1. Strong and Weak Acids and Bases And Dissociation Constants
Strong and Weak Acids and Bases
And
Dissociation Constants

2. 10.3
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

3. 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.

4. 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

5. A strong acid essentially ionizes 100%.
An example of a strong acid is hydrochloric acid, HCl (aq)
HCl(g) H2O(l)  H3O+(aq) Cl-(aq)
0.10 mol mol 0.10 mol
100% ionization
few molecules many ions

6. 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

7. 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

8. Weak Base Most bases are weak
They dissociate only slightly in a water solution
Example: NH3

9. Strong acids are strong electrolytes and weak acids are weak electrolytes

10. A strong base dissociates 100%.
An example of a strong base is sodium hydroxide, NaOH.
NaOH(s) H2O(l)  Na +(aq) OH-(aq) mol mol mol
100% dissociation few formula units (NaOH) many ions

11. 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

13. Strong bases are strong electrolytes and weak bases are weak electrolytes.

14. Examples of Strong Acids and Bases
Strong Acids Strong Bases
HClO4 perchloric acid LiOH lithium hydroxide
HCl hydrochloric acid NaOH sodium hydroxide
HNO3 nitric acid KOH potassium hydroxide
H2SO4 sulfuric acid RbOH rubidium hydroxide
HBr hydrobromic acid CsOH cesium hydroxide
HI hydriodic acid Ca(OH)2 calcium hydroxide
Sr(OH)2 strontium hydroxide
Ba(OH)2 barium hydroxide

15. 10.4

16. Dissociation Equation
A balanced chemical equation showing all ions produced when an ionic compound dissolves
Example:
HSO4-(aq) + H2O(l)  SO42-(aq) + H3O+(aq)

17. The Acid-Dissociation Constant (Ka)
Strong acids dissociate completely into ions in water:
HA(g or l) + H2O(l) H3O+(aq) + A-(aq)
In a dilute solution of a strong acid, almost no HA molecules
exist: [H3O+] = [HA]init or [HA]eq = 0
Qc =
[H3O+][A-]
[HA][H2O]
at equilibrium, Qc = Kc >> 1
Nitric acid is an example: HNO3 (l) + H2O(l) H3O+(aq) + NO3-(aq)
Weak acids dissociate very slightly into ions in water:
HA(aq) + H2O(aq) H3O+(aq) + A-(aq)
In a dilute solution of a weak acid, the great majority of HA
molecules are undissociated: [H3O+] << [HA]init or [HA]eq = [HA]init
Qc =
[H3O+][A-]
[HA][H2O]
at equilibrium, Qc = Kc << 1

18. The Meaning of Ka, the Acid Dissociation Constant
For the ionization of an acid, HA:
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Since the concentration of water is
high, and does not change significantly
during the reaction, it’s value is absorbed
into the constant.
Kc =
[H3O+] [A-]
[HA] [H2O]
Therefore:
Kc =
[H3O+] [A-]
[HA]
The stronger the acid, the higher the [H3O+]
at equilibrium, and the larger the Ka:
Stronger acid higher [H3O+] larger Ka
For a weak acid with a relative high Ka (~10-2 ), a 1 M solution
has ~10% of the HA molecules dissociated.
For a weak acid with a moderate Ka (~10-5 ), a 1 M solution
has ~ 0.3% of the HA molecules dissociated.
For a weak acid with a relatively low Ka (~10-10 ), a 1 M solution
has ~ 0.001% of the HA molecules dissociated.

19. Figure 7.1: Graphical representation of the behavior of acids of different strengths in aqueous solution.
A Strong Acid
A Weak Acid

20. The Extent of Dissociation for Strong and Weak Acids