produced is a weaker acid than H2SO4.
The conjugate base HSO4− is a weaker base than water.

15. Direction of Reaction: CO32−
The carbonate ion from carbonic acid, H2CO3, reacts with water.
Water donates one H+ to carbonate, CO32− to form HCO3− and OH−.
From Table 11.3, we see that HCO3− is a stronger acid than H2O.
We also see that OH− is a stronger base than CO32−.
To reach equilibrium, the strong acid and strong base react in the direction of the weaker acid and weaker base.
CO32− (aq) + H2O(l) OH−(aq) + HCO3−(aq)
Weaker Weaker Stronger Stronger
acid base base acid

16. Study Check
Identify each of the following as a strong or weak acid or base:
A. HBr
B. HNO2
C. NaOH
D. H2SO4
E. Cu(OH)2

17. Solution
Identify each of the following as a strong or weak acid or base:
A. HBr strong acid
B. HNO2 weak acid
C. NaOH strong base
D. H2SO4 strong acid
E. Cu(OH)2 weak base

18. Study Check Using Table 11.3, identify the stronger acid in each pair.
A. HNO2 or H2S
B. HCO3− or HBr
C. H3PO4 or H3O+

19. Solution Using Table 11.3, identify the stronger acid in each pair.
A. HNO2 or H2S HNO2 is the stronger acid.
B. HCO3− or HBr HBr is the stronger acid.
C. H3PO4 or H3O+ H3O+ is the stronger acid.