1. Strength of Acids & Alkalis

2. Strong vs Weak Acids
• The strength of an acid depends on
the degree of ionisation/
dissociation of the acid in water.
• Strong acid
– Acid that Ionises completely in
water to give high concentration of H+
ion.
• Weak acid
– Acid that partially ionises in water to
give low concentration of H+ ion

3. Example of Strong & Weak
Acid
• Strong acid
– Hydrochloric acid
– All hydrogen chloride molecule ionises
completely in water into H+ ions and Cl- ions
– HCl  H+ + Cl-
– No hydrogen chloride molecules in acid
• Weak Acid
– Ethanoic acid
– Only few CH3COOH molecules ionises to form
H+ ions and CH3COO- ions
– CH3COOH H+ + CH3COO-

4. Ethanoic Acid Hydrochloric Acid
CH3COOH H-Cl
CH3COOH H-Cl
CH3COOH H-Cl
CH3COOH H-Cl
CH3COOH CH3COOH H-Cl H-Cl
H+ CH3COOH H+ Cl- H+
CH3COO-
CH3COOH
Cl- H+
CH3COOH
Cl-
H+
CH3COOH CH3COOH
H+ Cl-
Cl-

5. Strong vs Weak Alkali
• The strength of an alkali depends on
the degree of ionisation of the alkali
in water.
• Strong alkali
– Alkali that Ionises completely in water
to produce high concentration of OH-
ion
• Weak alkali
– Alkali that Ionises partially in water to
produce low concentration of OH- ion

6. Strong vs Weak Alkali
• Strong alkali
– Sodium hydroxide ionises completely when
dissolved in water
– NaOH  Na+ + OH-
• Weak alkali
– Only some ammonia molecules ionises in
water to produce ammonium ions, NH4+ and
hydroxide ions, OH-
– NH3 + H2O ⇄ NH4+ + OH-

7. Sodium hydroxide Ammonia solution
Na+ OH- Na+ OH- NH3
NH3
OH- Na+ OH- Na+
NH3
NH3
Na+ OH- Na+ OH-
OH- Na+ OH- Na+ NH3l NH3
Na+ OH- Na+
NH4+ NH3
+
OH- Na
OH- OH- NH3
Na+ NH3
Na+ OH- NH3
OH- NH3

8. Compare hydrochloric acid
& ethanoic acid
Type of acid pH value
Hydrochloric acid 1
Ethanoic acid 3
Between two acids,
• Which one is more acidic?
• Which one has higher concentration of H+ ions?
• Which one has higher degree of ionisation?
Compare the strength of the two acids and the degree of ionisation.
What are the difference between strong acid & weak acid?

9. Strong acid vs Weak Acid
Strong acid Weak Acid
Lower pH value Higher pH value
High degree of ionisation Low degree of ionisation
Has higher concentration Has lower concentration
of H+ ions of H+ ions
Example: Example:
hydrochloric acid Ethanoic acid
Nitric acid Carbonic acid
Sulphuric acid Sulphurous acid

10. Compare sodium hydroxide
solution & ammonia solution
Type of alkali pH value
Sodium hydroxide 14
solution
Ammonia solution 11
Between two alkalis,
• Which one is more alkaline?
• Which one has higher concentration of OH- ions?
• Which one has higher degree of ionisation?
Compare the strength of the two alkalis and the degree of ionisation.
What are the difference between strong alkali & weak alkali?

11. Strong alkali vs Weak Alkali
Strong alkali Weak Alkali
higher pH value lower pH value
High degree of ionisation Low degree of ionisation
Has higher concentration Has lower concentration
of OH- ions of OH- ions
Example: Example:
Sodium hydroxide Ammonia solution
Potassium hydroxide methylamine
Barium hydroxide hydrazine